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CHAPTER 4. SCREENING ANALYSIS

TABLE OF CONTENTS

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

4.2 DISCUSSION OF DESIGN OPTIONS ..................................... 4-2

4.2.1 Design Options that Passed Screening ................................. 4-2

4.2.1.1 Improved Heat Exchanger Effectiveness ........................ 4-2

4.2.1.2 Modulating Operation ..................................... 4-5

4.2.1.3 Increased or Improved Insulation ............................. 4-7

4.2.1.4 Condensing Secondary Heat Exchanger ........................ 4-7

4.2.1.5 Electronic Ignition ......................................... 4-8

4.2.1.6 Induced and Forced-Draft Combustion Systems .................. 4-9

4.2.1.7 Infrared Burner ......................................... 4-10

4.2.1.8 Direct Vent ............................................ 4-11

4.2.1.9 Fuel Filtration ......................................... 4-12

4.2.1.10 Pulse Combustion ....................................... 4-12

4.2.1.11 Dampers .............................................. 4-13

4.2.1.12 Air-atomized Burner with Modulation ......................... 4-13

4.2.1.13 Delayed-Action Oil-Pump Solenoid Valve ..................... 4-14

4.2.1.14 Increased Motor Efficiency ................................. 4-14

4.2.1.15 Increased Blower Impeller Efficiency ......................... 4-15

4.2.2 Design Options Eliminated from Further Consideration .................... 4-15

4.2.2.1 Self-Generation of Electric Power ............................ 4-16

4.2.2.2 Fuel-Driven Heat Pumps .................................. 4-16

4.2.2.3 Flue-Gas Recirculation .................................... 4-17

4.2.2.4 Smart Valve ............................................ 4-18

4.3 SUMMARY OF SCREENING .......................................... 4-18

LIST OF TABLES

Table 4.3.1 Screening Results for Design Options by Product Class .................... 4-19
4-1

CHAPTER 4. SCREENING ANALYSIS

4.1 INTRODUCTION

The purpose of the screening analysis is to identify those design options that the Department will

consider for the engineering analysis and to screen out those design options that the Department will not

consider further. The Department consulted with industry, technical experts, and other interested

parties to develop a list of design options for consideration. The Department then applied a set of

screening criteria to determine which design options were unsuitable for further consideration in the

rulemaking. These screening criteria are based on the Process Rule, 61 FR 36974 (July 15, 1996). 

The criteria are summarized here:

C Technological feasibility: Design options used in commercial products or in working prototypes

are considered feasible.

C Practicability to manufacture, install, and service: Design options should be able to be massproduced and be reliable to install and service on the scale necessary to serve the relevant

market at the time of the effective date of the standard. 

C Adverse impacts on utility or availability to consumers: Design options should not create

adverse impacts on product utility, or result in the unavailability of any covered product type

with performance characteristics (including reliability), features, sizes, capacities, and volumes

that are substantially the same as products generally available in the U.S.

C Adverse impacts on health or safety: Design options must not adversely affect health or safety

aspects of the regulated product.

Based on past analysis, and on public comments on the September 8, 1993, Advance Notice

of Proposed Rulemaking (ANOPR), the Department prepared a draft screening analysis and presented

it for review and comment at the Furnaces and Boilers Standards Framework Rulemaking Workshop,

held in Washington, D.C. on July 17, 2001.1 This chapter presents the screening analysis of design

options that DOE carried out in response to comments received during and after the Framework

Workshop. The subsequent analyses do not consider or incorporate technologies that do not pass

these tests. 
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4.2 DISCUSSION OF DESIGN OPTIONS

This chapter presents the design options in two categories. The first category is design options

that met the screening criteria and that DOE used in the subsequent analyses for at least some product

classes. The second category is design options that DOE eliminated from further consideration for all

product classes. The discussions under both categories respond to the screening criteria described in

the Process Rule.

4.2.1 Design Options that Passed Screening

4.2.1.1 Improved Heat Exchanger Effectiveness

Heat exchanger effectiveness can be improved in many ways.2 To accomplish this, furnace

manufacturers optimize the heat exchanger size and geometry, gas input rate, combustion air delivery

system, heat transfer coefficient, and heat exchanger mass, and may apply electrohydrodynamic

enhancements to provide the greatest comfort, reliability, and safety. By adjusting the heat exchanger

effectiveness, a gas furnace can be designed with an annual fuel utilization efficiency (AFUE) rating

ranging from 75 percent to 98 percent. 

Between roughly 83 percent and 89 percent AFUE, condensate problems can occur and,

because of the high temperature of the flue at these efficiencies, stainless steel is needed to vent wet flue

gases to the outdoors. Above 89 percent AFUE, flue-gas temperatures drop below 155°F, and

polyvinyl chloride (PVC) pipe can be used for venting.3, 4

Heat Exchanger Area. Heat exchanger effectiveness can be improved by increasing the heat

exchanger area for a given burner input rate. This is a common way to improve the on-cycle efficiency

of the furnace. Increasing the size of the heat exchanger improves the steady-state efficiency and the

AFUE of the furnace. However, it also can increase the pressure drop on the flue side and/or air side

of the heat exchanger, so the blower system must be rebalanced to deliver the same flows. A general

rule is that doubling the heat exchanger area will increase efficiency by half the difference between the

current efficiency and 100 percent AFUE.5 For example, going from 80 percent to 90 percent AFUE

requires doubling the heat exchanger area.

Derating. Derating, which usually refers to reducing the burner input of the furnace while

keeping the heat exchanger area constant, is a direct way to improve furnace performance because it

does not involve design changes. Decreasing the gas input rate to a furnace improves the steady-state

efficiency and AFUE.

