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CHAPTER 4. SCREENING ANALYSIS

TABLE OF CONTENTS

4.1 INTRODUCTION ..................................................... 4-1

4.1.1 Stakeholder Comments ............................................ 4-1

4.2 DISCUSSION OF DESIGN OPTIONS ..................................... 4-3

4.2.1 Design Options Used in the Engineering Analysis .......................... 4-3

4.2.1.1 Heat Traps ............................................... 4-4

4.2.1.2 Plastic Tank .............................................. 4-5

4.2.1.3 Increased Jacket Insulation ................................... 4-5

4.2.1.4 Insulating the Tank Bottom (Electric Only) ........................ 4-6

4.2.1.5 Improved Flue Baffle/Forced Draft ............................. 4-6

4.2.1.6 Increased Heat Exchanger Surface Area ......................... 4-7

4.2.1.7 Flue Damper (Electromechanical) .............................. 4-8

4.2.1.8 Side Arm Heater ........................................... 4-9

4.2.1.9 Electronic (or Interrupted) Ignition .............................. 4-9

4.2.2 Design Options Eliminated from Further Consideration .................... 4-10

4.2.2.1 Flue Damper (Buoyancy Operated) ............................ 4-11

4.2.2.2 Submerged Combustion .................................... 4-11

4.2.2.3 Directly Fired ............................................ 4-12

4.2.2.4 Condensing Option ........................................ 4-12

4.2.2.5 Condensing Pulse Combustion ................................ 4-13

4.2.2.6 Advanced Forms of Insulation ................................ 4-14

4.2.2.7 U-Tube Flue ............................................. 4-15

4.2.2.8 Thermophotovoltaic and Thermoelectronic Generators .............. 4-15

4.2.2.9 Reduced Burner Size (Slow Recovery) ......................... 4-16

4.2.2.10 Heat Pump Water Heater Options ............................ 4-16

4.2.2.11 Timer Controlled ......................................... 4-29

4.2.2.12 System Application Options ................................ 4-29

4.2.2.13 Sediment Removal Features ................................ 4-30

4.2.2.14 Two-Phase Thermosiphon (TPTS) Design ...................... 4-30

4.2.2.15 Air-Atomized Burner (Oil-Fired Only) ......................... 4-31

4.3 RESULTS ........................................................... 4-31

TABLES

Table 4.1 Design Options Used in the Engineering Analysis ......................... 4-32

Table 4.2 Design Options Eliminated from Further Consideration .................... 4-33
a

 Water heaters fueled by natural gas and LPG are considered as one product class for the purpose of identifying

design options. They are treated separately with respect to manufacturing cost, markup, retail price, and fuel price in

the Life-Cycle Cost and subsequent analyses.

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CHAPTER 4. SCREENING ANALYSIS

4.1 INTRODUCTION

The screening of design options is the first step in the water heater rulemaking process. The

purpose ofscreening isto identify those design optionsthattheDepartment will considerforthe engineering

analysis. The screening analysis also provides a discussion of the criteria for eliminating certain design

options from further consideration. On July 15, 1996, the Department issued the Process Improvement

(Interpretive) Rule.1 Section 4 ofthe Interpretive Rule articulatesfactorsto take into account in screening

design options. These factors are as follows:

C Technological feasibility,

C Practicability to manufacture, install and service,

C Adverse impacts on product utility or product availability,

C Adverse impacts on health and/or safety.

This chapter discusses various design options for improving the energy efficiency of electric, gasfired (natural gas and LPG),a

and oil-fired storage-type water heaters. Building on past analysis and

incorporating public commentsto the 1994 NOPR, a draft screening analysis was prepared and presented

at the Water Heater Standards Rulemaking Workshop held in Washington, D.C., on June 24, 1997, for

review and comment from stakeholders. This chapter addresses comments received at three different

stages of the rulemaking process since that initial draft was issued. These later comments have been

incorporated into the technical review of all design options. Based on this technical review, the design

options have been screened and pared down to a list of options used by DOE for the engineering and

economic analyses. 

4.1.1 Stakeholder Comments

Section 4 ofthe Interpretive Rule, whichestablishesthe processfordevelopingenergyconservation

standards,providesfor greater andmoreproductive interactionbetweenDOEand stakeholdersthroughout

the process. It is also designed so that key analysis is performed earlier in the process, with early

opportunitiesforpublic reviewand comment on that analysis. The process is consistent with the procedural

requirements of the law, but adds some important enhancements. 

There have been several opportunities for public comment on the residential water heating

standards process. DOE has considered all the comments it has received and presents below those
4-2

comments that specifically discuss DOE’s screening of design options.

After the June 1997 workshop, DOE received several written comments from stakeholders

pertaining directly to the design options discussed. There were a total of 15 sets of comments from

manufacturers, associations, gas and electric utilities, a consumer group, and a consumer. (All comments

are available for viewing in DOE’s Freedom of Information Reading Room.)

LacledeGasCompanyand theAmericanGas Association(AGA) advised DOEtofollowstatutory

requirements and not to screen out any design options.2, 3 Section 4 of the Interpretive Rule articulates

factors DOE will take into account in screening design options, selecting candidate standard levels for

further analysis, and selecting proposed and finalstandard levels. Therefore, DOE is following the criteria

listed in the Interpretive Rule and will proceed to reject certain design options from the analysis based on

these considerations. In reference to Section 4(a) of the Interpretive Rule, the Gas Appliance

Manufacturers Association (GAMA) provided a list of design options to keep and a listto screenout and

included reasons for screening them out.4 Bradford White Corporation supported the GAMA design

options discussed at the workshop.5 Edison Electric Institute (EEI), Southern Company, and Virginia

Power supported DOE’s screening analysis and list found in Appendix B, Supplement to the Water

Heater Rulemaking 

Framework. 6, 7, 8, 9

In accordance with the Interpretive Rule, the Department published the results of a preliminary

screening of the water heater design options in Technology Assessment and Screening Analysis,

Appendix BSupplement totheWaterHeater Rulemaking Framework, inJanuary 1998.9 Stakeholders

were notified of the availability of this document in the Federal Register on January 14, 1998. 10 The

Notice further requested comments on the Technology Assessment and Screening Analysis document.

DOEreceived 28 comments on the Technology Assessment and Screening Analysis document.

Six stakeholdersincludingGAMA supported, withmodifications, the Department’s position regarding the

screened design options. These comments addressed air atomized oil burners, power vents, and tank

bottom insulation for gas- or oil-fired water heaters. 

The majority ofcomments dealt with the Department’s decision to eliminate the heat pump water

heater design optionfromconsideration. AGA claims DOEmust consider alldesign options because DOE

is mandated by law to consider all technologically feasible design options. AGA further claims that DOE

disregarded the positionit tookin 1994 on heat pump water heaters. 11 Okaloosa Gas District, Clearwater

Gas System, the City of Mesa, Arizona, Milza S. Barley and thirteen gas utilities recommend that DOE

consider all design options, including heat pump water heaters.12, 13, 14, 15, 16 Laclede Gas objects to the

elimination of the heat pump water heater from standards consideration because it claims that DOE is

protecting electric utility market dominance and it could lead to fuel substitution away from gas water

heaters. 17 Laclede Gas further claims that heat pump water heaters should be considered as a design

option because DOE is introducing a new heat pump water heater which may overcome several critical
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market barriers including cost.18 Okaloosa Gas District believes that DOE should screen-in heat pump

water heaters because a recently published GAMA directorylistsfive models. 12 AGA and Okaloosa Gas

District claim that DOE relied on unsubstantiated statements by Southern Company regarding product

reliability and inadequate infrastructure to eliminate heat pump water heaters.11, 12 AGA claims that heat

pump water heaters are technically feasible and that reliability and infrastructure issues are not a sufficient

reason to eliminate them.19 DOE eliminated design options based on the criteria in the process

improvement rule (see the 61 FR 36974, section III. 4).1 DOE believes that the heat pump water heater

is technically feasible but DOE eliminated the heat pump water heater design option because it is not

practicable to manufacture, install, and service heat pump water heaters on the scale necessary at the time

of the effective date of this standard and because of adverse impacts on product utility. 

GAMA claims DOE was justified in screening out heat pump water heaters and the decision is

supported by criteria listed in 42 USC 6295(o).20 Furthermore, GAMA supports DOE’s decision to

eliminate specific design options.4, 21. EEI, Southern Company and Virginia Power agree with DOE’s

decision to screen out heat pump water heaters. 7, 22, 23, 24 DOE has continued to screen out heat pump

water heaters and several other design options listed in the screening document.

In October, 1998, DOEpublished Results and Methodology of the Engineering Analysis for

Residential Water Heater Efficiency Standards.25 This document, an overview of the preliminary

engineering analysis for Residential Water Heater Efficiency Standards, was available to the public prior

to the November 1998 Water Heater RulemakingWorkshop. The document contained a section updating

the design options screening process based on comments received on the Technology Assessment and

Screening Analysis. DOE did not revise its position on heat pump water heaters, but further elaborated

on the rationale for eliminating heat pump water heaters as a design option. 

4.2 DISCUSSION OF DESIGN OPTIONS

The design options considered here are grouped in two categories. The first category is design

options used by DOE in the engineering and economic analyses. The second category contains design

options that have been eliminated from further consideration. The discussion under this second category

also includes the screening criteria used by DOE.

4.2.1 Design Options Used in the Engineering Analysis

DOE used the nine design options listed belowinthe engineering and economic analyses because

they are currently (or have been recently) applied to commercial or residential water heaters and pass all

of the screening criteria. In the engineering analysis phase, certain ofthese design options were combined

to meet an efficiency level based on a simple payback estimate. 
4-4

• Heat Traps

• Plastic Tank

• Increased Jacket Insulation

• Insulating the Tank Bottom (Electric Only)

• Improved Flue Baffle/Forced Draft

• Increased Heat Exchanger Surface Area

• Flue Damper (Electromechanical)

• Side Arm Heater

• Electronic (or Interrupted) Ignition.

