NBS

PUBLICATIONS

NBSiR 79-1783 A 1 1 1 5 DMBlbE

Evaluation of Energy-Conserving

Modifications for Water Heaters

Robert L. Palia, Jr.

Center for Consumer Product Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

July 1 979

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QC

U.S. DEPARTMENT OF COMMERCE

100

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79-1733

1979

Jlu?

NATIONAL BUREAU OF S7ANCA.RCS

Kationai Bureau FEB 1 1980

NBSIR 79-1783

EVALUATION OF ENERGY-CONSERVING

MODIFICATIONS FOR WATER HEATERS

Robert L Palla, Jr.

Center for Consumer Product Technology

National Engineering Laboratory

National Bureau of Standards

Washington, D.C. 20234

July 1979

Final

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Luther H. Hodges. Jr., Under Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

ABSTRACT

The effects of various energy-conserving modifications on water

heating energy consumption were evaluated based on laboratory tests. Nine

storage-type residential water heaters, representative of standard and

"energy-saving” electric, gas, and oil fueled models currently on the

market, were obtained for testing. Federally-promulgated water heater test

procedures were used to measure the energy consumption of each unit before

and after modifications. Energy-conserving modifications and corresponding

projected reductions in energy consumption included: reduced thermostat

settings (10°C), 12 percent savings for standard electric, gas, and oil

fueled water heaters (where appliance performance does not degrade below an acceptable level, and water heater capacity is still sufficient to meet hot

water needs); and improved insulation, 9 percent savings for all water

heater fuel types. Also considered for gas-fired units were reduced pilot

input rate from 220 W to 60 W (750 Btu/h to 200 Btu/h), less than 2 percent

savings; use of thermal dampers, 3 percent savings; use o£ intermittent

ignition, 5 percent savings; and use of intermittent ignition and

mechanical flue dampers, 11 to 16 percent savings. Modifications to

energy-saving models resulted in somewhat smaller reductions. Multiple

modifications were found to offer energy savings slightly less than the

sum of the individual savings.

Key Words: Energy conservation; energy consumption; heat loss; insulation;

modifications; gas pilot; recovery efficiency; residential; standby loss;

test procedures; water heaters

i
TABLE OF CONTENTS

Page

ABSTRACT i

LIST OF TABLES iv

LIST OF FIGURES v

NOMENCLATURE vi

1. Introduction 1

2. Energy Conserving Modifications 2

2.1 Reduced Thermostat Settings 2

2.2 Insulation Improvements 3

2.3 Decreased Pilot Input Rates 3

2.4 Intermittent Ignition and Flue Dampers 4

2.5 Thermal Losses From Water Heater Fittings 5

3. Laboratory Facilities 5

4. Test Procedures 6

5. Test Results 10

5.1 Reduced Thermostat Settings 11

5.2 Insulation Improvements 12

5.2.1 Added Insulation 12

5.2.2 Added Insulation and Reduced Thermostat Settings 13

5.2.3 Decreased Thermal Conductivity of Insulation 13

5.2.4 Decreased Thermal Conductivity, Increased Thickness 13

of Insulation

5.3 Decreased Pilot Input Rates 13

5.4 Intermittent Ignition and Flue Dampers 14

5.4.1 Intermittent Ignition for Gas-Fired Units 15

ii
Page

5.4.2 Intermittent Ignition and Mechanical Flue Dampers 16

for Gas-Fired Units

5.4.3 Thermal Flue Dampers for Gas-Fired Units 16

5.4.4 Mechanical Flue Dampers for Oil-Fired Units 17

5.5 Thermal Losses From Water Heater Fittings 17

6. Summary and Conclusions 18

7. References 20

Tables 21-28

Figures 29-34

Appendix A - Determination of Standby Loss from Cool-Down 35

and Recovery Tests

Appendix B - Effect of Thermostat Reduction on Water 39

Heating Energy Requirements

iii
LIST OF TABLES

Table 1

Table 2

Table 3

Table 4

Table 5

Table 6

Table 7

Table 8

Table 9

Table 10

Table 11

Table 12

Characteristics of Water Heaters Tested

Summary of Potential Energy Savings

Effect of Reducing Thermostat Settings

Effect of Adding 33 mm of Extra Insulation

Effect of Adding Extra Insulation and Reducing Thermostat

Setting

Replacement of Original Insulation With Improved Insulation

Replacement of Original Insulation With Improved Insulation

of Increased Thickness

Standby Loss at Various Pilot Input Rates

Replacement of Gas Pilot With Intermittent Ignition

Replacement of Gas Pilot With Intermittent Ignition and

Mechanical Flue Damper

Use of a Thermal Damper With Gas-Fired Water Heaters

Use of a Mechanical Flue Damper With an Oil-Fired Water

Heater

IV
LIST OF FIGURES

Figure 1 Schematic of a Typical Gas-Fired Water Heater

Figure 2 Temperature Profile of a Typical Water Heater Cycle During

Standby

Figure 3 Recovery Efficiency Versus Water Temperature Rise Per Cycle

For One Gas-Fired Water Heater Tested

Figure 4 Standby Loss as a Function of Gas Pilot Input Rate

Figure 5 Heat Loss Measurement Apparatus

Figure 6 System Power Input Versus Equilibrium Temperature Above

Ambient, With and Without Temperature-Pressure Relief

Valve (TPRV) Installed

v
NOMENCLATURE

C

P

E

* T

a

t

specific heat of water (J/kg°C)

total input energy consumed per day by water heater

(energy content of unburned fuel and electric input), (J/day)

standard recovery efficiency (dimensionless)

cyclic recovery efficiency (dimensionless)

total input energy consumed during the recovery efficiency test (J)

total input energy consumed during the standby loss test (J)

energy input rate to main burner or heating element (kW)

water heater standby loss (%/h)

stored water temperature at time t (°C)

stored water temperature at end of test (°C)

stored water temperature at start of test (°C)

stored water temperature at thermostat cut-off (°C)

stored water temperature at thermostat cut-on (°C)

average ambient air temperature during test (°C)

average stored water temperature during test (°C)

difference between water heater inlet and outlet temperatures

before thermostat setback (°C)

difference between water hgater inlet and outlet temperatures

after thermostat setback (°C)

difference between stored water temperature gnd ambient

air temperature before thermostat setback ( C)

difference between stored water temperature and ambient

air temperature after thermostat setback (°C)

difference between ambient air temperature and (°C) inlet

water temperature (°C)

time (h)

vi
t - duration of cool-down period in standby mode (h)

t^ - duration of burner heating period in standby mode (h)

U - daily hot water usage rate before thermostat setback (L/day)

? U - daily hot water usage rate after thermostat setback (L/day)

V - tank volume (L)

P - density of water (kg/L)

t - water heater cool-down time constant (h)

6 - fraction of hot water going to clothes washer and dishwasher

(dimensionless)

1. INTRODUCTION

In the United States, domestic hot water is supplied almost

exclusively by automatic, storage-type water heaters using electricity,

gas, or fuel oil as an energy source. These storage-type water heaters

perform two basic functions in supplying hot water: (I) they heat the

incoming cold water, and (2) they store this heated water until it is

delivered. Thermal losses occur in each of these processes and add

substantially to the energy requirements for water heating.

Several investigators [1,2]* have analyzed heat flows in typical

residential water heating systems. Their results indicate that thermal

losses account for approximately 30 percent and 50 percent of all water

heating energy consumption, for electric and gas-fired water heaters

respectively. (Oil-fired water heaters operate at an efficiency near that

for gas-fired units of similar design). These losses can be substantially

reduced through water heater redesign or modification, resulting in higher

water heater operating efficiencies and net reductions in total water

heating energy consumption.