A small derate of up to10–15 percent will not impair the heat exchanger function, but a larger

reduction will have a negative impact on the performance characteristics of the heat exchanger. A
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similar impact on efficiency may be achieved by increasing the heat exchanger area and keeping the

burner input rate constant.

Combustion Air Delivery System. Reducing the excess air fraction improves efficiency

because it changes the flue gas temperature and flow rates in the heat exchanger. A reduction in excess

air from 50 percent to 40 percent should improve AFUE by about 1 percent. Factors to consider

when reducing the excess air to the burners include size and alignment variations in the drilled gas

orifices, pressure and temperature variations within the gas manifold, and pilot gas feed into only one

heat exchanger tube.

Heat Transfer Coefficient. Increasing the heat transfer coefficient (on either the air side or

the gas side of the heat exchanger) can improve heat exchanger efficiency. This can be achieved

through special surface treatments, the addition of fins or dimples, or otherwise modifying the air- or

gas-flow characteristics. Increasing the heat transfer coefficient can increase the pressure drop, which

may result in increased electrical power demand.

The industry achieved a major improvement in the gas-side heat transfer coefficient when it

shifted from natural-draft heat exchangers to induced-draft combustion. This allowed manufacturers to

substantially reduce the size of the flue gas passageway, thus increasing the gas velocities. The heat

transfer coefficient and pressure drop both increase exponentially with velocity.

The heat transfer on the internal surface of the heat exchanger offers an area for improvement. 

As flue gases pass through the heat exchanger of a furnace, they cool from about 2000°F to less than

400°F. With this large drop in temperature, the specific volume of the gas changes by a factor of three. 

Thus, the combustion gas passageway of the heat exchanger can be reduced in size by the same factor

of three at the outlet, while maintaining approximately constant gas velocity.

Air-side heat exchanger coefficients also can be improved by increasing the air velocity over the

heat exchanger. This can be accomplished by making the air passages smaller or by increasing the total

air flow. Increasing the air flow also will reduce the change in the air temperature as it crosses over the

heat exchanger.

There are several other ways to improve the heat transfer coefficient. In the case of the

serpentine clamshell heat exchanger, this can be accomplished by decreasing the channel area and

perimeter in the direction of the gas flow, and creating dimples in the latter passes of the channel. Each

dimple consists of two half-spherical indentations that reduce the local cross-sectional area of the

internal passage.6 Gas-side performance also may be enhanced by adding external fins to the heat

exchanger tubes. These are frequently used in the condensing heat-exchanger section of condensing

furnaces.
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Heat Exchanger Mass. Lower heat-exchanger mass allows the furnace to heat up and cool

down faster during each heating cycle. This effect is due to the reduced thermal storage capacity of the

heat exchanger. The overall impact is a reduction of off-cycle energy losses.

Because of the advantages of low heat-exchanger mass in terms of AFUE as well as

manufacturing costs, the forced-air furnaces of today have much lighter heat exchangers than did the

natural draft furnaces common 20 years ago. The blower-on time delays of today’s furnaces run about

30 seconds, whereas, with natural draft furnaces, blower-on time delays of two minutes or more were

common.5 Computer simulation studies found that the flue-gas-side heat transfer in higher-mass heat

exchangers could be improved by 24 percent before reaching a steady-state efficiency where

condensation problems may arise.7

Electrohydrodynamic Enhancement. Electrohydrodynamic enhancement is a technique for

increasing heat transfer in heat exchangers for single-phase or phase-change processes. A highvoltage, low-current wire or plate electrode mixes fluid (or air, in the case of furnaces) of different

temperatures by increasing turbulence and destabilizing the thermal boundary layer near the heat

transfer wall. The effect, a “corona wind,” is caused by accelerated ionized particles colliding with gas

molecules, generating a flow that can quickly reach a speed of a few meters per second.8 A corona

wind interacting with the main gas flow intensifies mixing of the gas, increasing heat transfer rates.

Electrical power consumed by this operation is a few watts or less, additional manufacturing

costs are minimal, and installation and maintenance are no more difficult than for other active heattransfer techniques.9

 The principal cost is the high-voltage power supply. Enhancements to heat

transfer can be as high as 300 percent. Safety concerns are minimal. The electrical hazard is low due

to the small current and, given no moving parts, the mechanical hazard is non-existent.

Summary. All of the above technologies to increase heat exchanger effectiveness have been

demonstrated and incorporated in residential or commercial products and are therefore technologically

feasible. There are no known barriers to manufacturing, installing, or servicing products with these

technologies. Since heat exchanger technology today is mature, there is no reason to expect adverse

impacts on product utility or availability. There are, however, practical limitations to the heat exchanger

technologies discussed above. For example, for a given furnace output capacity, it is mainly the heat

exchanger’s material properties that dictate a minimum heat exchanger size and mass. The known

health and safety risks associated with heat exchangers include carbon monoxide (CO) exposure and

fire danger. However, these problems are not related to a specific heat exchanger technology or

attribute. Rather, they arise for several reasons, including poor design and/or fabrication, and improper

furnace set-up, installation, and maintenance. In summary, improved heat exchanger effectiveness

passes all of the screening criteria.
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4.2.1.2 Modulating Operation

A modulating control is any control that uses either gradual or step-wise adjustment of the

furnace input rate in response to changes in the heating load. Two different types of modulating

controls that can be applied to furnaces and boilers—two-stage and step control—could increase

AFUE and reduce electricity consumption. 