4.2.1.1 Heat Traps

The heat conducted and convected through the fittings (water pipes, drain valve, pressure relief

valve, and thermostat) accountsfor about 15% ofthe totalstandbylossina typicalresidential-size gas-fired

water heater.26 A heat trap is a device or arrangement of piping that keeps the buoyant hot water from

circulating through the piping distributionsystembecause ofnaturalconvection. When there is no draw of

hot water, this device prevents water in the hot water outlet line fromgetting back into the tank as it cools

off; and prevents hot water in the tank from circulating back into the cold water inlet line. Thus, by

containing the hot water in the storage tank, the heat trap minimizes standby loss. These devices can be

integral to the tank design or independently attached to the inlet and outlet pipes during installation at the

site. Paul et al. have shown that heat traps can increase the energy factor (EF) of residential-size water

heaters by 1%.26 Also, the effectiveness of a heat trap can be increased by insulating its exposed portion.

Conventional heat traps are currently made in two styles. In the first style, a floating plastic ball

blocks the cold water inlet. The buoyancy of the plastic holds it in place until water is drawn. The force

ofwater isstrong enough to push the ball out of the wayas water entersthe tank. The second style is used

for the hot water outlet. In this heat trap, the ball is denser thanwater, and the weight of the ball seals the

outlet until hot water is drawn and the water pressure lifts it out of the way.27 A small bypass channel is

left for water to escape back into the inlet line from the tank as it expands after a large draw fills the tank

with cold water. Heat traps could also be of metallic design. Manufacturing costs appear to be roughly

comparable betweenthe metaland plastic heat traps, but plastic heat traps are considerably more effective

in reducing water heater standby losses.

Other heat trap designs have also been invented and produced. These include U-shaped pipes,28

flexible seals,29 flaps, springs, and other mechanisms. 

Bradford White Corporation concludes that heat traps are not a design option because they are

noisyand viewed as a deterrent for product utility by consumers and cost more thanthe energy savings can

repay.5 On the other hand, GAMA recommends that heat traps be considered in the DOE analysis
4-5

because this technology is already implemented in most models.30

DOEagreedwithGAMA’scomment and considered this design optioninthe engineering analysis.

4.2.1.2 Plastic Tank

Plastic water heater tanks can be constructed using a seamless, blow-molded polybutylene inner

tank with a filament-wound fiberglass outer tank, similar to the fabrication of tanks for water softeners.31

A second method consists of constructing a thin steel shell withaninternalplastic tank. The steel exterior

is constructed first; then, plastic powder isinjected into the shell, and the tank is rotated in a furnace to coat

the interior with the plastic. The steel exterior serves as the primary structural support for the tank. The

lower heat conductivity of plastic compared to metal reduces the amount of heat conducted through the

tank wall to the insulation and to the feed-throughs.

However, the plastic tank cannot be used withstandard center-flue gas-fired water heaters or with

oil-fired water heaters, because the plastic cannot withstand the hightemperaturesproduced by the flames.

This option can be used onlywithelectric water heaters or withindirect water heating techniques (e.g., the

side arm water heater or heat pipe technologies) that avoid flame temperature problems.

GAMA recommends eliminating plastic tank water heaters because it is notanefficiencyimproving

design option. GAMA believes DOE should rely on increased insulation thickness for efficiency

improvements.4

DOEbelievesthat the plastic tank construction method enables an improved process ofinsulating

the tank bottom. Therefore, this design option may, in fact, reduce standby losses in certain applications

and should be investigated further. DOE considered this design option in the engineering analysis.

4.2.1.3 Increased Jacket Insulation

Manufacturersinsulatewater heaters by fillingthe cavity between the jacket and the tank (top and

sides) with polyurethane foam insulation or fiberglass insulation. Because polyurethane foam is a more

effective insulation than fiberglass, it is more widely used. Most water heaters on the market today have

at least 1-inch thick foam insulation, while some models have 2- or 3-inch thick insulation. The baseline

electric unit for the present analysis has 1.5 inches ofinsulationinorder to achieve the minimumEF levels..

The Gas ResearchInstitute (GRI)reportsthatfor a gas-fired water heater witha40-gallonstorage

tank with 1-inch polyurethane foam insulation around the jacket and fiberglass insulation around the

combustionchamber, the jacket losses are about 50% of the total standby losses.32 For a tank withmore

than 40 gallons of storage volume, the jacket losses as a percent of the total standby losses will be even

higher. Although increasing the insulation thickness reduces the standby loss, the increase in the overall
a

 In June 1999, Allied Signal and Honeywell announced their merger; in October 2000, GE acquired the newly

combined company.33, 34

4-6

diameter of the water heater may pose some installation problems. There may also be an increase in

shipping costs because fewer water heaters will fit in a truck.

Bradford White Corporation commented that the uncertainty over the blowing agent for foam

insulation be handled by limiting the cavity size.5 DOE has limited cavity size to 3 inches. 

DOE has used the blowing agent HFC-245fa in its analysis. On September 13, 1999, Allied

Signalaannounced in a press release that it was planning to manufacture HFC-245fa at its Geismar,

Louisiana facility.35 Allied Signal has received EPA approval for HFC-245fa. At the July 23, 1999

workshop AlliedSignalannounced a plant would be online by mid 2002.36 It appears that HFC-245fa will

be available by the effective date of this rulemaking, therefore, DOE chose this blowing agent for the

engineering and economic analyses. 

4.2.1.4 Insulating the Tank Bottom (Electric Only)

The bottomofthe tank of an electric water heater can be insulated as an alternative design option

to reduce the standby heat loss. This is not conventionally done. As evidence see the losses visible in the

infrared thermograph presented by NISTat the November 1998 workshop.37 DOEbelievestank bottom

insulation will significantly reduce these losses. DOE included this design option in the engineering and

economic analyses.

4.2.1.5 Improved Flue Baffle/Forced Draft 

The standard flue baffle is a twisted strip of metal inserted into the flue that increases the 

turbulence of flue gases and improves heat transfer to the walls ofthe flue. The geometry of the flue baffle

canbe modified to increase its effectiveness. One manufacturer uses a flue with many small rectangular fins

attached on itsinside surface. The arrangement and size of the fins in various models increases the recovery

efficiency of the water heater. Other baffle configurations that increase recovery efficiency are also

available.

A researchproject funded by the GRI reviewed technical literature, manufacturers' literature, and

patentsto determine what new technologies are applicable to heat exchangersthat involve flue gasesfrom

combustion of natural gas.38 The conclusion was that significant increases in the convective heat transfer

coefficient could be achieved with the use of heat transfer enhancement devices. The study suggested that

insome cases, an increase in heat transfer coefficient might be accompanied by anincrease inthe pressure

drop (due to an increase in the friction factor). The studyidentified twisted-tape inserts as a potentialheat

transfer enhancement device for water heaters. 
4-7

Beckermann and Goldschmidt 39 investigated experimentally and empirically the effects ofvelocity

of the flue gases, the twist (i.e., number ofturns) of the tape, and the surface emissivities on the total heat

transfer (convection and radiation) in a fuel-fired water heater. They reported that compared to an empty

tube, the flue tube with twisted tape enhances the overall heat transfer performance by as much as 50%.

This design optionof animproved flue baffle canincreasethe recoveryefficiencyto about 80-85%,

depending on the specific geometry. At the upper end of the recovery efficiency range, the water heater

would require power venting or induced draft and corrosion resistant flues for safe operation. Because of

the potential for condensation caused corrosion, recoveryefficiencies above 80% were not considered in

this analysis.

In addition to an increase in efficiency, there is also a reduction in standby loss. The off-cycle

standby loss is reduced by the additional restrictions to airflow due to the increased baffling.

Several manufacturers currently make water heaters with induced-draft blowers. However, this

feature is usually added to allow sidewall venting and may not be accompanied by any increase in flue

baffling.

Some manufacturers make water heaters with induced draft fans that, in addition to pulling the

combustion products through the water heater, also draw excess air into the flue gases prior to venting.

The additionalair cools flue gasesleaving the water heaterto a low enough temperature so standard plastic

piping canbe used for venting. This eliminates any problems with corrosion. Plastic piping is often cheaper

and easier to install than sheet metal or masonry chimneys. 

While this technique of flue gas dilution does not necessarily increase water heater efficiency by

itself, when combined with an improved flue baffle that increases recovery efficiency, it can help avoid

venting problems. DOE included the improved flue baffle/forced draft design option in the engineering

analysis.

4.2.1.6 Increased Heat Exchanger Surface Area

There are two fundamental ways to improve the heat transfer from the flue gas to the water

including the use of increased heat exchanger surface area and improved flue baffle. The improved heat

transfer leads to an increase inthe thermalefficiencyofthe water heater. However, the thermal efficiency

is limited to about 84%, at which point condensation of the flue gases begins to occur in the flue or vent

pipe that may cause corrosion of the surfaces. To avoid such problems, use of this design option will be

limited to an 80% recovery efficiency (or about 82% thermal efficiency) level.

The design option of increased heat exchanger surface area canbe achieved using, among others,

the following two modifications of the basic design of the standard gas-fired storage water heater: (1)
4-8

submerged combustion chamber and (2) multiple flues. The first is to surround the combustion chamber

with water; the other is to use a number of smaller flue tubes instead of one large flue to vent the exhaust

gasesfromthe combustionprocess. This design option can increase recovery efficiency sufficiently so that

condensation (and hence corrosion) is likely to occur in venting systems so in our analysis we limited the

recovery efficiency to 80% or lower.

 

Submerged Combustion Chamber. The combustion chamber in a standard gas-fired storage

water heater is below the water tank, and the bottomof the tank (below the burners) is seldom insulated.

Therefore, the water heater loses heat from the bottom of the tank. The sides and bottom of the

combustionchamber are not surrounded by water. By inserting the combustion chamber into the storage

tank, more of the combustion energy can be recovered. Standby losses are reduced somewhat because

of restrictions on the air flow through the combustion chamber and flue.

Multiple Flues. The multiple-flue design usesseveralsmallerfluesinplace ofone large centralflue

inthe middle ofthe storage tank. The increased surface area for heat exchange between the flue gases and

the water in the tank yields an increase in recovery efficiency. One manufacturer is currently offering this

design in a residential gas-fired water heater.