It was the objective of this investigation to evaluate several water

heater modifications that might reduce water heater losses and energy use. The energy-saving options selected for study were reduced thermostat

settings, increased and improved thermal insulation, intermittent ignition

(for gas-fired units), and flue dampers (for gas- and oil-fired units).

Projected energy savings were determined from tests on a selected group of

nine water heaters. Because most available residential water heaters

differ substantially only in the source of heat and the storage volume [1],

test results are expected to be representative of typical water heaters in

the field.

Evaluation of each modification was based on laboratory tests,

utilizing the Department of Energy (DOE) water heater test procedure C3J

where applicable, and other means of testing as required. In addition, the

thermal losses from typical water heater fittings were measured. Results

obtained in this study were in good agreement with energy-saving estimates

reported in the literature [1,2].

All testing was conducted at the National Bureau of Standards in the

Product Performance Engineering Division of the Center for Consumer Product

Technology. This work was performed in conjunction with work sponsored by

the Department of Energy (formerly the Federal Energy Administration)

pertaining to water heater test method development.

* Numbers in brackets indicate references in Section 7

-1-
2. ENERGY CONSERVING MODIFICATIONS

2.1 Reduced Thermostat Settings

Most residential water heaters are equipped with adjustable

thermostats. Without any extra cost or modification to the water heater,

energy savings can be realized by reducing the thermostat setting for

normal operation and turning the thermostat down even further or turning

the water heater off during extended non-demand periods.

Water at approximately 60°C to 71°C (140°F to 160°F)* is recommended

by most dishwasher manufacturers for reasonable automatic dishwasher

performance. Studies have shown that with some dishwashing machines,

satisfactory performance can be obtained with water temperatures as low as

38 C (100 F) [4]. All machines however, will not offer acceptable

cleaning at lower temperatures. In households equipped with such units

thermostat reductions will be limited to the minimum temperature at which

satisfactory performance is obtained. In homes without automatic

dishwashers, water heater thermostats can be set even lower for normal

operation, perhaps as low as 50 C (120 F). Minimum water heater storage

temperatures are ultimately limited to the point at which the amount of

thermal energy which can be delivered at the lower temperature is just

sufficient to meet hot water needs.

Lowering the thermostat setting reduces water heater surface

temperatures and thus all heat losses from the tank jacket, flue, exposed

fittings, and service connections. Additional energy savings are realized

due to a reduction in thermal energy delivered to fixed volume demands at

the lower water temperature.

To consider the effect of thermostat setback on hot water energy

delivery it must be recognized that a water heater satisfies two types of

hot water demands: (1) an energy related demand in which a quantity of

water at a desired temperature is obtained by mixing—for personal use such

as a shower, and (2) a volume demand by which appliances such as clothes

washers and dishwashers are supplied. The volume of hot water required to

satisfy the energy related demand is dependent on the hot water supply

temperature but the energy required in the delivered water is not. With a reduced storage temperature, water at any desired temperature and flow rate

will still be obtained at a point of use by varying the amount of hot water

used in proportion to the cold water. In contrast, the volume of hot water

required to satisfy the volume demand is independent of supply temperature

since the size of a draw is controlled by either a timer or volume control.

It is here that additional savings are realized through thermostat setback.

The effect of thermostat setback on water heater standby loss was

demonstrated in laboratory tests on several electric, gas-, and oil-fired

water heaters, both before and after adding extra insulation.

*In cases where they are normally encountered in common usage, customary

dimensional units have been included for clarification.

-2-
2.2 Insulation Improvements

Thermal losses from stored hot water, through the water heater

insulation and jacket, occur in all storage-type water heaters. These

losses, termed jacket losses, occur continuously and constitute a major

source of energy inefficiency. Through improvements in the water heater

insulation via increasing insulation thickness, or improving insulation

quality (lowering thermal conductivity), these losses may be reduced

substantially.

Insulation thickness can be increased either by adding extra

insulation (external to the jacket) after water heater installation, or by

using thicker insulation during water heater manufacture. The former

modification may be easily performed by a homeowner, using commercially

available insulation retrofit kits or other types of appropriate insulation

and following approved installation practices. The latter improvement must

by made by the water heater manufacturer.

Improved insulation is offered by several insulation manufacturers in

various grades and thicknesses. Through the use of low conductivity

insulation of standard thickness during water heater manufacture,

significant reductions in jacket losses are possible. This insulation

substitution requires no change in water heater dimensions or design, and

would be a simple and cost effective means of energy conservation. Several

water heater manufacturers now offer energy efficient models with improved

insulation. The use of low conductivity insulation of increased thickness

offers even greater energy savings.

The effects of three different insulation changes on water heater

losses were evaluated in laboratory tests. Each modification resulted in

substantial energy savings.

2.3 Decreased Pilot Input Rates

The primary function of a pilot light in all gas appliances is to

ignite a burner. In a gas-fired water heater (Figure 1), however, the

continuous pilot also performs a second function—supplying heat to the

stored water. Thermal energy is transferred from the pilot flame and hot

products of combustion, through the tank bottom and flue walls, to the

stored water. The transfer process is a complex one, dependent on several

parameters. These include combustion chamber geometry, flue size and

baffling, and pilot energy input rate. (See Section 6.4 for an assessment

of the effects of pilot input rate). Each of these parameters influence,

to varying extents, the amount of heat transferred to the stored water, and

therefore water heater standby loss and energy consumption.

The pilot flame, as well as the hot wall of the flue column, induces

air movement through the flue column. Air flow occurs as products of

combustion and heated air in the flue rise up the flue due to increased

buoyancy. Rising excess air (above that required for combustion of the

pilot gas) carries away thermal energy from the stored water and pilot

flame and contributes to water heater standby loss. Through a reduction in

-3-
pilot energy input rate, a reduction in air flow and thus thermal losses

may be realized. This modification was evaluated in laboratory tests on two different gas-fired water heaters.

2.4 Intermittent Ignition and Flue Dampers

Flue losses from gas- and oil-fired water heaters typically account

for about one-third of water heating energy consumption. These losses can

be reduced by several means, including elimination of the continuous pilot

in gas-fired water heaters, and the use of flue dampers in gas- and oilfired water heaters.

The pilot’s contribution to energy consumption in gas-fired water

heaters may be eliminated by replacing the pilot with an intermittent

ignition system. An intermittent ignition device performs the function of

igniting the main burner, without requiring continuous energy input during

burner "off" periods as the pilot does. Use of intermittent ignition by

itself does not eliminate flue losses. The flue remains unrestricted and

continues to lose heat as a result of a buoyancy effect within the flue

column and exhaust duct. Through use of the intermittent ignition system

however, losses due to pilot-induced air currents may be eliminated.

Pilot-induced air flow may be reduced by a second means, namely, the

addition of a thermal flue damper in the water heater exhaust vent (see

Figure 1). Operation of the thermal damper is quite simple. When the main

burner is off, the thermal damper restricts air passage through the water

heater flue column, while still permitting pilot combustion products to

safely leak through the damper. This leakage or venting occurs through

small slits in several flexible, thermally-actuated damper plates, which

are closed at the time. ’When the main burner ignites, the heat of

combustion products causes the damper plates to open, allowing unrestricted

flow of exhaust gases. Because the thermal damper does not create an airtight seal, it is only partially effective in reducing losses through

the vent. It will operate in conjunction with a continuous pilot however,

and offers energy savings with or without the use of an intermittent

ignition system.