 

Two-stage control refers to a modulating control that cycles a burner between a reduced heatinput rate and off, or between the maximum heat-input rate and off. Two-stage controls are limited to

these two operations. They are not capable of operating from reduced heat input to maximum heat

input, nor of operating from maximum heat input to reduced heat input.

Two-stage control can be achieved by a variety of means. Use of two-stage gas valve

modulation by itself is rarely considered in practice because, if gas flow is reduced without reducing

combustion air flow, the excess air increases and the steady-state operating efficiency of the furnace

decreases. By reducing the firing rate for longer periods, off-cycle losses are diminished and off-period

efficiency can increase.10 A two-speed circulation fan control is also rarely considered without

modulating the gas input, because increasing circulating air flow beyond a certain point reduces comfort,

increases noise, and increases electrical operating cost. The heating speed of the blower should be

selected to provide optimum comfort and minimum operating cost. 

Another control configuration, the two-stage gas valve with two-stage combustion fan control,

requires appropriate design considerations to ensure that the flue products do not condense in the

primary heat exchanger. The AFUE of such a system may improve, but the furnace would run for a

longer time.5 The two-stage gas valve and two-stage circulating blower design, on the other hand, is

similar to two-stage gas valve modulation, except that using an electronically commutated motor (ECM)

would result in reductions in the electrical operating cost that would more than compensate for the

increased cost of gas, and the AFUE rating would be lower.

The two-stage gas valve, two-speed combustion fan, and two-speed supply fan configuration is

a standard way to design a modulating gas furnace. Combustion efficiency is maintained because the

burners operate at high and low input rates with equal excess air rates. A considerable amount of

electrical energy is saved if an ECM motor is used.

Step modulation can provide a number of performance improvements. Furnaces that operate

at substantially reduced output over longer periods of time can provide more uniform space

temperatures, quieter operation, greater efficiency, and reduced emissions. Achieving these objectives,

however, requires that the combustion stoichiometry (the proper fuel/air mixture to assure clean

combustion) be carefully controlled at all firing rates to ensure safe operation and minimum emissions.
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The primary difference between two-stage and step-modulating control is that the two-stage

control must operate between either the low firing rate and off, or the high firing rate and off, whereas

the continuous modulating control can operate between multiple firing rates (e.g., off to low-fire to

high-fire to low-fire to off). According to manufacturer literature,11, 12 a continuously modulating

furnace contains several specific components: modulated output thermostat, modulating gas valve,

integrated furnace control, two-speed or variable-speed blower, two-speed induced draft blower,

supply-air temperature sensor, and return-air temperature sensor.

Step modulation can be achieved by the same variety of means described for two-stage

modulation. This system would have all the advantages of the modulating gas valve, modulating

combustion fan, and modulating supply fan configuration, with the further advantage of slightly better

efficiency and less electrical consumption.

According to manufacturers, step modulation requires a fully modulating gas valve, a variableoutput induced draft fan, a variable-output circulating air fan, and a combustion controller, to maintain

proper control of a modulating gas-fired appliance.12 This is the key to a fully modulating appliance,

which interfaces with all of the furnace controls, including the thermostat, the ignition and flame-proving

devices, the two motors, the gas valve, and all of the safety controls. 

Based on current information, modulating technologies do not pose any known safety hazards. 

The manufacturer installation manuals13 state that the venting system for modulating gas furnaces (at 80

percent and 81 percent AFUE levels) should use double-wall vent connectors, which are widely used

in venting heating equipment. The double-wall vent connector ensures that condensation does not

occur in the vent connector, which is the most likely point in the vent system for condensation to occur.3

Brookhaven National Laboratory is currently testing prototypical air-atomizing oil burners (fanatomizing burners) for potential two-stage firing rates. However, the conventional pressure-atomizing

burners used in residential applications operate with firing rates that are typically too high to be effective

in a staged-firing application.14

Summary. Modulating gas furnaces and boilers, both two-stage and step, are currently on the

market, which demonstrates the technological feasibility and the practicability to manufacture, install,

and service them. There are no known published discussions on adverse impacts on product utility. In

fact, one of the advantages of modulating furnaces and boilers is that they provide additional utility to

consumers (e.g., quieter operation, less variation in air temperatures). Based on the fact that most

major furnace manufacturers currently market modulating models, which use standard technologies and

production processes, there are no adverse impacts on product availability. This technology appears to

have no adverse impacts on health or safety. Thus, two-stage modulating and continuous modulating

technologies, as applied to gas-fired equipment, pass all the screening criteria.
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4.2.1.3 Increased or Improved Insulation

Many furnaces and boilers are equipped with a layer of insulation designed to reduce the

amount of heat transferred to the appliance's environment through the jacket. Increasing the thickness

of the insulation layer reduces the jacket losses in isolated combustion systems (ICS) (outdoor

installations). For indoor installations, jacket losses are considered useful heat and do not affect the

product’s efficiency. Fiberglass is the most commonly used insulation material. Using materials with a

higher R-value (i.e., thermal resistance, the measure of how quickly heat can be lost through the

insulation) would further reduce the jacket losses.

An increase in insulation affects the width but not the height of the furnace. Whereas an

increase in height can present a problem in some installations, the increase in width considered for this

design option does not pose any problems with respect to product utility.

Many ICS and weatherized furnaces (and outdoor boilers) already include the jacket insulation

design option, so adding or improving jacket insulation is technologically feasible. Similarly, the

capability to manufacture, install, and service equipment with increased or improved jacket insulation is

practical using today’s processes. Jacket insulation is already used in existing products and it is

therefore a technologically feasible design option. Jacket insulation poses no health or safety risks. 