DOE considered the use of these design modifications in the engineering analysis.

4.2.1.7 Flue Damper (Electromechanical)

Gas-fired storage water heaters are equipped with a draft hood connecting the flue pipesto a vent

pipe or chimney. During off-cycle, the water heater loses heat by natural convection and conduction

through the vent pipe or chimney. A damper can be installed either at the flue exit or in the vent pipe to

minimize the off-cycle heat losses. Aflue damper isinstalled upstreamofthe draft diverter, while the vent

damper is installed downstream of the draft diverter.

Electric flue dampers are activated by an external source of electricity. The dampers open when

combustion starts and close immediately after combustion stops. Therefore, there is a greater reduction

in off-cycle losses compared to buoyancy activated dampers [see Flue Damper (Buoyancy Operated)].

Whenthe damper reachesthe openposition, aninterlock switchenergizesthe solenoid and enablesthe gas

ignitioncircuit. Therefore, the burner cannot be ignited when the damper is in the closed position. Because

the dampers open and close immediately, no bypass is needed. A knockout is provided to vent the flue

gases from a standing pilot. The electric flue damper needs a 24-volt electric source and consumes about

5W when the gas supply is off.

Flue/vent dampers have no effect on the steady-state performance of the water heater. However,

in a field test of a 70-gallon gas-fired storage water heater with rated input capacity of 36,000 Btu/h, the

addition of an electric flue damper reduced the standby loss from 113,000 Btu/day to 46,000 Btu/day.

The overall system service efficiency increased from 61% to 65%, a significant increase in energy
4-9

efficiency.40

Itshould be noted there is anincreased installationcostfor electricalsupply and maintenance costs

are expected to be higher. DOE included the design option of the electromechanical flue damper in the

engineering analysis.

4.2.1.8 Side Arm Heater

The side armheater design avoidslarge flue losses by removing the flue fromthe center ofthe tank.

Water is withdrawn from the bottom of the tank and heated over a burner in a small, separate heat

exchanger. Water is returned to the top of the tank. A small circulation pump moves water through the

heat exchanger when the burneris on. The burner could have electronic ignition, which would reduce the

pilot light losses. Auxiliary power is supplied by a low-voltage plug-in transformer.

A water heater using this design in combination with electronic ignition and a plastic tank was

commercially available until 1999. DOE included this design option in the engineering analysis.

4.2.1.9 Electronic (or Interrupted) Ignition

Themostcommonly used ignitionsysteminstorage water heatersis a standingpilotignition system.

The disadvantage is it burns gas continuously at a rate of about 450 Btu/h, and only part of this heat is

converted to useful energy. Inadditionto a standing pilot, three electronic ignition devices are commonly

used in gas-fired equipment:

Intermittent Pilot Ignition. Thisis a device that lightsapilot by generating a spark, whichinturn

lights the main burner.

Intermittent Direct Ignition. This system lights the main burner directly by generating a spark.

Hot Surface Ignition. This is a system that lights the main burner directly from a very hot surface.

Unlike standing pilot ignition systems that consume gas continuously, these devices operate only

at the beginning of eachon-period. Although there is no increase in the steady-state efficiency with use of

electronic ignitiondevices, the overall fuel consumption may be reduced. Burner on-time mayincrease to

make up for the heat the standing pilot would have supplied during standby periods.

The “interrupted ignition” systemfor anoil-fired burner activatesthe spark only until a steadyflame

is established. The oil consumption is not affected by interrupted ignition, nor is there an improvement in

the recovery efficiency of the water heater. However, this design option not only reduces the igniter’s

electricity consumption, but also reduces its maintenance costs because the electrodes do not have to be

replaced as often. In additionto changesincontrols, the igniter can be made from solid-state electronics,
4-10

instead of an iron core transformer. This improves performance and also reduces power consumption. 

DOE included the electronic ignition design option in the engineering analysis.

4.2.2 Design Options Eliminated from Further Consideration

DOE may eliminate design options in accordance with the mandate of the process improvement

(Section 4 of the Interpretive Rule, “Process for Developing Efficiency Standards and Factors to be

Considered”) per the Interpretive Rule.1 Technologically feasible design options (i.e., those design options

that are already in use by industry or options that are proven by research and progressing towards the

development of a prototype) have been identified in this report. In additionto technologicalfeasibility, the

other screening criteria for the design options are:

• Practicability to manufacture, install, and service,

• Impacts on product utility or product availability, and

• Impacts on health and safety.

If production of a technology in commercially available products and reliable installation and

servicing of the technology could be achieved on the scale necessary to serve the relevant market at the

time of the effective date of the standard, then that technology will be considered practicable to

manufacture, install and service. If a technology is determined to have significant adverse impact on the

utility of the product to significant subgroups of consumers, or if adoption of a technology results in the

unavailabilityof any covered product type with performance characteristics(including reliability),features,

sizes, capacities, and volumesthat are substantially the same as products generally available in the U.S. at

the time, it will not be considered further.Ifit is determined that a technology will have significant adverse

impacts on health or safety, it will not be considered further.

DOE eliminated the following design options fromfurther consideration because they do not meet

the screening criteria as described in eachoftheir individualdiscussions. Some of the issues are highlighted

by the fact that the design options, if implemented, can conflict with national building codes, plumbing

codes, or othertechnicaltesting criteria. Also, in some cases, they do not meet the criteria of practicability

to install and service, they have adverse impact on product utility or product application, or they are not

an efficiency improvement feature.

• Flue Damper (Buoyancy Operated)

• Submerged Combustion

• Directly Fired

• Condensing Option

• Condensing Pulse Combustion

• Advanced Forms of Insulation
4-11

• U-Tube Flue

• Thermophotovoltaic and Thermoelectronic Generators

• Reduced Burner Size (Slow Recovery)

• Heat Pump Water Heater Options

• Timer Controlled

• System Application Options

• Sediment Removal Features

• Two-Phase Thermosiphon (TPTS) Design

• Air-Atomized Burner (Oil-Fired Only)

4.2.2.1 Flue Damper (Buoyancy Operated)

This flue damper is a small, very lightweight aluminum dome-shaped poppet that slides up and

down in an enclosure placed at the top of the flue of a gas-fired water heater. The buoyancy of the

combustion products lifts the poppet, allowing flue gasesto enterthe venting system. Working prototypes

have been tested by AGA.

This design option would reduce off-cycle standby losses, but would have no effect on recovery

efficiency. This flue damper may not work with high recovery efficiency water heaters because there may

not be enough waste heat in the combustion products to provide sufficient buoyancy to lift the poppet. 

The standard for gas water heaters 41 requires the burner to shut off if the flue gets blocked for

some reason. Thus the effects of a failure of the flue damper to open should be mitigated by the burner

controls. More field tests, however, are planned to address other safety and operational concerns.

GAMA statesthat the buoyancyoperated flue damper is only a concept that needs more research

and testing to determine that it will not cause safety problems. A water heater with a buoyancy operated

flue damper must also demonstrate compliance with existing appliance safety standards.4

DOE agreed withGAMA and believes that additional information on safety issues and long-term

use is required.

4.2.2.2 Submerged Combustion

Inthis design option, the flue products arebubbledthrough a small volume ofwater by the pressure

fromthe burner blower. This small amount of water is heated by direct contact with the flue products. The

heated water is re-pressurized by a circulating pump and returned to the storage tank. 

The direct contact heat exchange process as described above is a more efficient means of

transferring heat than in a conventional tube-and-shell heat exchanger, but can also lead to the
4-12

contamination of the domestic water by the flue products. This can cause health and safety problems for

the end users. This design option can also cause conflict with local plumbing codes (e.g., International

AssociationofPlumbing and MechanicalOfficials (IAPMO) 199142) with respect to water quality. Local

code restrictions for health reasons may also conflict with the atmospheric emissions from the unit.

Therefore, DOE eliminated this design option because of its likely adverse impacts on health and safety.

4.2.2.3 Directly Fired

Inthis design option, water is sprayed through a series of baffles above the burner. Flue products

are in direct contact with the water, which is re-pressurized by a circulating pump and returned to the

storage tank. 

The direct contact heat exchange process as described above is a more efficient means of

transferring heat than in a conventional tube-and-shell heat exchanger, but can also lead to the

contamination of the domestic water by the flue products. This can cause health and safety problems for

the end users. This design option can also cause conflict with local plumbing codes (e.g.,IAPMO 1991)

withrespect to water quality. Therefore, DOE eliminated this design option because of its likely adverse

impacts on health and safety.

4.2.2.4 Condensing Option

Condensing the combustionproductsinthe flue gas extracts more heat inthe formoflatent energy,

leading to an increase in the thermal efficiency of the water heater. The flue-gas condensate is corrosive

and often contains acids. Therefore, special corrosion resistant heat exchangers and vent linings are

required for safe and reliable operationofthe water heater. A number of studies and field tests have been

conducted to quantify the corrosion characteristics of condensing gas appliances. 43 Since 1979, Battelle

has conducted research on corrosion-resistant heat exchangers for condensing appliances for GRI and

DOE. Based on this research, a set of guidelines was developed for the design of condensing heat

exchangers. The guidelines consist of minimizing condensation in noncondensing regions of heat

exchangers, reducing the corrosiveness offlue-gas condensate and usingmaterials having a high corrosion

resistance to flue-gas condensate in the condensing region.44 European experience with condensing

appliances has also shown that in addition to metal corrosion, problems could occur in rubber seals and

in flue pipes.45 Corrosion due to condensation of combustion gases limits the thermal efficiency of a fuelfired water heater with a standard flue and ventsystemto 84%. Using corrosion resistant heat exchangers

or side-wall venting, or lining the vent/masonry systems with corrosion resistant material, can extend the

thermal efficiency limit beyond 84%. 

Condensing gas appliances can be of two types: fully-condensing or near-condensing. Fullycondensing appliances will have flue gastemperatureslessthanthe dew point (130/F to 140/F) of the flue

products. Condensation is expected in both the heat exchanger and the vent system. Near-condensing
4-13

appliances will have flue gas temperatures greater than the dewpoint of the flue gas. Condensation is

expected inthe ventsystembut not inthe heat exchangers. The thermal efficiency of fully condensing water

heaters can be as high as 99%; for near condensing water heaters, it is generally between84% and 90%.