Greater reductions in water heater flue loss are possible with a mechanical-type flue damper. A mechanical damper, similar to a butterfly

valve, is installed at the flue exit of the water heater. When installed in

location B shown in Figure 1, the damper is commonly referred to as a flue

damper, whereas when installed at location A it is called a vent or stack

damper. The damper is automatically actuated when the main burner cycles.

When open (burner on), the damper allows products of combustion to be

safely vented, but when closed it blocks air flow through the water heater

flue, thus decreasing flue losses.

Ideally, the damper plate completely blocks all air passage through

the flue when the main burner is off. For this reason an intermittent

-4 -
ignition system would be required for gas-fired water heaters equipped with

such a damper.

Use of any type of flue damper with water heaters is currently

prohibited by ANSI standards. However, a mechanical-type damper, which

requires that the flue be open before gas will flow to the burner, may soon

be permitted.

The energy saving potential of intermittent ignition systems was

demonstrated in laboratory tests on three gas-fired water heaters. The

effect of adding flue dampers was also determined for these units and for

an oil-fired water heater.

2.5 Thermal Losses From Water Heater Fitting

Current water heater manufacturing practices provide for a layer of

thermal insulation between the water tank and outer jacket. But this

insulation is not extended to cover fittings or connections external to the

jacket, such as temperature-pressure relief valves, tank drain valves, and

exposed thermostat housings. These fittings and connections are often

metallic and in effect constitute "thermal leaks" or low resistance paths

through which heat escapes. The elimination of these losses offers

potential savings in both energy and operating expense. In order to

determine their contribution to total losses during standby, the thermal

losses from typical fittings were measured in the laboratory. Energy

saving modifications to these fittings were not attempted.

3. LABORATORY FACILITIES

Water heater tests were conducted in a laboratory designed

specifically for this purpose. The laboratory consisted of four separate

water heater test stations; two for electric water heaters and two for gasor oil-fired units. The latter stations were equipped with exhaust vents

which reduced the clearance above the test units to less than 1.5m.

Because of this reduced clearance, gas-fired water heaters were tested

without a vent pipe installed above the draft diverter — a minor deviation

from the DOE test procedure.

A variety of controls and instrumentation were utilized in the water

heater studies. Power was supplied to each electric water heater from a regulated power supply at 240 V, in accordance with manufacturers’

installation instructions. Electrical energy input to each water heater

was measured using a watt-hour meter having a 10 watt-hour resolution.

Natural gas consumption of each gas-fired water heater was measured

using a precision wet-test meter equipped with a pressure gage and

thermometer. The heating value of the natural gas was measured and

recorded by a continuously operating calorimeter. The higher heating value*

ranged from 37.7 to 38.8 kJ/L (1010 to 1040 Btu/ft*5 ) but was typically

37.9 kJ/L (1015 Btu/ft-3 ).

*The higher heating value is that measured when the products of combustion

in a calorimetry test are condensed and cooled to the initial temperature

-5-
Fuel oil (No. 2) was supplied to oil-fired water heaters from a reservoir mounted on a precision electronic scale. The specific gravity

and heating value of the fuel oil was determined by calorimetry tests on a sample taken from the main fuel storage tank. The higher heating value of

the fuel used was approximately 45.3 MJ/kg (19,500 Btu/lbm).

Stored water temperatures, as well as ambient air temperature, were

recorded for the duration of each test using either a multipoint

temperature recorder or a minicomputer data collection system. The

multipoint recorder employed Type T (copper-constantan) thermocouples and

sampled temperatures several times per minute for the duration of each

test. Type K (chromel-alumel) thermocouples were used with the

minicomputer system, which measured and recorded temperatures at 2 minute

intervals during recovery efficiency tests and at 15 minute intervals

during standby tests.

4. TEST PROCEDURES

Each water heater tested was equipped with six thermocouples. These

were located within the water tank, one at the level of the center point of

each of six sections of equal volume, from top to bottom of the tank. The

stored water temperature was computed as the arithmetic mean of the six

thermocouple readings. Average temperatures during tests were computed as the arithmetic means of temperature samples taken at equally spaced

intervals.

Water heater performance was evaluated in the laboratory by

determining the efficiency at which the water heater performed each of two

primary functions, namely, heating water and storing the hot water. Inservice water heating energy requirements were projected based on these

tests.

The recovery efficiency test is used to measure the efficiency at

which input energy to the water heater is utilized in raising the stored

water temperature^ In this test the water heater is filled with cold water

at about 21 C (70 F). The heater is then energized and allowed to operate

through a temperature rise of approximately 50°C (90°F). Recovery

efficiency for gas- and oil-fired water heaters is calculated from the

equation

VPC 0>T.) (1)

E - P f 1 Q

r where E^ = recovery efficiency (dimensionless),

V = tank volume (L),

P = density of water—taken to be 0.989 kg/L (8.25 Ibm/gal),

0^

= specific heat of water — taken to be 4180 J/kg°C (1.0 Btu/lbm°F),

-6-
= stored water temperature at end of test (°C),

T\ = stored water temperature at start of test (°C), and

Q = total input energy consumed during the recovery efficiency

r test (J).

Thermal losses through the water heater jacket and fittings occur

during the recovery efficiency test. As a result, recovery efficiency

reflects combustion efficiency, thermal losses through the vent, and jacket

losses during recovery.

A recovery efficiency of 100 percent was assumed for all electric

water heaters tested (all had immersed heating elements). The recovery

efficiency of an electric water heater is actually less than 100 percent

because heat is lost from the stored water during recovery; however, these

losses constitute only a small fraction (typically less than 2 percent) of

the total energy added during the recovery test.

All water heater modifications evaluated in this study were aimed at

reducing water heater storage (standby) losses, and did not significantly

alter the efficiency at which heat is added to stored water. Hence,

recovery efficiency needed only to be evaluated initially.

The losses incurred in storing hot water (standby losses) were

evaluated in accordance with the DOE test procedure for measuring standby

loss [33. Standby loss for electric, gas- and oil-fired water heaters is

calculated from the equation

Q

s +

V pC (T.-T

f

)

S = tv pK C (T -T ) p w a

x 100

(2)

where

S = water heater standby loss (%/h),

Q = total input energy consumed during the standby loss test (J),

o

t = duration of standby loss test (h),

T = average stored water temperature during test (°C), and w

T = average ambient air temperature during test (°C).

3

In this form, standby loss,S, represents the total input

energy required per hour to maintain an average stored

water temperature; it is given here as a percentage of the heat

content of the stored water above ambient air temperature.

-7-
An alternate method of determining standby loss was utilized when

evaluating the effect of gas pilot elimination and the use of mechanical

flue dampers. This method, used in lieu of manually operating the main

burner and blocking the flue column, is based on the construction of a typical water heater standby cycle from water heater recovery and cool-down

profiles (Figure 2). It results in the following expression for standby

loss

S = T - T . max min

(T - T ) (t + t. )

' w a c bE.

x ICO (3)

where

T

max

= stored water temperature at thermostat cut-off (°C),

T

min = stored water temperature at thermostat cut-on (°C),

t = duration of water heater cool-down period in standby

mode (h),

»

The importance of Equation 3 is that it lends itself to the

calculation of standby loss based only on water heater operating

temperatures and the results of two simple independent water heater tests,

namely cyclic recovery efficiency and cool-down tests. A derivation of this

equation is presented in Appendix A.