Increasing or improving jacket insulation would not have any adverse impacts on health or safety. 

Therefore, the jacket insulation design option passes all the criteria.

4.2.1.4 Condensing Secondary Heat Exchanger

While a primary heat exchanger captures most of the heat available in the combustion process,

a secondary heat exchanger is designed to condense the water vapor in the flue gas, thus harnessing the

latent heat associated with the phase change of the water. The heat of vaporization of water in the

combustion products of natural gas represents about 9 percent of the total energy in the gas.

This analysis focuses on the most common way to condense water vapor in the flue gases, by

adding a secondary condensing heat exchanger to the dry primary heat exchanger. Because sulfur,

chlorine, and other impurities can combine with water vapor to create acids and may be present in the

gas and combustion air, it is essential that the condensing heat exchanger materials be highly corrosionresistant. Most designs that meet this criterion use stainless steel. Another solution is to use low-cost

carbon steel with a plastic coating that is impervious to condensate and to the flue gas.

The water vapor in the flue gases condenses if the temperature of the heat exchanger is

sufficiently low. The heat exchanger area of a condensing furnace must be about double that of the heat

exchanger of an 80 percent AFUE furnace having equivalent heating capacity. Most condensing

furnaces operate with higher inducer pressure than non-condensing furnaces, because of the higher
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density of the low-temperature flue products going through the inducer fan and the greater pressure

drop across the larger heat exchanger. 

Another way to condense flue gases is to increase the heat-transfer coefficient in the heat

exchanger. In a condensing heat exchanger, mass transfer (condensing water vapor) takes place

simultaneously with heat transfer, which improves the heat-transfer coefficient.

Operating in a condensing mode is not a practical near-term option for oil-fired furnaces. The

use of low-sulfur fuel oil may make this a more attractive option in the future, but considerable research

that includes evaluation of safety concerns is needed.

Condensing gas furnaces account for nearly one quarter of annual gas furnace shipments. 

Condensing hot-water boilers are also available from several manufacturers. The high volume of

shipments demonstrates the technological feasibility of condensing secondary heat exchangers. 

Likewise, the ready availability of models of condensing non-weatherized gas furnaces and hot-water

boilers confirms the practicability to manufacture, install, and service this design option, and indicates no

adverse impact on product availability from an increased reliance on this technology. Condensing

technology seems to pose no adverse impacts on product utility or on health and/or safety. Therefore,

the Department determined that this technology option meets all screening criteria for the nonweatherized gas furnace and hot-water boiler product classes.

4.2.1.5 Electronic Ignition

The baseline model ignition system used in hot-water gas boilers and mobile home furnaces

(MHFs) is a standing pilot. A type of electronic ignition system, the hot-surface ignitor (HSI), is also

available for these products. Unlike standing-pilot ignition systems that consume gas continuously, HSI

devices operate only at the beginning of each “on” period. Although electronic ignition devices do not

increase the steady-state efficiency, burner on-time may increase slightly to make up for the heat the

standing pilot would have supplied during standby periods. HSIs use a hot surface to ignite the main

burners. This type of igniter uses 400 Watts (w) for 37 seconds during startup and 2.5 W continuously

during on-cycle. This design does not require maintenance. However, because the igniter itself does

not last indefinitely, igniter replacement is occasionally necessary.

A design option that DOE considered, the “hot-surface pilot,” is a modification of the HSI

system. It is a small, hot-surface igniter that lights a pilot, which in turn lights the main burner. It uses

much less electrical power than the typical HSI and is much more reliable (i.e., less likely to burn out).

Residential oil burners are currently ignited using an intermittent-duty ignition. Underwriters

Laboratories, Inc. (UL) defines intermittent-duty ignition as "ignition by an energy source that is

continuously maintained throughout the time the burner is firing.”15 This design has been the standard of

the industry for many years. Reductions in electric consumption are possible through “interrupted
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ignition,” which is available for residential oil burners.14 The UL defines interrupted-duty ignition as an

"ignition system that is energized each time the main burner is to be fired and de-energized at the end of

a timed trial period or after the main burner is proven to be established.16 In other words, the ignitor

comes on to light the flame, and then, after the flame is established, the ignitor is turned off and the flame

keeps burning.

The burner industry promotes the interrupted-duty ignition strategy. The industry is currently

transitioning away from electromechanical controls and iron core transformers, and toward solid-state

controls with interrupted-duty capability and electronic igniters. So far, the reaction from installers and

consumers has been positive, since these devices are smaller, weigh less, offer improved performance

with cold-oil or delayed-spark conditions, are less sensitive to line voltage fluctuations, and extend the

life of the igniter and the electrodes.17, 18

Since this design option is already being used, it is clearly technologically feasible and practical

to manufacture, install, and service. There is no reason to expect that widespread adoption of this

design option would have an adverse impact on product availability. No adverse impacts on product

utility or on health and/or safety have been reported. This design option passes all the screening

criteria.

4.2.1.6 Induced and Forced-Draft Combustion Systems

Induced-draft and forced-draft combustion systems are two alternatives to natural-draft

systems. Induced- and forced-draft systems use a fan to provide the air flow necessary for

combustion, without the need for a draft hood or draft diverter.

The combustion-air blower in an induced-draft system is located downstream from the burner

and heat exchanger. This is the safest way to obtain the advantages of a power-draft furnace. An

induced-draft system maintains negative pressures throughout the flue gas stream within the appliance,

thus ensuring that any leakages will be directed into the flue gas stream and away from the surroundings. 