 

Severalmanufacturers offernear-condensingwater heaters and a few offerfully-condensing types.

Currently these are all commercial water heaters. A large-scale field test, sponsored by GRI, compared

the performancecharacteristicsof condensing withconventionalnon-condensing commercialwaterheaters

46. A detailed inspection of the nine prototype units after 2 years of field operation uncovered no serious

concerns regarding reliability or durability. The overall results confirmed the technical feasibility of the

condensing design and that it provided a substantialefficiencyimprovement over conventional equipment.

Although the field testshowed no corrosion in the heat exchangers after 2 years offield operation,

Lennox Industries noted, for certain regions of the country, a high incidence of problems with heat

exchangers in their condensing pulse combustion warm-air furnaces.47 The secondary heat exchangers

(primarily used to extract latent heat) had very high sulfur concentrations because ofthe highsulfur content

inthe supply fuel. Similar limitations may exist in gas-fired condensing water heaters. DOE is co-funding,

with GRI and American Water Heater Products, a research project into inexpensive residential-sized

condensing water heaters. The objective of the researchisto find a lowcost method to resolve problems

with corrosion in the condensing heat exchangers. Completion date for this research is April 2001. 

There are also limitations with respect to the servicing and maintenance infrastructure in retrofit

situations. To install a condensing flue gas water heater, the vent systems have to be replaced or modified

to prevent corrosion or damage from flue-gas condensate. Whentwo or more gas appliances are vented

through the same flue, replacing the standard gas water heater with a condensing gas water heater could

orphan a standard gas furnace. Essentially, the flue becomes oversized for the remaining appliance.

DOE is not aware of any commercially available residential size condensing water heaters, or of

any prototypes and doesn’t expect information on whether commercially viable products are possible until

itsresearchproject iscompletedinApril 2001. Mass productionofthistechnology incommercialproducts

on the scale necessary to serve the gas-fired residential water heater market at the time of effective date

of this standard is not possible. Therefore, this product is not considered further. 

4.2.2.5 Condensing Pulse Combustion

Pulse combustionburners are another condensing technology. Pulse combustionburners operate

on self-sustaining resonating pressure waves that alternately rarefy the combustion chamber (drawing a

fresh fuel/air mixture into the chamber) and pressurize it (causing ignition by compression heating of the

mixture to its flash point). This process is initiated by a blower supplying an initial fuel and air mixture to

the combustion chamber. The mixture is ignited with a spark. Once resonance is produced, the process

becomesself-sustaining.48 Pulse frequencies are on the order of 100 cycles per second. Pulse combustion

systems feature high heat transfer rates, are capable of self-venting, and can draw outside air for
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combustion even when installed inside. The emissions from pulse combustion burners are 50% to 66%

lower than those of a conventionalburner.48 Because the pulse combustion process is highly efficient, the

burners are generally used withcondensing appliances. AGA Laboratories built several prototypes as part

of a study of pulse combustion residential water heaters.49 They reported that the prototype models had

a recovery efficiency of more than 90%. However the costs of pulse combustion control systems and

condensing heat exchanger materials were too high for manufacturing water heaters for the residential

market, and no further development activities were considered.

Thistechnology has not been developed for oil-fired equipment. Also, as noted in the condensing

option discussion, Lennox Industries has experienced problems with heat exchangers in their condensing

pulse combustion warm-air furnaces. The primary concerns with using this design option are the likely

condensation of flue gases and the need to reline or replace vent systems to prevent corrosion.

While prototype residentialsize pulse condensing water heaters have beenbuilt,DOEis not aware

of any commercially available residential size pulse condensing water heaters, or of any plans to develop

them. Without further research and development mass production of this technology in commercial

products on the scale necessary to serve the gas-fired residential water heater market at the time of

effective date of this standard is not possible. Therefore, this product is not considered further. 

4.2.2.6 Advanced Forms of Insulation

Alternate ways of reducing the jacket losses without increasing the diameter of the water heater

include the use of advanced insulation materials or the use of evacuated panels. Some of the advanced

materials or methods ofinsulationconsidered here involve the use ofvacuum, inertgases,aerogel, or partial

vacuums.

Vacuum Insulation. A “hard” vacuum between internal reflective surfaces is a very good

insulator. It has been used for years in Thermos bottles and Dewar tanks for cryogenic applications.

Durabilityand the difficultyof maintaining the seal over the life of the water heater are some problems with

this technology that have to be resolved.

Gas-filled Panels. Gas-filled panels are thermal insulating devices that retain a high concentration

of a low-conductivitygas, at atmospheric pressure, within amultilayerinfraredreflective baffle. The thermal

performance of the panels depends on the type of gas fill and the baffle configuration. Calorimetric

measurements have shown totalresistance levels of about R-12.6 for a 1-inchthick kryptonpanel, R-25.7

for a 2-inch krypton panel, and R-18.4 for a 1-inch xenon panel. Gas-filled panels are flexible, selfsupporting, and can be made in a variety ofshapes and sizesto thoroughly fillmost types of cavities. This

technology has not been demonstrated for water heater applications.

Aerogel Insulation. An example of advanced insulation materials is silica aerogel, which is

composed of96% air and the remaining 4% of a wispymatrix ofsilica (silicondioxide). Aerogels are more
a

 A torr is a unit of measure for vacuum levels. 1 torr equals 133 Pascals (0.0193 psi).

4-15

efficient and weigh less than the polyurethane foam that is currently used in most water heaters. The Rvalue ofthe aerogelat atmospheric pressure iscomparable to that ofthe polyurethane foam, but when90%

ofthe air is evacuated froma plastic-sealed aerogelpacket,itsresistance nearly triples. Another advantage

of the aerogel insulation over foam insulation is avoiding the use of chlorofluorocarbons to blow the

polyurethane foam into the heater jacket. New manufacturing processes have been developed that can

produce flexible blankets or clamshell forms of this material. The aerogel material is vulnerable to shock

and vibration, however, and materialhandling becomes anissue. Because it is hygroscopic, it also requires

a thorough sealing of the cavity betweenthe water heatertank and the outside cover. The material has not

been demonstrated for use with water heaters.

Evacuated Panels. Inadditionto aerogels, other materials with a lightweight open structure can

create verygood insulators at “soft”or lowvacuums. The materials can be enclosed with metals or plastic.

At least one company sells evacuated panels for use with special purpose, marine applications,

refrigerators.50 A vacuum (10-3 torra) is drawn in this panel before sealing, and a lightweight, rigid foam

keeps the vacuum from compressing the panel. This technology has not been demonstrated for curved

surface applications, such as water heater tanks.

GAMA recommends elimination of advanced insulation materials because it would require the

industry to completely restructure their production lines to accommodate a radically different way of

insulatingwater heaters. GAMA claimed that manufacturers have not established methodsto manufacture

water heaters with these types of materials.4 DOE agrees with GAMA on the issue of manufacturability,

particularlyinthe context ofrestructuring water heater productionlines. Moreover, reliability and durability

overthe life ofthe water heater ofsome ofthese advanced forms ofinsulationfor water heater applications

have not been demonstrated. Nor has such insulation been applied to water heaters as prototypes or in

commercially available products. Therefore, these have not been demonstrated to be technically feasible.

4.2.2.7 U-Tube Flue

One comment on the 1994 NOPRmentioned anold water heater design that used an inverted Ushaped flue within the tank of the water heater51

. This design could increase the recovery efficiency and

would reduce standby losses. No working prototypes or commercially available products are currently

available and DOE believesthat this design would not meetmodernsafety standards, because of possible

flue-gas condensation in the U-tube. Lack of working prototypes precludes DOEfromtesting the option

further and validating the applicability or usefulness of this particular design option. Therefore, this design

option is considered not technologically feasible.
4-16

4.2.2.8 Thermophotovoltaic and Thermoelectronic Generators

A thermophotovoltaic water heater uses a special light-emitting burner coupled with silicon

photovoltaic cells that generate auxiliary power to run a fan, operate the electronic ignition and controls,

and charge a battery. This avoids the requirement of auxiliary electrical supply, while offering the efficiency

advantages of electronic ignition and forced-draft combustion. DOE has funded the development of

prototypes of this design option in the past, but there has beenno recent activity. This technology has not

been demonstrated for widespread applications, such as in water heaters.

Another method ofgenerating electricityat the water heateris based onthermocouple technology.

Thermoelectric generators are available, but none has been used in water heaters. Furthermore, DOE

understandsthisoptionand the thermophotovoltaic generator optionneedfurtherdevelopmentforcommon

day-to-dayapplications before prototypesfor water heaters canbe developed and tested. Therefore, this

design option is considered not technologically feasible. 

4.2.2.9 Reduced Burner Size (Slow Recovery)

Reducing burner size while keeping flue baffle and tank geometry the same will increase the ratio

of heat transfer surface area per Btu of input, thereby increasing the recovery efficiency. The lower input

means thatrecoverywould be slowerthanwithconventionalburners. The first hour rating would probably

be reduced, as well, although some of this first hour rating reduction could possibly be offset by changing

the dip tube design, to betterstratify the water inthe tank and therebyreduce mixing. Slower recovery also

implies reduced product utility by the consumer and hence will not meet one of the key screening criteria.

GAMA believes that slow recovery is another name for derating.4 By derating, the water heater

is not able to provide as much hot water. This is particularly an issue for households larger than three

persons because normal hot water use may result in frequently running out of hot water. Therefore, DOE

eliminated this design option from further consideration because of the adverse impact on product utility.

4.2.2.10 Heat Pump Water Heater Options

Because the vast majorityof electric water heaters use resistance elementsto heat the water, there

are few available options for substantially improving efficiency. With current technology, a heat pump can

achieve the greatest efficiencyimprovement for heating water. A heat pump water heater can double the

EF of a resistance water heater but it uses heatfromthe surrounding air, which, depending on the location

of the air heat source, may need to be made up by the household heating system.