It should be noted that the value of cyclic recovery efficiency used

in Equation 3 is obtained for a temperature rise from T . to T rather

than the 50 C temperature rise specified in the DOE test procedure for the

standard recovery efficiency. The variation in recovery efficiency with

temperature rise per cycle is illustrated in Figure 3 for one gas-fired

water heater tested. As shown, the cyclic recovery efficiency is about 5

percentage points lower than the standard recovery efficiency for this

water heater.

For the purpose of determining water heater standby loss, use of the

cyclic recovery efficiency, E , with Equation 3 gives good agreement with

values of standby loss determined using E in Equation 2.* Hence, Ej, was

utilized in all simulated standby loss test calculations.

= duration of burner heating period in standby mode (h), and

= cyclic recovery efficiency — value of recovery efficiency

for a water temperature rise from T . to T

V pC (T - T ) min maX

p max mm

*Any error which might be introduced by using the standard rather than the

cyclic recovery efficiency in the energy correction term in Equation 2 is

minimized by beginning and ending the test calculation at T. “ T^. Under

these conditions, standby loss calculations using Equation 2 are essentially

independent of small variations in measured values for recovery efficiency.

3-
Upon laboratory determination of water heater standby loss and

recovery efficiency and the assignment of values to water usage variables,

the energy consumption of a water heater, in actual use, can be estimated.

Daily water heating energy requirements are related to water heater

characteristics and hot water use conditions through the relation

Energy required

to heat U liters

of water and to

make up water Energy required

Daily Energy heater storage to rnaKe up water

consumption = losses during heater storage

these heating losses during

periods _ —

standby uerioas

PC at, u

D 1 r pc at if

i on r x

p o A T^ vo I ^

E

i „ r

q u 100

r r where

E = total input energy consumed per day by water heater (energy

content of unburned fuel and electric input), (J/day),

U = daily hot water usage rate (L/day),

A T = difference between water heater inlet and outlet

temperatures (°C),

A T 9 = difference between average stored water temperature

ana ambient air temperature (°C),

q

= energy input rate to main burner or heating element (kW).

using the DOE test proceaure values of

U = 243 L/day (64.3 gal/day),

A T

1

= 50°C (90°F), and

AT

p

= 50°C (90°F),

in Equation 4 allows the daily energy requirements to be expressed as a function of water heater characteristics alone.

The energy saved through a thermostat setback was determined using the

following equation which gives daily water heating energy requirements as a function of water heater usage parameters. This equation is a modification

of Equation 4 and is derived in Appendix B. For a 0°C thermostat setback,

that is, AT* = ATr

Equation 5 reduces to Equation 4.

_Q_
E = [A-B(AT’-AT ) ] [<j>AT- ( 5 )

f +(l-<f>) AT] + C (At'-AT ) 1 a 1 1 la

where the constants A, B, and C are given by the expressions

PC U

A = Er

•

(p C ) uvs

B =

1 and

100 Q Z

C = 0.24 p C VS

P

with

U = daily hot water usage rate before thermostat setback —

taken to be 243 L/day (64.3 gal/day)

,

AT, = difference between water heater inlet and outlet temperatures

before thermostat setback — taken to be 50°C (90°F), and

» AT^ = difference between water heater inlet and outlet temperatures

after thermostat setback (°C),

AT = difference between ambient air temperature and inlet

water temperature — taken to be zero, and

(j) = fraction of hot water going to clothes washer and

dishwasher — taken to be 0.3.

Water heating energy consumption, as calculated from Equation 4 or 5,

does not account for any interactions with the space heating or cooling

systems. Separate calculations involving standby loss, flue loss, duration

of space heating and cooling seasons and other parameters would be required

to calculate the impact of the water heating system on the total energy

consumption of a building.

In the laboratory, water heater standby loss was determined both

before and after each modification. Recovery efficiency was essentially

unaffected by any of the modifications performed and was therefore measured

only initially. The effect of each modification on energy usage was then

evaluated using the previous equations.

5. TEST RESULTS

Nine water heaters, representing standard and "energy-saving” models

currently on the market, were obtained for testing. This collection was

comprised of several standard models of each water heater type — electric,

gas-fired, and oil-fired — as well as one electric and one gas-fired

’’energy saving" model. Some characteristics of these water heaters are

-10-
presented in Table 1. Notable features of the energy-saving models

included increased thermal insulation and an integral heat trap for the

electric model, as well as improved insulation, reduced pilot rate, and

reduced burner input rate for the gas-fired model.

A summary of potential energy savings is given in Table 2 for the

various modifications which were evaluated. Results indicate that through

a combination of two easily performed water heater modifications —

reducing thermostat setting and adding extra insulation — energy

consumption may be reduced dramatically. Energy savings of about 18

percent were obtained for the gas- and oil-fired models tested, while an 11

percent reduction was realized for the energy-conserving electric model.

Energy savings slightly higher than 11 percent are expected for standard

electric water heaters undergoing this combined modification. Additional

savings may be realized for gas- or oil-fired units by adding flue dampers.

Energy requirements for these models might be reduced by as much as 30

percent with all three modifications.

It should be noted that all estimates of energy savings presented in

this study are based on a small sample size (in many cases one test unit).

Most (standard) residential water heaters do not differ substantially with

regards to energy features such as thermal insulation or flue design

however, and a large sample size is not necessary. Good agreement between

test results obtained and estimates of energy savings presented in the

literaure [1,2] reinforce this fact.

5.1 Reduced Thermostat Settings

To evaluate the effect of reduced thermostat settings on water heater

losses, standby loss tests were conducted on electric, gas-, and ojl-fired

water heaters operating at thermostat settings of approximately 70 C

(160°F). The thermostat settings were then reduced to about 60°C (140°F)

and the tests were repeated. Only the setting of the lower thermostat was

reduced for electric units since this thermostat controls stored water

temperature. Standby loss tests were performed on water heaters both with

and without extra insulation added. Test results are presented in Table 3

along with total reductions in water heating energy requirements as determined using Equation 5. These reductions include energy savings due

to both reduced thermal losses during standby and reduced thermal energy

delivered for fixed-volume demands. The latter accounts for approximately

1/3 of the total energy savings that are realized through thermostat

setback ^see Agpendix B). As indicated in Table 3, a thermostat setback of

about 1CTC (18 F) resulted in energy savings of 11, 13, and 12 percent for

standard electric, gas-, and oil-fired water heaters (without extra

insulation) respectively. Reductions slightly less than these were

obtained for the water heaters with added insulation, after thermostat

setback. Energy savings of 9 percent were realized for the energy

conserving electric unit after undergoing this change.

-11-
It can be seen from Table 3 that lowering thermostat settings does

not significantly change the water heater standby loss (S). This indicates

that the heat transfer resistance between the energy of the stored water

and the ambient surroundings is essentially temperature independent^, as would be expected for temperature changes of only 10°C. For a 5.6°C (10°F)

thermostat setback then, energy savings of 5 percent and 6 percent are expected for standard electric, and gas- and oil-fired water heaters.

5.2 Insulation Improvements

The effect of insulation improvements on water heater standby loss and

energy consumption was demonstrated in laboratory tests on several water

heaters. The original insulation in all standard water heaters tested was

"single density" fiberglass of nominal 25 mm (1 in) thickness. The thermal

conductivity of this type insulation is about 0.042 W/m»K (0.024 Btu/h.ft *°F) C 1]

.

Water heater insulation was improved through three different

changes, namely: (1) the addition of extra insulation, (2) the replacement

of the original water heater insulation with improved insulation (lower

thermal conductivity) of the same thickness, and (3) the replacement of the

original water heater insulation with improved insulation of increased

thickness.