This provides an added measure of safety, because a negative pressure is established in the heat

exchanger as well, preventing the combustion gases inside the heat exchanger from contaminating the

conditioned air, should the heat exchanger develop a crack or opening during its lifetime. 

The forced-draft combustion system provides almost the same efficiency advantages as an

induced-draft furnace. The difference is that the combustion-air blower in a forced-draft system is

located upstream of the burner. This difference may affect heat loss from the collector box assembly

because the combustion-air blower is not located in the flue-gas stream in forced-draft systems. 

Although once more common, forced-draft systems are not widely used today, as they cannot maintain

negative pressures in the flue-gas stream.
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The induced-draft technology is commonly used for non-weatherized and weatherized gas

furnaces. Oil-fired furnaces and oil-fired boilers typically use forced-draft combustion. Mobile home

furnaces and gas hot water boilers typically use natural draft systems. Therefore, the Department

considers induced-draft combustion as a design option for mobile home furnaces and gas hot-water

boilers. 

In the case of gas-fired furnaces, both the induced and forced-draft combustion technology

have a significant efficiency advantage compared to the natural draft combustion system. A Gas

Research Institute (GRI) study7 points out that using an induced draft system increases efficiency by 6.6

AFUE points compared to the natural draft system. Electricity consumption increases slightly, but the

total site energy consumption decreases considerably.

In the case of gas-fired boilers, the induced-draft combustion technology also has an efficiency

advantage compared to the natural draft combustion system. The GRI study7 points out that using the

natural draft system instead of induced draft system would increase the efficiency by 2.3 AFUE points. 

Electricity consumption increases, but the total site energy consumption decreases considerably.

Induced-draft combustion systems are used in most gas furnaces currently sold in the U.S., and

forced-draft systems were once common, demonstrating that this technology is feasible and practical to

manufacture, install, and service. The Department is aware of no reports of adverse impacts on

product utility or product availability, or of adverse impacts on health and/or safety, that are attributable

to the use of these technologies. This design option passes all of the screening criteria.

4.2.1.7 Infrared Burner

Infrared burners are a design option for gas-powered burners in furnaces and boilers. Infrared

burners are typically premix burners that produce a high radiant-heat flux with low emissions. Infrared

burners have a small efficiency advantage over blue-flame burners because they can operate at lower

levels of excess air. However, because these burners use 100 percent primary air, they are susceptible

to uneven operation (flashback). To prevent this from becoming a safety problem, flashback-sensing

devices must be added to each burner, thus increasing the cost. Low nitrogen oxides (NOx)

production is a strong advantage of all infrared burners.5

Several infrared burner materials have been developed, including ceramic, stainless steel, and

glass fiber. Most ceramic burners are flat plates and have not been used in gas furnaces, although they

have been used in boilers. Recently, a ceramic burner design that consists of ceramic fibers formed into

a mat structure has been reported.19 This design satisfies the heating load requirements for residential

furnaces, has very low NOx emissions, and is capable of operating within a very wide turndown ratio

(the ratio of the maximum to minimum output). Perforated high-temperature stainless steel and woven

high-temperature wire are formable and adaptable to various configurations. However, metal gets
a

 These results are obtained under the current DOE test procedure conditions, which specify relatively short vent runs

(five-foot venting section). These conditions reflect the installations of mobile home furnaces and do not require the

concentric vent to be made of special stainless material. However, relatively longer venting sections are required in most

homes, which may cause condensate problems in a concentric vent system in the colder parts of the country. Therefore,

corrosion-resistant vent materials should be considered with this design option for applications other than mobile homes.

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brittle with time, creeps out of shape, and cracks. Glass fiber is the most successful material, since it

can be formed and it reliably maintains its shape.

Infrared burners are currently used in many applications, demonstrating technological feasibility

and practicability to manufacture, install, and service. Experience shows no adverse impacts on

product utility. The existence of several technological options to produce infrared burners (as

described above) indicates that there are no adverse impacts on product availability. No reports have

been found indicating adverse impacts on health and/or safety attributable to infrared burners. None of

the criteria would screen out this design option.

The infrared burner design option is not currently applicable to oil burners, because the oil fuel

requires fuel atomization and vaporization prior to combustion. Ongoing research on alternative fuel-oil

atomization and combustion methods may someday permit the development of radiant oil burners. 

However, due to the lack of current technological feasibility, this is not a viable option for oil-burning

appliances at this time.14

4.2.1.8 Direct Vent

Direct-vent appliances use ducts to provide outdoor air for combustion. Non-direct-vent

appliances typically use air from the appliance's environment for combustion. Direct-vent systems may

use a combustion-air preheat system that passes the outdoor combustion air through a heat exchanger

in contact with the flue gases. The combustion air does not mix with the flue gases. The so-called

direct-vent with preheat design uses a concentric vent/combustion air system in which the flue gases

pass through a central-vent pipe and the combustion air passes through a concentric duct surrounding it. 

This arrangement creates a counter-flow heat exchanger that recovers some heat from the flue gases to

preheat the combustion air.

The direct vent with preheat technology provides an efficiency advantage compared to nondirect vent systems. The GRI study7

 points out that using the combustion air preheat technology instead

of non-direct vent system would increase the AFUE by 3.4 AFUE points.a

The direct-vent-without-preheat design is the same as the direct-vent-with-preheat design,

except that the combustion air is not preheated by the flue gases. There are separate vent and

combustion air systems and the combustion air is not preheated. The current test procedure does not
4-12

differentiate between the performance of direct-vent with and without preheat technology; both are

evaluated as ICS installations.