Inits Technology Assessment and Screening Analysis, Appendix B Supplement tothe Water

Heater Rulemaking Framework ofJanuary 1998, DOEeliminated heat pump water heaters as a design

option based on poor reliability and lack of an infrastructure for servicing of heat pump water heaters.9
4-17

DOE received the following comments on heat pump water heaters from the Technology

Assessment and Screening Analysis document. AGA claimed DOE must consider all technologically

feasible design options.11 Okaloosa Gas District, Clearwater Gas System, the CityofPalo Alto, the City

of Mesa Arizona, Milza S. Barley and 13 gas utilities recommended that DOEconsider heat pump water

heatersinitsstandards analysis.12, 13, 14, 15, 16 LaClede Gas claimed DOE is protecting electric utility market

dominance and it could lead to fuel substitution away from gas water heaters.17 LaClede Gas further

claimed that heat pump water heatersshould be consideredas a design optionbecause DOEisresearching

a new heat pump water heater which may overcome several critical market barriers including cost. 18

Okaloosa Gas District claimed DOE must explain why it eliminated heat pump water heaters since the

recent GAMA directorylistsfive models.12 Following the November 1998 workshop, AGA claimed that

neitherElectric Power Research Institute (EPRI) nor American Council for an Energy-Efficient Economy

(ACEEE)identified any infrastructure or reliabilityproblems in their public information on heat pump water

heaters and that reliability and infrastructure issues are not a sufficient reason to eliminate them.19

GAMA, EEI, Southern Company and Virginia Power agreed withDOE’s decision to screen out

heat pump water heaters.4, 7, 8, 20, 21, 22, 23 Southern Company claimed that heat pump water heatersshould

be eliminated as a design option because of product reliability issues and an inadequate infrastructure.7

The 1994 rulemaking process produceda litanyofcommentsdescribingproblems withestablishing

anelectric water heaterstandard based on heat pump water heaters(see docket EE-RM-90-201). Many

of the comments discussed issues that the screening process was designed to address early in the

rulemaking process. Among the comments received: 

• 216 comments cited the immature infrastructure for manufacturing, sales and repair,

especially in rural areas.

• 107 comments feared increased electric rates due to loss of revenues in rural electric

cooperatives.

• 66 comments questioned the inconvenience and possible price hikes for larger tank size

due to the lower recovery rate, 40 % slower than conventional electric water heaters.

• 33 comments questioned the reliability of heat pump water heaters.

• 33 comments expressed concerns about increased noise.

• 22 comments claimed under-the-counter applications would not be cost effective.

• 21 comments believed it would distort the marketplace and lessencompetitionamong fuel

sources.

• 15 comments cited the space restrictions ofsmall homes, and the increased payback time

due to reduced electricity use.

DOE has reviewed these comments as well as otherissues associated withthe installation and use

of residential heat pump water heaters. The following issues form the basis of DOE's decision to screen

out heat pump water heaters as a design optionpursuant to10 CFR 430, Subpt. C, App. ASection5(b):52
4-18

• Practicability to Manufacture (on the scale necessary to serve the relevant market at the

time of the effective date of the standard),

• Practicability to Install (reliably on the scale necessary to serve the relevant market at the

time of the effective date of the standard),

• Practicabilityto Service (reliably on the scale necessary to serve the relevant market at the

time of the effective date of the standard), and

• Impacts on Product Utility to Consumers (significant adverse impact to significant

subgroups of consumers).

Description of Technology. A heat pump water heater represents a merging of two otherwise

unrelated technologies: anelectric resistance water heater withtank and controls, and a refrigerationcircuit

similar to that found in a residential air-conditioner. To understand the issues confronting manufacturers

inbringing a heat pump water heaterinto the market, it isimportant to understand howthe heat pump water

heater is similar to and how it is different from other residential appliances. What follows is a description

of how a heat pump water heater works and how it differs from the standard electric resistance water

heater. 

Heat pump water heaters transfer heatfromair, typically at roomtemperature, to water at a higher

temperature. The heated water can then be used to provide the residential hot water supply. Since heat

does not naturally transfer from a low temperature to a higher temperature, a “heat pump” is used. The

terminology “heat pump” is used here to mean a mechanical system consisting primarily of a closed

refrigeration loop containing a refrigerant vapor compressor, an evaporator (a type of heat exchanger), a

condenser (another heat exchanger), and an expansion device. These are the four basic components of

standardvapor-compressionrefrigerant circuitrythat exist inany household refrigerationor air-conditioning

system.

The refrigerationprocess normally removes heatfroma heatsourceand pushesthatheat to another

location. A good example is the household refrigerator. The refrigerator removes heatfromthe contents

of the refrigerator box and rejects it to the surrounding kitchen air. In common parlance, we refer to the

systemas a refrigerator, but the thermodynamic cycle upon whichit is based is oftenreferred to as a “heat

pump.” A heat pump is a system which can extract heat from a low temperature heat source (i.e., the

refrigerator contents) and reject it to a higher temperature “sink” (the surrounding air). Note that the heat

pump is usuallydesignedto help inmoving heatfroma lowtemperature heatsource to a highertemperature

heat sink. However, it can also be used to simply speed up the rate of heat transfer from a high

temperature heat source to a low-temperature heat sink.

So, howdoesthe systemwork? The refrigeration process relies on the behavior of the refrigerant

fluids. Like most fluids, refrigerants can exist as both a liquid and as a vapor. Which state they are in

depends on their temperature and the pressure to which they are subjected. 

A common starting point inanalyzing the refrigerant loop isthe compressor. The compressor uses
4-19

electricalenergy to turn the compressor motor and compress refrigerant vapor. As the compressor does

mechanical work on the refrigerant vapor, the vapor’s temperature rises. The refrigerant leaves the

compressor as a hot, high pressure gas. This gas moves to the condenser. The condenser is a heat

exchanger which allows the hot gas to lose heat to a heat sink. Typically, it does this by passing the gas

along one side of a metal wall while some other, cooler fluid (air or water depending on the appliance in

question) absorbs the heat across the metal wall. As the high pressure refrigerant vapor is cooled in the

condenser, it condenses into a liquid. This high pressure liquid moves from the condenser to the expansion

device.

As the high pressure liquid flows through the expansion device, it quickly moves from a region of

highpressure to low pressure. Some of the liquid evaporates in the process, drastically cooling the rest of

the liquid to the liquid’s saturation temperature. The saturation temperature is the temperature at which

liquids evaporate or condense at a specified pressure. Since the suction side of the compressor keeps the

pressure lowat thispoint,the saturationtemperaturewill also be correspondingly low(typically below 20/F

in a household appliance). The mixture of cold, saturated liquid and vapor moves onto another heat

exchanger, the evaporator.

In the evaporator, heat transfers from a warmer surrounding fluid to the cold refrigerant liquid,

which causes the remaining refrigerant to evaporate. The temperature of the refrigerant remains

approximately constant until all the refrigerant is evaporated, and the heat transfer takes place as long as

the heat source is at a higher temperature than the saturation temperature of the refrigerant. Thus the

evaporator is what is used to remove heat from the heat source (like the air inside a refrigerator), with the

heat absorbed by the refrigerant. 

The vaporizationinthe evaporator occurs underlowpressure and the volume ofthe resultant vapor

is high and the temperature relatively cool. The cool, low pressure vapor flows from the evaporator to the

suction side of the compressor. As it enters the compressor, it is recompressed again, restarting the

refrigeration cycle. Throughout the operation of the system, heat is transferred from the fluid surrounding

the evaporator to the fluid surrounding the condenser. 

In a heat pump water heater, the evaporator is a refrigerant-to-air heat exchanger and the

condenser is a refrigerant-to-water heat exchanger. The refrigeration cycle is used to remove heat from

ambient, household air at room temperature and use that heat to heat a tank of stored water to a much

highertemperature (typically 120/F or greater). Electricity powers the compressor whichenablesthe heat

pump water heaterto operate. At the evaporator and condenser temperatures used for a typical heat pump

water heater, the “heat pump” extracts approximately twice as much energy from the air (in the form of

heat) as is used to operate the compressor (in the formof electricity). Additionally, much of the electrical

energy used to operate the compressoreventuallywindsupheating the refrigerant directly and istransferred

to the water inthe condenser. When used continuously, typical heat pumps appear to heat water at roughly

2.5 to 3 times the rate of energy input (in the form of electricity).
4-20

Electric resistance water heaters, in contrast, directly produce heat inelectric resistance elements.

The conversion from electricalenergy to heat energy occurs at essentially 100% efficiency, i.e., the energy

leaving the electric resistance elements in the form of heat is essentially equalto the electrical energy input

to the elements. This is in contrast with what appears to be greater efficiency in the heat pump water

heater. However, the fundamental difference between the two designs is that the majority of heat that

entersthe tank from a heat pump water heateris heat that has beenextracted fromsome othersource such

asroomair.Comparisonof the apparent efficiency ofthe two productsis not possible without establishing

the cost to heat the air that the heat pump water heater eventually transferred to the hot water.

Note, inpractice, actualtested residentialheat pump water heater designs typically yield anenergy

factor of about 2.0. This lower energy factor accounts for cycling losses, heat escaping from the heat pump

water heater system, piping, orstorage tank, and any use of backup electric resistance heat in the system.

This compares with typical energy factors of about 0.86 for electric resistance systems. Note the EF of

2.0 inthe heat pump water heater design does not account for where the originalheat came from. A direct

comparisonofthe EFs of the two water heater designs only makes sense ifthe heat extracted fromthe air

by the heat pump water heaterisfree and extracting this heat does not produce another energy penalty on

the household. This isthe case insome verywarmclimates(e.g., Hawaii). In most of the U.S., homes are

heated for substantial portions ofthe year and operating a heat pump water heater that extracts heat from

the household air increases the load on the home heating system during these periods. For this reason,

accounting for the net energy or energy cost impact of the heat pump water heater requires analyzing the

energy cost impact on the entire house, not just the electrical energy input to the water heater.

There are two basic designsforresidentialheat pump water heaters: add-on and integral. An addon heat pump water heater is a product designed to be added to a separately manufactured storage-tank,

oftena standard electric resistance water heater. Asmall pump circulates water fromthe tank through the

heat pump. 