5.2.1 Added Insulation

Extra insulation was added in the form of commercially available

insulation retrofit kits. These kits consist of a piece of 38 mm (1.5 in)

thick fiberglass insulation faced with vinyl fabric. The thermal

conductivity of £he insulation blanket is estimated to be about 0.050 W/m-K

(0.023 Btu/hr • ft- F) [5] . The extra insulation was installed according to

manufacturers' instructions, that is, each insulation blanket was cut to

size, wrapped around the water heater jacket, and then taped in place. The

insulation thickness, as installed, was about 33 mm (1.3 in).

Two electric (one an "energy conserving" model), one gas-, and two

oil-fired water heaters were tested before and after adding extra

insulation. Results of these tests are presented in Table 4 along with

corresponding reduction in energy usage, as determined from Equation 4. On

the basis of these findings it appears that reductions in energy usage of

approximately 7 to 9 percent may be realized for most standard electric,

gas-, and oil-fired water heaters through the addition of extra insulation.

Improvements obtained by adding extra insulation to energy conserving

models (originally equipped with thicker insulation) may be much lower

than for standard water heaters. A 2.6 percent reduction in energy usage

was obtained for the energy conserving electric model as compared with a 7

percent reduction for the other electric water heater undergoing the same

change.

-12-
5.2.2

Added Insulation and Reduced Thermostat Settings

After adding extra insulation, several water heaters were tested at

reduced thermostat settings. The effect of the combined modification —

decreased thermostat setting and increased thermal insulation — yielded

impressive energy savings. As indicated in Table 5, reductions in energy

consumption of 11 and 19 percent were realized for the energy saving

electric and standard gas- and oil-fired water heaters tested. Energy

savings somewhat higher than 11 percent are expected for standard electric

models undergoing this combined modification. Note that the reduction in

energy requirements for the combined modification is not equal to the sum

of the reduction for each individual change, but is slightly less.

5.2.3 Decreased Thermal Conductivity of Insulation

In the second insulation modification, the original water heater

insulation was replaced with low conductivity insulation of the same

thickness. The thermal conductivity of this insulation is about

0.031 W/m»K (0.016 Btu/h-ft *°F)[&J . No extra insulation was used in conjunction

with the improved insulation for this first test. The benefit of

replacing original water heater insulation with improved insulation of the

same thickness was demonstrated in tests on electric, gas- and oil-fired

water heaters equipped with 25 mm (1.0 in) thick insulation. Results of

these tests are presented in Table 6 and show that decreases in standby

loss,S, of about 15, 9 and 5 percent were realized after this change , for

electric, gas- and oil-fired water heaters respectively. These decreases

correspond to reductions in energy usage of 3.6 percent for the electric,

2.9 percent for the gas-fired, and 1.8 percent for the oil-fired water

heater. It should be noted that since the improved insulation used was the

same thickness as original insulation, its utilization would require no changes in water heater design or dimensions.

5.2.4 Decreased Thermal Conductivity, Increased Thickness of Insulation

The original insulation in two water heaters was replaced with

commercially available insulation of 50 mm (2 in) thickness and thermal

conductivity of 0.029 W/m.K (0.017 Btu/h.ft* F ) [6 ] . After making this

modification, the water heater jacket could net be reinstalled due to the

increased insulation thickness. The insulation was taped in place and

subsequent tests were conducted with the jacket removed. Test results,

presented in Table 7, indicate a significant potential for energy savings.

For the electric, a decrease in standby loss,S, of 35 percent was observed

after the insulation change. This corresponds to an 8.5 percent reduction

in energy use. Similarly, a decrease in standby loss of 29 percent,

corresponding to a 9.6 percent reduction in energy usage, was obtained for

the gas-fired water heater. These improvements are of the same magnitude

as those obtained by adding 38 mm (1.3 in) of extra insulation.

5.3 Decreased Pilot Input Rates

The potential for energy savings through reductions in gas pilot input

rate was assessed based on tests on two gas-fired water heaters. The pilot

-13-
input rate of each water heater was first measured "as received." The

original thermostat was then replaced with one having a pilot adjustment

capability, and tests at the original and several different pilot rates

were conducted. The minimum pilot input rate of heater Gl, 88 W (300 Btu/h),

was limited by a pilot flame sensor (thermocouple) cut-off, but by

repositioning the sensor closer to the pilot flame, a pilot input rate of

59 W (200 Btu/h) was possible. The pilot input rate of water heater G2 was

adjustable to 59 W (200 Btu/h) without modifications to the flame sensor. Pilot input rates below 88 W (300 Btu/h) may not be feasible in practice,

however, due to chimney downdrafts which could extinguish the pilot flame.

ANSI standards of updraft and downdraft tests, as well as wind tests, will

ultimately determine the minimum pilot size necessary.

Test results for both water heaters are presented in Table 8 and

Figure 4 for various pilot rate reductions. These results indicate a trend

toward lower values for standby loss at decreased pilot input rates, but

the energy savings realized were not significant. For a reduction in pilot

input rate from 200 W to 88 W (680 Btu/h to 300 Btu/h), the standby loss,S,

of water heater Gl decreased from 6.9 percent per hour to 6.7 percent per

hour. This decrease corresponds to only a 0.8 percent reduction in water

heating energy requirements. A further reduction in pilot input rate to 59

W (200 Btu/h), if safety requirements can be met, would result in water

heating energy savings of 1.0 percent for this unit.

Water heater Gl was also tested at a pilot input rate greater than the

original rate. As shown in Table 8, except for pilot input rates below 130

W (440 Btu/h), standby loss for this water heater was essentially

unaffected by changes in pilot input rate.

For water heater G2, a reduction in pilot input rate from 220 W to 88

W (750 Btu/h to 300 Btu/h) resulted in a decrease in standby loss,S, from

6.1 percent per hour to 5.9 percent per hour. This reduction corresponds

to a 1.4 percent energy savings. A further reduction in pilot input rate

to 59 W (200 Btu/h) resulted in only a 1.8 percent decrease in water

heating energy consumption.

5.4 Intermittent Ignition and Flue Dampers

It was the original objective of this investigation to evaluate the

effect of gas pilot elimination and mechanical flue dampers through actual

water heater modification, using commercially-available electric ignitors

and dampers. As such items were not available for domestic water heaters,

it was necessary to simulate the effect of these devices. Thermal dampers

were obtained commercially and were evaluated in accordance with DOE

standby test procedures.

Two different methods were utilized in determining water heater

standby loss. In the first method, pilot elimination for gas-fired water

heaters was simulated in the laboratory by manually igniting the main

burner at the thermostat cut-on temperature (T . ) and extinguishing the

-14-
burner and pilot at the thermostat cut-off temperature (T ). The effect

of a mechanical flue damper combined with intermittent ig?i?£ion was

simulated by additionally blocking the flue during periods in which the

main burner was off.* Conducting the DOE standby loss test under these

conditions, however, is a time-consuming task because water temperature

must be monitored frequently, and burner ignition may be required during

times when personnel are not available. For this reason, a second means of

determining standby loss was utilized. This method is based on an energy

balance on the water heater during a typical standby cycle (Figure 2) and

results in an expression for standby loss in terms of water heater cooldown rate and cyclic recovery efficiency. These parameters may be measured

independently and then substituted into Equation 3 to determine water

heater standby loss.

The accuracy of Equation 3 was demonstrated in three separate tests.

In the first test, the effect of pilot elimination on the standby loss of

water heater G1 was determined by manually igniting and extinguishing the

burner at the proper time. Standby loss determined using this method was

6.23 percent per hour as compared to a value of 6.03 percent per hour

calculated from Equation 3. The use of a mechanical flue damper in

conjunction with electric ignition was then simulated by manually operating

the burner and blocking the flue completely when the main burner was off.