The fact that direct-vent designs are currently available for many models of furnaces and boilers

shows that this design option is technologically feasible and practical to manufacture, service, and install. 

Long experience with direct-vent systems has shown no adverse impacts on product utility, product

availability, or health or safety. Thus, this design option meets all of the screening criteria.

4.2.1.9 Fuel Filtration

An option for oil-fired equipment is the use of modern fuel-filtration technology to improve

system efficiency by maintaining clean flames and reducing the rate of fouling in the heat exchanger.14

Fuel filtration is currently available for some models of oil-fired furnaces and boilers, and

therefore this design option is technologically feasible and practical to manufacture. The Department is

aware of no reports of adverse impacts on product utility, on product availability, or on health or safety

that are attributable to the use of fuel filtration technology. This design option meets the screening

criteria.

4.2.1.10 Pulse Combustion

Pulse-combustion burners 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 initiated, the process becomes self-sustaining.20 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 combustion even when installed indoors. The CO

emissions from pulse combustion burners are 50–66 percent lower than those of a conventional

burner.20 Because the pulse combustion process is highly efficient, the burners generally are used with

condensing appliances.

In contrast to other forced-air furnaces utilizing natural draft and induced draft technologies,

pulse-combustion furnaces generate positive pressure on the flue side of the heat exchanger. This

creates a potential safety problem, because any leak in the heat exchanger results in the potential for

combustion products contaminating the circulation-air stream.

Pulse-combustion gas furnaces were available in the United States for more than two decades,

but they were withdrawn from the market because the manufacturers found that competing technologies

such as condensing flue gases using a secondary heat exchanger cost significantly less to manufacture

and operate. Regarding oil-fired pulse combustion of boilers and furnaces, at least one prototype
4-13

version has been developed, but no commercially available units have been introduced for residential

applications in the United States.

The length of time this technology was available demonstrates the technological feasibility and

the practicability to manufacture, install, and service products with this design option. The Department

is not aware of any adverse impacts on product utility or health or safety that are attributable to the use

of pulse combustion technology. Given the long years of production of this design option, there is

enough experience available to assure product availability. For gas furnaces, this design option passes

all the screening criteria.

4.2.1.11 Dampers

Burner-box dampers and flue dampers restrict the exhaust gas flow on the flue-gas side of the

heat exchanger during the off cycle, thereby trapping residual heat in the heat exchanger. Burner-box

dampers are installed at the combustion-air inlet to the appliance and flue dampers are installed in the

flue, where the flue gases leave the appliance. Stack dampers may be used with appliances equipped

with draft hoods; they are installed downstream of the draft hood. The stack damper closes during the

off cycle to restrict the flow of dilution air through the vent system when the appliance is off. The

dampers are most useful with natural-draft combustion systems.7

A high-quality flue damper may be used in lieu of a fan-assisted design.7 In order for such a

furnace to achieve performance which matches or exceeds the current minimum efficiency requirements,

the damper closure ratio must be close to 100 percent.

Dampers currently are installed on many models of gas boilers, which demonstrates the

technological feasibility of this design option and its practicability to manufacture, install, and service. 

Since the design option is not a complicated technology and has been produced for at least two

decades, there is no reason to expect adverse impact on product availability. The Department is not

aware of any adverse impacts on product utility, or on health or safety, from damper designs. This

design option meets all the criteria.

4.2.1.12 Air-atomized Burner with Modulation

The residential oil burner market is currently dominated by the pressure-atomized retention

head burner. At low firing rates, pressure-atomizing nozzles suffer rapid fouling of the small internal

passages, leading to bad spray patterns and poor combustion performance.

To overcome the low input limitations of conventional burners, a low-pressure air-atomized

burner has been developed that can operate at firing rates as low as 0.25 gallons of oil per hour (10

kW). In addition, the burner can be operated in a high/low-firing rate mode. This burner is currently

being commercialized. It can operate with almost no excess combustion air (less than 10 percent) with
4-14

lean-burning, ultra-clean combustion. This burner design is capable of firing fuel at low input rates to

closely match the smaller heating loads of well-insulated modern homes. Tests performed at the

Brookhaven National Laboratory have demonstrated that the use of this technology produces

significant reductions in emission of nitrogen oxides (NOX) and particulates.21

This burner design has been available for more than a decade. The results from long-term tests

at Brookhaven National Laboratory indicate that there are no problems with technological feasibility or

practicability to manufacture, install, or service this technology. There are no reasons to expect adverse

impacts on product utility, and no indications of adverse impacts on health or safety. This design option

passes all the screening criteria.

4.2.1.13 Delayed-Action Oil-Pump Solenoid Valve

Delayed-action oil-pump solenoid valves can improve overall efficiency and lower particulate

emissions during burner on-off cycling. This device is installed to supplement the fuel pump regulator;

by delaying the fuel release by several seconds, it allows the pressure to build to the required level and

to be fully discharged in the combustion chamber without dripping. Testing at Brookhaven National

Laboratory indicates that the typical efficiency benefit of delayed-action solenoid valves is expected to

be less than 1 percent.

Delayed-action oil-pump solenoid valves for oil-fired equipment are already offered in at least

two fuel-pump models (Valcor Scientific and Taisan), which indicates the technical feasibility of this

design option. It also indicates the practicability to manufacture, install, and service products

incorporating this design option. The Department is aware of no reports of adverse impacts on product

utility or adverse impacts on health or safety caused by this design option. Therefore, this design option

passes all the screening criteria.