The second type of heat pump water heater is an integral heat pump water heater. In an integral

design, the heat exchanger (condenser) is built into the storage tank. This eliminates the need for a

circulation pump and increases efficiency. 

Bothdesigns have a refrigerationsubsystemcomposedof, at a bare minimum, anexpansiondevice,

evaporator, compressor, and a fan (to force air over the evaporator and enhance heat transfer). The

refrigerant condenser, also part of the refrigeration loop, can be located with the rest of the subsystem as

a separate unit (as is done with the add-ontype heat pump water heater) or located integralto the tank as

with the integral heat pump water heater design. In the add-on heat pump water heater, cold water is

removed from the tank, heated and pumped back into the tank. In the integral design, the heat pump

compressor moves refrigerant from the condenser coil, located in the tank to the evaporator coil located

outside the tank. Regardless of design, the expansiondevice, evaporator, fanand compressor have to be

located outside the tank proper. With the integral design, these components are most commonly located

on top of the water tank. With the add-on design, they can be mounted on top of the tank or as a
4-21

completely separate unit that can be placed on the floor or hung on the wall. For good efficiency, the addon heat pump water heater should be placed relatively close to the tank. As the unit is placed farther from

the tank, the water heating efficiency is degraded due to piping losses, predominantly the result of warm

water left in the piping between on-cycles.

Another component that is incorporated in most residential heat pump water heater designs is

electric resistance heating elements. These are needed because, for cost and space considerations with

practical residential heat pump water heaters, the heating capacityofthe refrigerationsystem in heat pump

water heaters is limited. Existing residential designs have between 40% and 80% of the heating capacity

of typical electric resistance water heaters. To compensate for this and allow faster water heating and

shorter recovery periods for the heat pump water heater during periods of high use, electric resistance

heaters are employed that can heat the water in the tank faster than the heat pump unit. Larger capacity

heat pump systems can be constructed, but the cost of larger systems generally outweighs the energy

efficiencybenefitsto the average consumer. Instead, practical systems are designed to heat water gradually

using the higher efficiency heat pump, but can, when required by the heating load, use a higher capacity

electric resistance backup. 

Practicability to Manufacture. From our analysis of the water heater industry, DOE has

determined that water heater manufacturers would not have the lead time to ramp up heat pump water

heater productionto present saleslevelsinthe three-year time frame established bythe NOPR. Since the

late 1970s, sales of heat pump water heaters have not exceeded 10,000 per year (<0.33% of electric

water heatersales, <0.17% of allwater heatersales). Sales have declined drastically since the mid-1980s.

Presently, two manufacturers of residential heat pump water heaters produce less than 4,000 residential

water heaters a year in categories covered by the present rulemaking. None of the five major

manufacturersofresidentialwater heaters currently have a heat pump design intheir residentialproductline,

and only two (State and Rheem) have had a heat pump water heater in their product lines in the last 10

years. 

Reliabilityissues are an ongoing concern that have not yet been adequately solved by manufacturers. As

evidence of reliability problems, Northeast Utilities (NU) claims they are the only utility to place a

substantial(2,000 units) order (in 1996) for heat pump water heatersinthe lastfive years. NU states it has

only installed 600 heat pump water heaters because of a failure rate in excess of 10%.53 

Althoughmostmanufacturers could develop, either alone or in partnership withothers, a working

heat pump water heater design in the next few years, there are significant difficulties in capitalizing and

building heat pump water heater manufacturing facilities to provide for the present 4 million plus electric

water heater sales annually. 

Heat pump water heaters would require the introductionofa completelynewproductto the market

witha market volume greaterthanthat of allroomair-conditionershipmentsin the U.S.54 In a 1994 Arthur

D. Little report, the estimated investment cost to convert to heat pump water heaters was $750 million.
4-22

Giventhe current levels ofprofitabilityofthe water heater industry and the limited capital resources, some

manufacturers will not be able to finance these costs.55

In addition, given the high initial cost for heat pump water heaters, poor reliability with past heat

pump water heater designs, and anticipated impact on consumer utility, initialsales of electric water heaters

after a heat pump water heater standard may be low as consumers look for other alternatives. These

alternatives could be switching to gas or oil-fired orsolar or photovoltaic water heaters orsimply replacing

the existing tank with a replacement electric resistance storage tank as allowed in the current DOE test

procedure.

 

With a government imposed time frame forshifting allproduction to heat pump water heaters and

a shifting market size, it is unclear how the electric water heater industry could planto satisfy anunknown

final market volume. 

The gas industry has commented that DOE should not screen heat pump water heaters out as a

design option because it is presently supporting the development of a residential heat pump water heater

product.18 The heat pump water heater design being pursued by DOE is an integral heat pump water

heater design which uses a small compressor with40% less heating capacitythanany used in existing heat

pump water heater products (and has about 25% ofthe heating capacityof a typicalelectric resistance hot

water heater). Thisshould assist ininstallationinsmallerspaces as it will physically use smaller components

(particularly the compressor and evaporator/fan system), and will likely be quieter in operation. Present

designs re-inject condensate back into the air to be recondensed in the evaporator. This may simplify

installation, at some expense to system capacity, efficiency and dehumidification of the residence. The

integral heat pump water heater design proposed by DOE uses a 50-gallon tank, but even the small

compressor and heat exchanger used inthat design adds approximately a foot in height to that tank. The

attached 50-gallonstorage tank issized to provide ample water for a typicalday’s use inmostresidences.

Smallertank sizes are not being proposed, asthe cost effectiveness ofthe heat pump decreasesrapidlywith

smaller tank sizes and characteristic lower water usage. Presently, the smallest integral heat pump water

heater design available in the U.S. is an 80-gallon unit. The design proposed by DOE would still need

access to the same amount of household heat any heat pump water heater would require to serve the

residence load, however the lower heat extraction rate of the DOE unit may allow for installation in

locations with smaller surrounding air volume than is required for existing designs.

The unit is being developed with input from DOE, Arthur D. Little, and Oak Ridge National

Laboratory and has been designed from the outset to address many of the known market barriers facing

the adoption of residential heat pump water heaters. Preliminary field tests of the DOE design are likely

to start in the spring of 2000. Larger scale utility testing is slated for late 2000 to 2001. Accelerated

reliability testing is also scheduled sometime after initial field testing has resulted in a more or less stable

product. If field and reliability testing are positive, limited commercial production and sales are possible

by 2003. Actual production and sales would be through an existing air-conditioning equipment

manufacturer who would likely purchase storage tanks from an existing water heater manufacturer.
4-23

Because of the issues that have plagued heat pump water heatersinthe past, DOEisrequiring its partners

to introduce the product cautiously, correcting problems encountered during field testing and fully testing

the corrections. A market study done by Arthur D. Little projected potential sales for the DOE design up

to 300,000 units/year 10 years after commercial introduction, or 7.5% of present electric water heater

sales.56 It is important to note that the time scales established for the DOE development effort represent

typical product design and production ramp-up schedules for newly introduced residential appliances. 

Whether this unit is successful will depend on how well it addresses both the marketplace and

technical issues facing heat pump water heater products and the final cost to the consumer. However, at

no time did developers believe this design to be a replacement for all electric water heater applications.

Its future marketplace success willsave some energy directly, but more importantlywill serve to introduce

consumersto benefits and successful applications ofthe technology, willbegin the development of a limited

installation and service infrastructure for heat pump water heaters, and may point other manufacturers to

successful product designs and marketing niches for heat pump water heaters.

There is also the additionalcumulative burden on the manufacturers of both gas and electric water

heaters who have to redesign and retooltheir manufacturing processesto address the issues of preventing

ignition of flammable vapors (for gas units) and the phaseout of HCFC-141b insulation blowing agent. 

Considering these issues with regard to manufacturability and achieving a scale of production

necessary to serve the electric water heater market at the time of the effective date ofthe standard, DOE

has concluded that the screening criterion of “practicability to manufacture” will not be met and has

eliminated heat pump water heaters as a design option from further consideration.

Practicability to Install. Heat pump water heaters cannot be used as direct replacements for

existing electric water heaters in many applications. For example, small electric resistance water heaters

are often used in mobile homes, under-counter applications, apartments, small condos and other

applications where space is limited. Approximately 30% of all electric water heater installations are in

residences smaller than 1,000ft2in size (average size: approximately 760 ft2).57 In many of these

installations, space restrictions would make it impossible to simply replace the existing electric resistance

water heater withany ofthe existing heat pump water heater designs sold todayor withfuturedesigns,such

as that being proposed by DOE. 

The physicalsize ofthe heat pump water heateris necessarily largerto accommodatethe placement

of the heat pump unit. A small (4,000-6,000 Btu/h) heat pump unit mounted on top of a tank will add

approximately 8-12 inches on top of the tank for compressor, evaporator coils, and evaporator fan. In

addition, to match the same water use profiles, the size of the storage tank itself may need to be larger to

compensate forthe smaller heat input rate ofthe heat pump water heater and the particular controlstrategy

used in the heat pump water heater design. Even assuming no change in tank size from the electric

resistance model, this extra height of the heat pump design will present installation problems where the

existingwater heater enclosure is height limited,suchasisthe case withmany existing lowboywater heater
4-24

installations. It is estimated that lowboy electric water heaters make up approximately 18% of all water

heatersales according to GAMA estimates,58 and maymake up as muchas25%ofthe market inparticular

regions.59 Since there is a cost premium attached to a lowboy design, it is believed that most of these sales

reflect some type of height restriction in the space. Additionally, the space in which the heat pump water

heater is located must be large and must provide ready access to change the air filters on the heat pump

evaporator coil. Lack of regular filter changes has been reported as a primary reason for product failure

in field studies of heat pump water heaters.60

As mentioned previously, it is possible to install the heat pump components of the add-on heat

pump water heater away from the tank and pipe water between the heat pump unit and the tank. In

practice, this introduces efficiency losses from the connecting piping and does not remove the need for a

place to put the heat pump unit.

Hiding the plumbing betweentank and heat pump unit may require extensive remodeling expenses

for many residences, and cannot be considered a practical installation. In addition, finding a location for

the refrigeration subsystem is more complicated than simply finding the physical space to place the unit.