The result was a standby loss of 4.52 percent per hour. Standby loss

obtained from Equation 3 was 4.53 percent per hour. Finally, the standby

loss of water heater 01 (oil-fired) was calculated using Equation 3. A

value of 6.27 percent per hour was predicted. This was in excellent

agreement with test results of 6.28 percent per hour. Equation 3 was used

in the remaining tests.

Test simulations were performed on three gas and one oil-fired water

heaters. Prior to simulating any modifications, the recovery efficiency

and standby loss of each model was measured using DOE test procedures. The

cyclic recovery efficiency of each water heater was then measured at

various temperature rises. Cool-down experiments were conducted, and the

time constants were evaluated with both the flue open and the flue blocked.

Standby loss was predicted using Equation 3.

5.4.1 Intermittent Ignition for Gas-Fired Units

The results of tests simulating replacement of the continuous gas

pilot with intermittent ignition are presented in Table 9 for two standard

and one energy-saving water heater through pilot elimination. Through

pilot elimination, reductions in total water heating energy use of 4

percent and 6 percent were realized for the standard gas-fired water

heaters tested. A reduction of about 3 percent was obtained for the energy

saving model tested. These estimates do not include any energy required to

operate the ignitor.

It is interesting to note here that if the results of standby loss

versus pilot input rate tests (Figure 4) were extrapolated to a zero input

*This is equivalent to a 100 percent effective flue damper, since no air

passage occurs.

-15-
rate, standby loss values of approximately 6.7 percent per hour and 5.7

percent per hour would be predicted for Units G1 and G2 respectively.

Through actual tests on these units however, standby loss values of 6.0

percent per hour and 5.0 percent per hour were measured when the pilot was

eliminated — significantly lower than the predicted values. This

difference seems to indicate a discontinuity or extreme non-linearity at or near the zero input rate axis of Figure 4. The net result is that just the

presence of a pilot has a greater effect on water heater standby loss than

variations in the pilot input rate.

5.4.2 Intermittent Ignition and Mechanical Flue Dampers for

Gas-Fired Units

' '

The use of mechanical flue dampers in conjunction with intermittent

ignition offers energy savings far greater than those obtained using

intermittent ignition alone. As shown in Table 10, reductions in water

heating energy use of 11 and 16 percent were realized for water heaters G1

and G2 after this modification. A decrease of 10 percent was measured for

heater G3 (energy-saving model). These estimates were obtained from tests

in which the flue was completely blocked during cool-down periods. Smaller

reductions are expected for flue dampers which do not restrict air flow

completely.

Comparison of the standby loss of heater G1 when the exhaust stack is

blocked above the draft diverter (simulating a stack damper) to that

obtained when intermittent ignition alone is used (Table 9), indicates that

the "flue damper'1 has a minimal effect in reducing water heater standby

loss when installed as a stack damper (above the diverter). In this

location however, it would be effective in reducing infiltration losses

through the stack.

5.4.3 Thermal Flue Dampers for Gas-Fired Waters Heaters

The energy saving potential for commercially-available, thermal-type

dampers was demonstrated in tests on each of the three gas-fired units.

Thermal dampers allow normal operation of the continuous gas pilot, hence

the DOE test procedure was utilized in all tests. The dampers were

evaluated in their two possible installation locations — above and below

the draft diverters (locations A and B in Figure 1) — that is, as stack

dampers and as flue dampers.

Test results are presented in Table 11. These findings indicate that

thermal dampers, like mechanical dampers, offer only small water heating

energy savings when they are installed as stack dampers — typically less

than 1 percent. When installed as flue dampers they offer substantially

greater water heating energy savings — 4 percent and 3 percent for

standard and energy saving gas-fired units respectively.

-16-
5.4.4 Mechanical Dampers for Oil-Fired Water Heaters

The use of a flue damper on an external-flue, oil-fired water heater

was also simulated in the laboratory. The effect of the damper on standby

loss was determined for two water heater conditions; as received, and with

extra insulation installed. Results are given in Table 12. Reductions in

energy use of 14 percent and 9 percent were realized after adding a damper,

for the original and insulated condition respectively. When equipped with

extra insulation and a flue damper, the standby loss for the water heater

was nearly half its original value.

5.5 Thermal Losses From Water Heater Fittings

Determination of the thermal losses from exposed water heater fittings

was made using an indirect, comparative method of measurement. In this

method the total heat loss from a 75-liter (20 gal) electric water heater

was determined both with the test fitting installed and with the fitting

removed. The difference in the measured heat loss was attributed to the

fitting losses.

The thermal losses from water heater temperature-pressure relief

valves (TPRV) and thermostats were measured in the laboratory using this

comparative method and the apparatus shown in Figure 5. In these tests, a variable power supply was used to apply constant electrical power to

the electric water heater, on which test fittings were installed. With the

application of power, the system temperature increases until an equilibrium

temperature (a function of power input and thermal losses) is reached. At

this point, ; the power input is equal to the total heat loss from the

system. By taking measurements at various power input levels then removing

the fitting and repeating the procedure again, the thermal losses of each

fitting were determined.

Heat loss test results are presented in Figure 6 for a temperaturepressure relief valve. In this Figure the power input to the test fixture

is plotted against the equilibrium tank temperature above ambient for both

the TPRV installed and TPRV removed. At a given tank temperature above

ambient, the heat lost by the TPRV is the difference between the power

input with TPRV installed and the power input with TPRV removed. Based on this measurement, the thermal losses from a TPRV are about 4 W (14 Btu/h)

at a stored water temperature of 40 C (72 F) above ambient. Losses from

metallic tank drain valves are expected to be about half the losses from a TPRV because the valve is located closer to the tank bottom where the

stored water is coolest, and because the exposed area of a drain valve is

approximately one-half that of a TPRV.

Losses from a gas-fired water heater thermostat and associated

plumbing were measured also using the comparative test method. These

losses are about 10 W (34 Btu/h) at a stored water temperature 40U

C above

ambient. An oil-fired water heater thermostat was also tested in the

-17-
laboratory. The losses from the unit were approximately 4 W (14 Btu/h) at

a stored water temperature of 40°C above ambient.

Thermal losses from exposed fittings and connections may be reduced

through use of thermal insulation, where safety is not compromised, or

through use of replacement fittings having decreased thermal conductivity.

These modifications were not evaluated in the laboratory.

6. SUMMARY AND CONCLUSIONS

Reducing water heater thermostat settings is one of the most effective

means of saving energy. For the most common types and sizes of domestic

water heaters, the expected energy saving is approximately 10 percent for

electric models and 12 percent for gas or oil-fired models when a 10°C

thermostat setback is utilized.

Standby loss and thus water heating energy usage is substantially

reduced by improving jacket insulation. Adding insulation blankets results

in energy savings of 7 to 9 percent for standard electric, gas-, and oilfired water heaters. The direct replacement of original insulation with

thicker and lower conductivity material offers similar reductions.

Potential savings for "energy-saving" water heaters, are about 3 percent —

somewhat less than for standard models.

Reductions in pilot input rate decrease water heater standby loss only

slightly. Energy savings of from 0.8 to 1.4 percent were realized for a pilot reduction from about 220 W to 88 W (750 Btu/h to 300 Btu/h). Further

pilot reductions to 60 W (200 Btu/h) resulted in additional savings of less

than one-half percent.

Replacement of gas pilots with intermittent ignition offers

considerably greater savings than pilot rate reduction. Energy savings of

about 5 percent were achieved through the elimination of continuous pilots

in standard gas-fired water heaters. Smaller improvements (3 percent) were

observed for the "energy-saving" model tested.