4.2.1.14 Increased Motor Efficiency

More-efficient electric motors could be used in furnace blowers, hot-water boiler pumps, and

draft inducers.

Most furnace-blower motors and hot-water boiler pumps use a permanent split capacitor

(PSC) design. PSC motors are reasonably efficient (above 70 percent) when operating at high speed. 

However, when these motors are operated at low speed, their efficiencies may drop into the 20 percent

range. The circulating blower for most gas furnaces is also used to circulate the supply air when the air

conditioner is operating. The air-conditioner evaporator coil needs a higher airflow than the furnace

heat exchanger; therefore, the blower motors operate at low speed, and thus inefficiently, for the

furnace operation.
4-15

Technologies that use power electronics offer dramatic improvements in efficiency at low

speeds. In the ECM—also known as the brushless, permanent-magnet motor—the rotor is a

permanent magnet. A rotating magnetic field is created in the armature by switching current in a

coordinated manner among six coils. This creates a three-phase motor with essentially no losses in the

rotor. The speed and torque of the motor can be accurately varied by controlling the frequency and

voltage applied to the armature coils. ECMs can operate at efficiencies above 80 percent across a

very wide range of speeds. A similar, slightly-the-less-efficient motor technology, the switchedreluctance motor, has a solid-steel rotor instead of a permanent magnet. The power electronics are

more complicated than for the ECM.

Designs to increase PSC motor efficiency are currently available; about 5 percent of furnace

shipments currently use ECMs, so there is apparently no issue with technological feasibility or

practicability to manufacture, install, and service. There have been no reports of adverse impacts on

product utility or availability. In fact, the electronic controls on ECMs allow manufacturers to offer

additional features such as continuous ventilation, reduced noise, and better control of air flow, which

improve the utility of furnaces. There have been no reports of adverse impacts on health or safety. In

fact, improvements that become possible with electronic controls may lead to a healthier indoor

environment due to the lower temperature variations, and also reduce noise. Increased motor efficiency

passes the screening criteria.

4.2.1.15 Increased Blower Impeller Efficiency

This technology discussion applies to furnaces only, since boilers do not have blowers. 

Improving the efficiency of the blower impeller (wheel) lowers the power requirements of the motor. 

The maximum efficiency of the commonly used forward-curved fan impellers is 65 percent, although

efficiency in operation may be much lower. A viable alternative design is a backward-curved impeller,

which has a maximum efficiency of 78 percent. It has been available in larger blowers for decades.

Impellers with improved efficiency are technologically feasible and practical to manufacture,

install, and service. As long as the blower moves the same amount of air as current designs, there will

be no impact on product utility, health, or safety. The improved designs are not expected to be

appreciably different to manufacture, so product availability will not be a problem. This design option

passes all the screening criteria.

4.2.2 Design Options Eliminated from Further Consideration

The Department eliminated the following design options from further consideration because they

do not meet the screening criteria as described in section 4.1.
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4.2.2.1 Self-Generation of Electric Power

It may be possible to use the heat generated by a furnace's combustion system to generate the

electricity needed to operate the various electrical components.22, 23 Known methods include

thermophotovoltaic (TPV) generators, thermoelectric generators, and thermionic conversion. Other

techniques use engines based on the Rankine cycle, Brayton cycle, Stirling cycle, or Otto cycle, where

the engine drives an electrical generator or provides direct mechanical power and the waste heat from

the engine is used for space heating. Several references provide detailed discussion of the selfpowered heating appliance technologies.22, 23

TPV devices convert radiant heat directly into electrical power by using a semiconductor cell

situated inside of the combustion zone. Currently, TPV systems are not available for oil- or gas-fired

equipment. Preliminary testing at Brookhaven National Laboratory has not yet produced an effective

prototype. Future advances in TPV technology may allow incorporation of this new technology into

control systems and power generation for residential equipment. However, this technology is not yet

available for residential use, and considerable research is needed before its potential savings and costs

can be evaluated.

Thermoelectric generators are thermocouples made from semiconducting alloys with power

conversion efficiencies capable of small-scale power generation. A thermocouple is a circuit formed by

two wires of dissimilar metals, where the junctions are kept at different temperatures, producing an

electromotive force in the circuit, also known as the Seebeck effect. For furnace designs, the

thermoelectric generator would form a cylinder around a firing flame. A rechargeable battery would

start the thermoelectric generator but, once hot, the thermoelectric generator would provide sufficient

excess capacity to recharge the battery.24 With no moving parts, a thermoelectric generator is

maintenance-free and can last the life of the furnace.

No self-powered furnaces or boilers are currently on the market. While there has been

research and development work that looks promising, at this point the Department is not convinced that

this design option is technologically feasible. There are no data to indicate the practicability to

manufacture, install, and service products incorporating this design option. The Department is aware of

no reports assessing the impacts on product utility or impacts on health or safety resulting from this

design option. Therefore, the Department excluded this design option from further consideration for the

purposes of this rulemaking.

4.2.2.2 Fuel-Driven Heat Pumps

Heat pumps extract heat from outdoor air or the ground and thus offer the potential for greater

heating output than energy input. This technology takes advantage of the thermodynamic principle that

energy can be extracted from the appliance's environment and delivered to the conditioned space. 
4-17

It is possible to manufacture fuel-fired heat pumps that use engine-driven vapor compression

cycles. The fact that, in the past, several manufacturers marketed this type of equipment shows that this

design is technologically feasible. Other designs use absorption cycles and adsorption cycles. (Several

manufacturers sold air conditioners utilizing an aqua-ammonia-based absorption cycle from the late

1960s up to the 1990s, but the reliability problems and related service costs were prohibitive, and the

manufacturers took these models off the market.) This option uses a fundamentally different

thermodynamic phenomenon from the process of convective and conductive heat exchange that occurs

in furnaces and boilers, and would require a complete redesign of the appliance. 