The refrigeration subsystem of a heat pump water heater needs sufficient flow of warm air across the

evaporator coils to serve as a heat source. Without this heat source, the heat pump cannot function and

must revert to electric resistance operation. Placing the heat pump water heater in a non-ventilated closet

willnot provide anadequate heatsource. Operation in heat pump mode will generally require on the order

of 100 cfm or more of warm air turnover between house and closet; unlikely to be achieved with natural

ventilation through a closet door. A heat pump water heater installed behind even a fully louvered closet

door is likely to experience short cycling of cold exhaust air, affecting efficiency and capacity, and thus

performance of the unit. 

Additionally, one must dealwithcondensate produced at the evaporator coil. Like all refrigeration

devices, the temperature ofthe evaporator coilmust be cold toextractheatfromthe surrounding air. When

air blows across the coil, moisture in the air is condensed to liquid. The amount of liquid produced is a

function of the humidity in the air blown acrossthe coil, the temperature ofthe evaporator coil, and the hot

water load. Heat pump water heaters installed in unconditioned spaces in the south, east, and midwestern

U.S. can be expected to produce between several quartsto severalgallons of condensate per day during

the cooling season. Locating the heat pump in the conditioned space will not eliminate all the condensate

buildup unless the air conditioning system can be counted on to dehumidify all house air, to a dewpoint

below the coil temperature. Since this is hard to ensure in practice, a condensate drain is essentially

required in present heat pump water heater designs. The need for a drain complicates installations where

the existing water heater is located in a hall closet, attic, garage, or other location where access to a

plumbing drain is not readily available. A pump can be used to move condensate to a drain if needed.

However, this represents not only added expense, but a potential source of failure and consequent

household damage.

There has been some discussion of techniques forre-evaporating the condensate, but no existing
4-25

heat pump water heater models incorporate this option and no prototypes exist. While perhaps simple in

concept, re-boiling of the condensate maydramatically reduce (by as much as 30-50%) the efficiency of

the heat pump water heater under humid conditions. The extent that this impacts the annual average

efficiency will depend on climate; however, the above figures could represent a common performance

degradation in southern and coastal regions of the U.S. 

Siting constraints are expected to be lesssevere withlarger apartments, where it is anticipated that

the greater available space has encouraged both larger rooms and more “traditional” looking water heater

design and placement, such as separate laundry/utility rooms. 

Siting ofheat pump water heatersis expected to be less of a problem with traditional single family

housing. With single family housing, the water heater is commonly installed in a basement or garage, or,

wheninstalled inside the residence, in a utility room where access to condensate drainage is available and

where cool air from the heat pump water heater is not likely to negatively impact the main living space.

However, installations under stairwells and in closets remain common and are expected to generate

installation and performance problems. Since these locations may be centrally located in the residence,

condensate pumps may be a necessary part of heat pump water heater installations in these areas. 

With manufactured housing, condensate drainage is generally not a problem, however finding

workable locations for the water heater and a heat pump unit may be difficult. This is particularly true in

smaller, older manufactured housing units, where the housing design focused on optimal use of space and

height- restricted closet installations were common. More modern manufactured housing relies on fullheight closets or utility rooms. Approximately 62% ofexistingmanufactured housing less than 1000 ft2

 in

size was built before 1980.50

Surveys fromthe Georgia Power and Alabama Power companiesreported that between20% and

30% of water heaters in their service territory are installed inlocations unacceptable for heat pump water

heaters, primarily due to air volume constraints.55 It is anticipated that specific subgroups, such as

apartment units, should expect even higher incidence of installation problems.

DOE believes that heat pump water heaters can be installed inmost locations. However, 18% of

sales of electric water heaters, or about 700,000 per year, are lowboy models. An integral heat pump

water heater would notfit into these locations. Perhaps 50% of the lowboy sales are for new construction

leaving 350,000 units per yearthat would require anadd-on heat pump unit. Additionally, over one million

standard sized electric water heaters per year are installed in residences of 1,000 ft2 or less. Perhaps as

many as 500,000 of these installations would also require an add-on heat pump unit. This is a total of

850,000 add-on heat pump water heaters per year. These add-onheat pump units require a space with

at least 100 cubic feet per minute of warm air and wiring and plumbing connections (probably through one

or more walls) for water pipes and a condensate drain. We would characterize this installation as

“difficult.” Without an extensive survey, we are unable to determine how many of these difficult

installations would be feasible, although costly, and how many would result in loss of product utility as
4-26

discussed later in this section. 

Since we have determined that almost a million households could be affected each year, we

eliminated heat pump water heaters as a design option from further consideration because of issues

concerning practicability to install on the scale necessary to serve the relevant market at the time of the

effective date of the standard. 

Practicability to Service. Two hundred sixteen comments to the 1994 rulemaking process

(docket EE-RM-90-201) claimed that “ . . . the infrastructure to service heat pump water heaters is not

capable of handling a large quantity of heat pump water heater units.” The issues faced in service and

maintenance of heat pump water heaters have not changed since 1994.

The present service and maintenance infrastructure for electric resistance water heaters, consists,

forthe most part, of plumbers. Service and maintenance of a heat pump water heater is, however, more

complex than service and maintenance of an electric resistance water heater. In addition, where most

electric resistance water heaters are similar in design, there are several different classes of heat pump

designs (integral versus add-on designs, ambient air versus exhaust air heat source) that all have different

service and maintenance requirements and are likely to be available in a mature market. 

A plumber will likely be the primary installer of a residential heat pump water heater. Any water

heater installation typically utilizes a plumber to attach the plumbing to the water heater. In most

jurisdictions, the electrical connection (a hard-wired 220 Volt connection) requires either a separate

electrician or a plumber with some electrical wiring skill and certification. 

However, if there is a problem with an installed heat pump water heater unit, a plumber may

currently not have the background to troubleshoot either the refrigeration system or controls. Note that

you really cannot reliably test the refrigeration system or controls of anintegrated heat pump water heater

untilthe tank isfullofwater and the plumbing is connected, whichmeans thatreplacement witha new water

heater is also an expensive option at that point. 

A further complication with heat pump water heater products is the distribution structure for

residentialwater heaters. Presently, several of the major residentialwater heater manufacturers distribute

their products through a distribution chain that extends through wholesale suppliers directly to plumbers

installing their products. Approximately 50% of the residential water heater market follows this pattern.

This distributionmethod providesanaturallinkfromthe manufacturerto the installationand productservice

personnel through which training could be accomplished.

The other 50% ofwaterheaters are sold primarily through retail outlets(hardware stores and some

major department stores). Particularly for sales through hardware stores, there is little link between the

manufacturer and the productservice personnel. This will complicate training of product service personnel

for any given manufacturer’s products. 
4-27

The distribution through hardware stores would also complicate repair and service of heat pump

water heaters. Heat pump water heaters are more complexindesign and based on fundamentally different

technology from electric resistance water heater designs. Because ofthis, theyrequire a broader range of

skills to service the units. The typical electric water heater requires two screw-in electric resistance

elements, a tank, and thermostat controls. The add-on heat pump water heater, the most common design

produced in the past, is composed of a refrigerant compressor, expansion device, water pump, piping,

refrigerant-to-air heat exchanger (evaporator), refrigerant-to-water heat exchanger (condenser), fan,

sensors and controls that can: turn the pump, compressor, and fan on together or independently, defrost

the evaporator, or even to provide freeze protection. Thisisinadditionto the need for electric resistance

backup elements and controls that have to operate inconjunctionwiththe heat pump controls. In addition

to the mechanical components, there is the refrigerant itself, whichcanleak out fromthe system gradually,

resulting in poor performance, or leak out of the system catastrophically, resulting in system failure. 

Any one of these system components can fail and cause the water heater to stop functioning as a

heat pump water heater. This is in contrast to the simple, relatively maintenance-free, gas or electric water

heater. Once installed, the most likely failure mode of either electric or gas water heaters is a leak from the

tank wall, leading to complete replacement of the water heater. There is little “service” necessary.

The add-on heat pump water heater, however, would arguably be one of the more mechanically

complex, and possibly more expensive, appliances found in a residence. The integral heat pump water

heater design simplifiesthe system by dispensing withthe pump,waterpiping, and pump controls; however,

theyare still substantially more complexthananelectric resistance water heater. For these reasons, where

repair of heat pump water heater components is possible, it may be an attractive alternative to simple

replacement. Repair of heat pump water heaters, however, will likely be costly. Since the heat pump

water heater is plumbed and hardwired into the residence, and since there really isn’t a “down period” for

hot water consumption in a residence, most repairs will have to be made in the field. This is particularly

true of existing integrated heat pump water heaters, where it may be impossible to separate the upper heat

pump section from the tank itself without breaking into the refrigeration circuitry. 

Plumbers generally do not have training or background in repair of appliances like a failing heat

pump water heater, nor would plumbers likely be a logical source of labor to do these repairs since they

are among the highest paid residentialcontractors. Premature (non-tank) heat pump water heater failures

are typically caused by the electrical components (fans, pumps, electronic controls, or wiring) or through

loss of refrigerant. Generally, this type of repair work is done by small appliance repair personnel who

repair refrigerators, freezers, room air conditioners, and other “white” goods (e.g., washing machines).

Theserepair personnelhave backgroundsintroubleshooting fans, pumps, wiring and controls and areoften

certified to repair low and high pressure, hermitically sealed, refrigeration equipment. According to the

Bureau of Labor and Statistics, of the approximately 71,000 home appliance repair workers in the U.S.,

2 out of 3 work directly for department stores or household appliance stores, where they service the same

products that they sell retail.61 These stores represent a fraction of sales of water heaters, but might be

logical sales outlets for heat pump water heaters. However, for some existing water heater manufacturers,
4-28

this would require opening up new distribution channels for electric water heater products.

Another possible option for repair and service of heat pump water heaters is air-conditioning

contractors. This is a vertically oriented industry that is completely separate and distinct from the water

heater industry. However, if the residential air-conditioning industry chose to build heat pump water

heaters, they could choose to sell these products through their already established distribution channels.