Use of mechanical flue dampers in conjunction with intermittent

ignition is one of the most attractive combined modifications for gas-fired

water heaters. Water heating energy savings of from 11 to 16 percent and

10 percent can be expected for standard and energy-conserving gas-fired

water heaters undergoing this combined modification. Thermal flue dampers,

operating in conjunction with continuous gas pilots, offer smaller energy

savings — about 4 percent for standard and 3 percent for "energy-saving"

gas-fired water heaters. When installed as stack dampers, both mechanical

and thermal type dampers offer little water heating energy savings.

Uninsulated water heater fittings and connections, although small

area compared to the total tank area, account for substantial losses.

Thermal losses at a stored water temperature 40°C (72°F) above ambient

in

are

-18-
approximately 4 W (14 Btu/h) for a temperature-pressure relief valve, 10 W

(34 Btu/h) for a gas-fired water heater thermostat and 4 W (14 Btu/h) for

an oil-fired water heater thermostat. Thermal losses of the drain valve

are estimated to be 2 VJ (7 Btu/h) under similar hot water storage

conditions.

All results obtained in this study are in general agreement with what

energy saving estimates are reported in the literature [1,2]. In

conclusion, it should not be assumed that the potential savings from making

all possible modifications will be the sum of the individual savings

measured. As results presented in Table 2 indicate, multiple modifications

have interacting effects and result in slightly smaller overall savings.

-19-
7. REFERENCES

(1) Arthur D. Little, Inc., Study of Energy Saving Options for

Refrigerators and Water Heaters, May 1976.

(2) Hoskins, Robert A. and Hirst, Eric, "Energy and Cost Analysis of

Residential Water Heaters," Oak Ridge National Laboratory, ORNL/CON10, June 1977.

(3) Energy Conservation Program for Appliances, Test Procedures for Water

Heaters, Federal Register, Vol. 42, No. 192, October 4, 1977.

(4) Karieka, T.M. and Compton, J. W. "Low Temperature Home Machine

Dishwashing, "Proceedings of the Conference on Major Home Appliance

Technology for Energy Conservation, Purdue University, February 27 - March 1, 1978.

(5) "Insulation Refit Kit for Domestic Water Heaters," Report prepared by

Johns-Manville Sales Corporation for ERDA under Contract Nos. WA-76-

3813 and WA-76-3814, March 23, 1977.

(6) Owens-Corning Fiberglass Corp., Thermal Conductivity Data.

-20-
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Energy

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nr

vo

min o

r~

VO CO nr • • • rH

rH rH

CN CN CN o

CN

00VO

VO

ro

uo

+J o o CN

02 t • r- • 00 • 03 0

vo in VO

+J 4J

(T3 03• r-• 00• 0U ro rH o 0

•H CN CN CN

04H

O

O -25- m

oo

ro

vo

co

oo

nr

nr

vo

in

in

CM

(N

IN

CM

CN o

CN

03

vo

r~vo

co

in

03

ro

CN

r-• rH

CN

CN rH

ta e> i *c

Burner

did

not

cycle
Table

8

-

Standby

Loss

at

Various

Pilot

Input

Rates

t'-» co cn \0 \0 \ooo cn no o

CTk oo x NO

>x xi

T3 X t""

OT !/) O'? NO

+-» o

CO X

oo CT> o pH

in in NO \o

a)§

-*-1 © (0

5-1 U 5-1 -PQ §Qj

CO E

in CO f=H (SI oo 1—

1

in NO in CN ©

cn r> to i—

4

• r—

t

CO !—

1

O (SI to

• (SJ CN

CO 5-0 M 0) © 4-5 > X

<c s

NO NO vO X in NO NO NO nO NO NO NO

u ©

<1 to o CN CTi 00 in r- CN NO NO CO

4-J n3 • • CN

* § feP

CN CN CN t-H r—

H

r-H CN r-4 t-H rH r-4 <N CN CN CN CN CN CN CN CN CN CN CN

CQ ©

4-> /-s

/ V /—

\

or- CON / N / X o o o O o o o o o O o o

3 o o 00 00- oo in in CN in in

4-> 4-> CN to •rr rr NO NO 00 CO CN hO m r- O CQ '—

X J ^—/ N > N / v—

/

V / v / N—

t

X ' i-4 '—

•H cn 03 o o o o O o to o O o a- 2: in CO to o o in in NO o NO CN

r—

I

r—

4

CN CN CN CN r—4 r—

i

CN

5-4

5-4 O

0) 4->

4-> C3

ctJ ©

3: 3: u cnO

•M

a5

+-> cn9

c

©

5

.5

00 6

1 < «

26-

-

Original

pilot

input

rate
c

.2

-U •H

+J

+J

Q) 4JC

jc4J•H4Jo o

•H

isSM5s

cnc8

ctf>

QJ 4JaUI

idc

•H&

•H

5-1 O

-P

fd -P0

rH•H

eu

u

pa

-Pc0) •H 23

jj

tdP

S.

JU

aj rH&c?

_ 1e. _u

C

B 0

•H

Q)Cn

co n*c so

00

CN o

VO o

in

vo

CO

CN in 00 • • • n* 00 00

CN CN co

U •BOp&

<d aPS$

OSvo

i“

H

VO o

rr

CO

vo

CN

CN

VO

m

vo

r-~• rH

CN

CO• rH

CN

CN

CO

CN

u

<d CN• •sr• as• rHcu

VO

in Pin oVO Cm$

COVO

VO

00

VO o

r»

O 4: cn cn U ID l 0i—t

d) f-l 1 I5

ix a*4-> cnc6 £P

-pq

<d 1p

cu P3

CQcn • • i—i uo

<n« in

oo

OS

in oo • . in

in

co

voCN

n* CN 00 in • • • • CN in rH CO

CO CN -sr in

ID CNa

COa -27-

.1I

cnC&P

d)I < i03

Damper

installed

above

draft

diverter.

Reported

energy

savings

do

not

include

decreased

space

heating/cooling

energy

requirements.
Table

11-

Use

of

Thermal

Damper

With

Gas

Fired

Water

Heaters co

-H

-P -HC8

<T3 C

•rlCn •H

cntn <#> 3s d CO E & H O

tn o

<0

M

U Q) 0 U £ S

05 *5.

0) ctf toss

gg5

OQ

Om

©no co

O CN

00 rH • • vo voin

in

CT\ VO • e co co

r- CV rH vo o o o

VO vo

vovo

-p *p

VO VO

cn CO

O cn in iH CN •P CO CO

CN CN CN CN CN

cnVO

rH

vo c

'p

cn

vo

CN

CNVCinvo

h s3

HJ

4Je r- 00 CN 5 a u e • • •H rH rH CO CN CN CN

rHC

CNc <coO

-28-

gs •H X O P ^0) 3 0 M-U

'O 4-J Q)

J

cN> c0

•H

+> •H1

rH

rd .5

cn •Ha

is.