Fuel-driven heat pump technology appears to be technologically feasible and has no adverse

impact on product utility or on health and safety. During the 1990s, one manufacturer marketed, in

limited localities, a design for a fuel-driven heat pump, but withdrew it after a few years. Based on the

past experience with fuel-driven technology,25 the Department found that residential fuel-driven heat

pumps would not be able to be mass-produced and be reliable to install and service on the scale

necessary to serve the relevant market at the time of the effective date of the standard. Therefore, the

Department excluded this option from further consideration.

4.2.2.3 Flue-Gas Recirculation

Flue-gas recirculation (FGR) design allows a fraction of the flue gas to be recirculated to the

inlet and mixed with the combustion air so that some of the products of combustion pass through the

burner more than once. This process reduces NOx, but potentially increases CO production. The

flame temperature and the temperature of the flue products are also reduced. This technology has been

implemented for pollution control in automobiles and large commercial boilers.

Computer simulations of furnace performance show that FGR with the current furnace design

improves the AFUE by 0.3 percent, due to the reduction of the excess air. However, the combustion

fan would consume 10 percent more electricity in order to overcome the pressure drop associated with

the flue-gas recirculation.7

In the case of boilers, a study showed that the addition of NOx-control systems, such as FGR

and low NOx burners, can reduce combustion efficiency (due to lower combustion temperatures),

resulting in higher CO and organic emissions relative to uncontrolled boilers. The FGR process reduces

the peak flame temperature because the flue gas that is reintroduced into the furnace's combustion and

flame zones is at a lower temperature and has a lower oxygen concentration.26 This process reduces

excess air (free oxygen) fractions, potentially compromising the furnace's ability to meet the

requirements of the American National Standards Institute (ANSI) Z21.47 overfire test, which

stipulates that the carbon monoxide level in the flue products must remain below 0.04 percent when the

furnace is fired at 12 percent over its rated input. Incorporating the FGR process would make it more

difficult for furnace (and boiler) manufacturers, who currently design their equipment to operate with
4-18

excess air fractions that are sufficiently high enough to account for the practical aspects of field

operations and maintain safe operation, to meet the ANSI CO limit.7, 27

Furnaces with FGR design are technologically feasible and practical to manufacture, install, and

service. This design is used extensively in commercial-sized equipment and, therefore, the product

availability is not a problem. There are no known negative impacts on the product utility. However,

this design may result in higher CO production, and therefore, given the potential negative impact on

health and safety due to an increased potential for CO production, the Department excluded this

technology from further consideration.

4.2.2.4 Smart Valve

One option for oil-fired furnaces and boilers is a “smart valve” or positive shut-off valve on the

fuel nozzle, which reduces smoke and soot production during burner start-up and shut-down. This

valve is generally installed directly in the nozzle tip and prevents oil from dripping into the combustion

chamber. This option can also be retrofitted on existing burners. Smart-valve design for oil-fired

equipment does not affect the efficiency of the appliance, but plays a role as an emission-control device. 

Smart valve design has no known adverse impact on product utility or on health and safety. 

Although this technology has been offered by at least one oil-burner manufacturer for commercial-sized

equipment, the Department is not aware of any prototypes for use in residential-sized equipment. Since

there is no proof that smart valve design would be able to be mass-produced and be reliable to install

and service on the scale necessary to serve the relevant market at the time of the effective date of the

standard, the Department excluded it from further consideration.

4.3 SUMMARY OF SCREENING

Table 4.3.1 presents the results of the screening of design options by product class. The design

options showing a “Y” (for “yes”) pass all of the screening criteria. Some options are not applicable

(n/a) for certain product classes. For example, improved or increased insulation is not applicable for

boilers because boilers are tested as indoor appliances according to the DOE test procedure.
4-19

Table 4.3.1 Screening Results for Design Options by Product Class

Gas Furnaces Oil-fired

furnaces Mobile- Home Hot Water

Nonweatherized Weatherized Gas Furnaces Boilers

Design Option Gas Oil

Improved Heat Exchanger Effectiveness Y Y Y Y Y Y

Modulating Operation Y Y Y Y Y Y

Increased or Improved Insulation Y Y Y Y n/a n/a

Condensing Secondary Heat Exchanger Y N N Y Y Y

Electronic Ignition b b b Y Y b

Induced or Forced Draft b b b Y Y b

Infrared Burner Y Y Y Y Y Y

Direct Vent Y Y Y Y Y Y

Fuel Filtration n/a n/a Y n/a n/a Y

Pulse Combustion Y Y N Y Y N

Dampers n/a b n/a Y Y n/a

Air-Atomized Burner with Modulation n/a n/a Y n/a n/a Y

Delayed Action Oil Pump Solenoid Valve n/a n/a Y n/a n/a Y

Increased Motor Efficiency Y Y Y Y Y Y

Increased Blower Impeller Efficiency Y Y Y Y n/a n/a

Self-Generation of Electricity N N N N N N

Fuel-Driven Heat Pumps N N N N N N

Flue Gas Recirculation N N N N N N

Smart Valve n/a n/a N n/a n/a N

Y = The design option is applicable to this product class and passes screening. 

N = The Department screened out this design option from further analysis for this product class.

b = The design option is already in the baseline model of this product class.

n/a = The design option is not applicable to this product class.
4-20

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