This would simplify the service and maintenance infrastructure forthese companies. Installation, however,

would require contracting with or hiring of local plumbing professionals. It is unknown if present water

heater manufacturers could have accessthrough these same residentialair conditioning industrydistribution

channels to distribute and service their own heat pump water heater designs.

To date, long term reliability in add-on heat pump water heater products has not been well

established infield studies of available products. In the U.S., the greatest field experience with residential

heat pump water heaters has been through utility demand side management or U.S. military housing

programs. In both cases, poor reliability, in programs and product designs going back 20 years has been

the single biggest reason for the demise of virtually all such programs. Additionally, NU claims they have

trained over 80 installers during two years of the program but only 19 continue to install the units at

reasonable prices. NU lists number of call-backs because offailures among the reasons contractors have

dropped out of the program.53

Presently, no mass-market installation and servicing infrastructure for heat pump water heaters

exists. It is noted that while there are several potential candidate industries who could provide servicing

capabilities, none have relationships with major water heater manufacturers or with plumbers who install

water heaters. There is no precedent in the history of the U.S. major appliance industries to suggest that

a new service and repair infrastructure could develop, on the scale of several million units per year, in a

roughly three-year time frame. This is particularly true given the virtual absence of available products in the

next two years from major manufacturers, the radical changesrequired inthe existingwater heater market

simply to develop heat pump water heater products, and the absence of any present consumer demand for

such products. A complete reversal of this situation would be required for an energy efficiency standard

at the heat pump water heater level to succeed. 

Therefore, DOEeliminated heat pump water heaters as a design optionfromfurther consideration

because ofproblems concerning practicabilityto service on the scale necessarytoserve the relevant market

at the time of the effective date of the standard.

Impacts on Product Utility to Consumers. Manufacturers of present ambient-air heat pump

water heaters recommend locating the evaporators in spaces of at least 1,000 ft3in volume to provide

adequate air circulation and minimize overcooling of the space which can impact performance.

Approximately 14% of all residences are smaller than 1000 ft2and presently use electric water heaters.

A 10' x 12' room with an 8' ceiling (1,000 ft3) is 12% or more of the floor area in a 1,000 ft2residence.

Formanyresidencesofthissize, this would force evaporatorsto be located inwhat is presently livingspace
4-29

of the residence, causing additionalfan, compressor, and pump noise. Cold air from the heat pump water

heaterlocated inthe livingspace mayalsocausesome dissatisfaction. Thisis particularly a concerninsmall,

slab-on-grade housing, mobile/manufactured homes, or apartments. 

Ifthere is no space to incorporate boththe water tank and the refrigeration subsystem in the same

location, a reduced tank size may have to be installed. This could cause a 20% to 25 % loss of tank

volume on a standard 50 gallonwater heater. Any substantial reduction in the tank size to accommodate

the heat pump would reduce the first hour rating, since first hour rating depends on tank size and reheat

capacity. The first hour rating is, “an estimate of the maximum volume of “hot” water that a storage-type

water heater can supply within an hour that begins with the water heaterfullyheated.”.62 We interpret lost

first hour rating as a loss of product utility. 

Because of this significant adverse impact to significant subgroups of consumers, the Department

has concluded that the screening criterion of “impacts on product utilityto consumers”willnot be met and

eliminated heat pump water heaters as a design option from further consideration.

In summary, DOE has eliminated residential heat pump water heaters as a design option for this

rulemaking. It fails to meet the screening criteria listed earlier, namely, practicability to manufacture, install,

and service and it has adverse impacts onproduct utility. There is no foreseeable means forthe technology

to advance enough inthe short termto allow heat pump water heaters to fill market needs and to continue

to provide a reasonable level of consumer utility.

4.2.2.11 Timer Controlled

This design optionlimitsthe time ofdaywhenthe elements of anelectric storage-type water heater

maybe energized. This is most often used as part ofanelectric utilitydemand-side management program

for load shifting (“demand avoidance strategy”). Field testsshow a few percent energy savings, because

the water in the tank remains at a reduced temperature for part of the day. However, the actual energy

savings will depend on the end-use profile, lifestyle of the consumer, and a basic desire to save energy.

GAMA claims that timer controls are used as a demand avoidance device to allowconsumersto

use lower priced electricity.4 Although this design reduces standby losses, it does not improve efficiency

because it only shifts the electric consumption from on-peak to off-peak times. Therefore, DOE did not

include this in its analysis. 

4.2.2.12 System Application Options

The following three techniques are applicable to the water heating systemratherthanto individual

water heaters and, assuch, are not considered inDOE's definitionofwater heaters. Therefore, DOE does

not consider these add-ons as design options and hence, they are not relevant for the present screening

considerations.
4-30

Solar Pre-Heat. This and the following two techniques may be relevant on a site-specific basis,

and are applied at a system level as opposed to anequipment level. Individual water heater manufacturers

do not controltheir use. This technique uses solar collectors as pre-heaters for a standard electric storagetype water heater. Many designs are currently available, with a wide range of installed costs.

Drain Water Heat Recovery System. This technique uses a heat exchanger to recover waste

heat from the drain. In effect, this becomes a pre-heater for a standard storage type water heater. A few

designs are currently available. 63, 64 However recent water conservation measures, such as low-flow

showerheads, may lead to fewer benefits than previously thought. 

Tempering Tank. A tempering tank is an un-insulated storage tank plumbed in the water line

before the water heater. When installed in a conditioned or semi-conditioned space, it may raise the inlet

water temperature to the ambient temperature. 

GAMA states that solar pre-heat, drain water heat recovery systems, and tempering tanks are

installationoptions independent ofthe water heater design.4 DOE agrees with GAMA and considers these

as system installation features and not necessarily as enhancing the design nor the individual appliance

efficiency.

4.2.2.13 Sediment Removal Features

Several manufacturers offer models with dip-tubes designed to create turbulent water flow that

prevents or reduces the buildup of sediment on the bottom of the tank. This may reduce the degradation

of efficiency and prolong the life of the water heater. This design feature relates more to equipment

reliability than efficiency improvement.

GAMA claims that sediment removal features are not a design option applicable to meeting a

minimum efficiency specification.4 DOE agrees with the comment and believes this only preserves asdesigned efficiency over time but does not increase energy efficiencyofthe water heater. Therefore, DOE

did not consider this design option for engineering analysis.

4.2.2.14 Two-Phase Thermosiphon (TPTS) Design

This is a heat-pipe mechanism to transfer heat from the burner to the storage tank. The TPTS is

a closed loop device consisting of an evaporator in which the working fluid (water) is heated, percolating

liquid and vapor into the condenser where heat is transferred into the water storage tank.65 At the

condenser, the vaporized working fluid is condensed and drains back through a separate restricted tube

to the evaporator, where it is reheated. The restriction prevents the heated vapor and liquid fromflowing

to the condenser through the return path. During off-cycle, there is very little heat transfer through the

TPTS system. This reduces standby losses to levels similar to those of electric water heaters.
4-31

GAMAbelievesthe TPTS systemwould cause a drastic redesign of allgas water heaters withlittle

increase in efficiency.4 DOEunderstandsthat working prototypes ofthe heat exchanger design have been

developed and delivered to RheemCorporationfor detailed performance and life testing. However, DOE

is aware that some designs of heat pipes used in furnace applications posed safety problems due to

contamination of the working fluid. DOE concludes that large scale production of these devices has not

been satisfactorily demonstrated. Therefore, DOE did not include this design option in the engineering

analysis on the basis of practicality to manufacture.

4.2.2.15 Air-Atomized Burner (Oil-Fired Only)

This is a different type of burner for oil-fired equipment. Instead of relying on pressure to create

a spray of fine oil droplets prior to combustion, this burner uses a stream of air to atomize the oil.

Compared to conventional burners, this design allows better control of droplet size and mixing with air at

lower oil flow rates. Downsizing the burner with the same flue and baffle geometry will give a higher

recovery efficiency.66 The combustion process is also cleaner with this burner.

Furnace prototypes have beenmade withthis design option. There could be a slight improvement

fromcleaner combustion. Even if the first hour rating were reduced because of the smaller burner, it would

still be larger than that of a gas-fired water heater. Other impacts on manufacturability, installation, and

service of a water heater are yet to be determined. 

GAMA believes that air-atomized burner technology is not yet sufficiently developed, especially

for a water heater application, to include it as a design option.4 DOEagrees and did not include this design

option in the engineering analysis on the basis of practicality to manufacture.

4.3 RESULTS

Based on the above discussion, the design options used in the Engineering Analysis are listed in

Table 4.1, and those that have been eliminated from further consideration are listed in Table 4.2.
4-32

Table 4.1 Design Options Used in the Engineering Analysis

Design Option Description Electric Gas Oil

Heat Traps X X X

Plastic Tank X X*

Increased Jacket Insulation X X X

Insulating the Tank Bottom (Electric Only) X

Improved Flue Baffle/Forced Draft X X

Increased Heat Exchanger Surface Area X X

Flue Damper (Electromechanical) X

Side Arm Heater X

Electronic (or Interrupted) Ignition X X

 * used in conjunction with the Side Arm Heater option
4-33

Table 4.2 Design Options Eliminated from Further Consideration

Design Option Description Criteria for Elimination

Flue Damper (Buoyancy Operated) Safety issues and lack of long-term use data

Submerged Combustion Conflict with health and safety codes

Directly Fired Conflict with health and safety codes

Condensing Option Fails practicability to manufacture test

Condensing Pulse Combustion Fails practicability to manufacture test

Advanced Forms of Insulation Not technically feasible within next few years

U-Tube Flue Not technically feasible within next few years

Thermophotovoltaic and Thermoelectronic

Generators

Not technically feasible within next few years

Reduced Burner Size (Slow Recovery) Adverse impact on product utility

Heat Pump Water Heater Options Fails practicability to manufacture, fails

practicability to install, fails practicability to

service, and has negative impacts on product

utility to consumers

Timer Controlled Not an efficiency improvement option

System Application Options System installation feature

Sediment Removal Features Not an efficiency improvement option

Two-Phase Thermosiphon (TPTS) Design Fails practicability to manufacture test

Air-Atomized Burner (oil-fired units) Fails practicability to manufacture test
4-34

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