I*

W

fq cn <*>

u 1 CO 3 I

4Jc Q I

r* cn 5 £ •rt s 8 x:

u o

in ov

co oo

co

co

CN

CO

in

00

'CP

in

vom

§8 <5. 4J Q tf(> CO t-H 1

COS dE 0 E-* ,, cn , U

ct3U

C

u a) a) -p £ £

CO

vo

r-T

00

VO cnvo

VO

CN

00d

CN

tn £ U C E-1 0£ U • B-> O

Sh 3

(T5 £

fl5

'p

VO

co

in

vo

CNrt-H O

(0 05

rH 1 05 0 cr1 92 > it! 3 Q o a) rH « n p ip

rd

a

cn 0 0 c mj T3 +J -H£

*8

<D cd c; rH 0 P r-< 04 CQ 0 a a C C 0 0 •H -r-i U S H 4-3 iH I <

53 QU ! Q

With

insulation

blanket
CAS FUEL

FIGURE 1. SCHEMATIC OF A TYPICAL GAS-FIRED WATER HEATER

-29-
<0 -H £h g g ^ &4 3yniv«3dW3j_ y3iv^ a3uoi$

30.

FIGURE

2.

TEMPERATURE

PROFILE

OF

A

TYPICAL

WATER

HEATER

CYCLE

DURING

STANDBY
0.8 0

0:LUOO=>IxIQ:>

C£t_>

ro

UJ

cn

co

-31
CDI

Ul!

I

<C

cd

<CO

CL. CO zc <C—

CO I

CO O

0-

J

i —

Q_ >-

CQ CO

ca<c

2: CD

<c

1— LL. COO

LU

oc

CD

IODQiJ> Wh<TZQffl> ae

32-
-

THERMOCOUPLE

x

-33-

FIGURE

5.

HEAT

LOSS

MEASUREMENT

APPARATUS
0H

LU

oc

<c I or: LU LU

Q_ CC I— <C

cez cd

sz LU LU

ZD

D I

cr: LU <C C23 I— !

1—1 CO a nr ^

LU

*—i— >* cl: CO IS Cl. ZD Hoo a w

oc

LU < LU I— 1— <C

=3

—• > CL. ^

2= LU —i > LU

I—

Cd ^ I LU LU LU

IS •—i CC O CQ

Q- SI LU < QC

SI 2D LU LU CO !— > CO

CO o LU >CQ CL

CO <c c_

CD

LU

QC

CD> < 0. 03USI HZQ.DH I 3 €hH(n
APPENDIX A

Determination of Standby Loss From Cool-Down and Recovery Tests

A typical water heater cycle, Figure 2, consists of two periods — the

period in which the burner is on (t^) and the period in which the volume of

stored water cools from T to T . (t ). From an energy balance on the

IT13X min o water tank, burner on-time is given by the expression

V pC (T - T . ) p max mm

E

r

q

(Al)

where

t^ = duration of burner heating period in,

standby mode (h),

V = tank volume (L),

p = density of water (kg/L),

c

C = specific heat of water—taken to be 4180 J/kg C

P (1.0 Btu/lbm F),

T = stored water temperature at thermostat

cut-off ( C),

T . min stored wgter temperature at thermostat

cut-on ( C),

q

= energy input rate to main burner or heating

element (kW), and

E = cyclic recovery efficiency — recovery

efficiency for a water temperature rise

iron xmin to T

max

V PC (T

p max

- T . ) min

The recovery efficiency used in Equation Al is a measure of the

efficiency at which energy is transferred to the stored water for a temperature rise from T . to T (typically about 10°C). This efficiency

i$ termed the "cyclic recovery efficiency" and is designated by the symbol

E . The cyclic recovery efficiency is dependent upon the water

-35-
temperature rise per cycle (temperauture differential) and water heater

characteristics. This dependency is shown in Figure 3 for one gas-fired

water heater tested.

The main burner is off during the period, t , when the stored water

cools from Tma

to T

^

. From a single lumped capacity analysis for water

heaters without pilots? water temperature during cool-down may be expressed

as

T = (T, - T )

• exp (-t/ t ) + T

1 a a

where

T = stored water temperature at time t (°C),

T^ = stored water temperature at start of test (°C),

T = average ambient air temperature during test (°C),

t = time from start of test (h), and

t = water heater cool-down time constant (h).

The rate of cool-down, represented by the inverse of the constant t,

is a function of (1) the overall heat transfer coefficient between the

stored water and ambient, (2) heat transfer surface area, (3) tank volume,

and (4) properties of the stored fluid. As such, the water heater time

constant is a design-dependent water heater parameter which may be changed

through water heater modifications such as the use of improved insulation,

flue dampers, etc.

The cool-down rate of a given water heater may be determined by a simple test, termed a cool-down test. In this test, stored hot water at

temperature T. is allowed to cool without the addition of heat by burner or

pilot. The temperature difference between the stored water and ambient

air, T - T , is recorded throughout the test. The time constant may then

be evaluated from the relation

t = t/ In (A2)

which is the slope of the line obtained from a graph of t versus In

Knowing the time constant for a particular water heater design, the

time required for water to cool from a specified maximum temperature, T „ , to a minimum temperature, T^.

n

, is given as

-36-
t t • In (A3)

C

The sum of t, and t , given by Equations A1 and A3, is approximately

equal to the duration of one complete cycle of a water heater. An

expression for standby loss in terms of these parameters may now be

derived. The standby loss of a gas- or oil-fired water heater is defined

as

S =

where

Q.

Vp VTi-V

E

r tV pC (T - T ) p w a

x 100

( A4)

S = water heater standby loss ( %/h

)

Q„ = total input energy consumed during the standby loss test (J),

o

E = standard recovery efficiency (for a 50°C temperature rise),

(dimensionless),

t = duration of standby test (h),

T^ = stored water temperature at start of test (°C),

T^, = stored water temperature at end of test (°C), and

T = average stored water temperature during test (°C) w

T + T . max min

2

Considering a typical water heater cycle beginning and ending at T ^ , the second term in the numerator of Equation A4 vanishes. For one cycle

oX

(without pilot), the energy term Q may be expressed as:

2

Q

s

V PC

P

(T - T . ) max min

T E

(A5)

Making these substitutions and noting that the time for one complete cycle

is simply the sum of the cool-down time and burner time, Equation A4 can be

rewritten as

S = T - T . max min

(t V(T„

T ) e'

a r

x 100 (A6)

-37-
Through the use of Equation A6, water heater standby loss may be determined

based on the water heater cool-down time constant and cyclic recovery

efficiency. Cyclic recovery efficiency, E , is obtained from recovery

tests in which the temperature rise is approximately equal to that which

occurs in a normal water heater cycle during standby, that is, T minus

T . . Equation A6 is valid for water heaters without pilots. I??a$as used

iPPSetermining standby loss with simulated intermittent ignition and flue

dampers.

- 38-
Appendix B

Effect of Thermostat Reduction on Water Heating Energy Requirements

To consider the effect of a thermostat setback on water heater energy

consumption, it must be recognized that a water heater has two supply

functions: an energy related function to supply hot water for personal use

such as a shower, and a volume function to supply such appliances as the

clothes washer and dishwasher. The latter have no control on the

temperature of the water delivered and accept what comes through the hot

water connection. The amount of water used by these appliances is

controlled either by a timer or volume control.

i The amount of flow through the water heater for any temperature rise AT^,

can be expressed as:

AT

U = [d> + -4- (1 - 4))] U (Bl)

h

AT

1 where

U = daily hot water usage rate before thermostat setback — base flow

(L/day)

,

T U = daily hot water usage rate after thermostat setback (L/day)

cf> = fraction of base flow water, U, going to clothes washer and

dishwasher (dimensionless).

AT = difference between water header inlet and outlet temperatures

1 before thermostat setback ( C), and

1 AT-^ = difference between water heater inlet and outlet temperatures

after thermostat setback (°C).

The breakdown of water consumption between fixed-volume and fixedtemperature demands is illustrated in Figure 31 for a base hot water usage

rate, U, of 243 L/day, base temperature use, ATp of 90°F, and a fixed flow

fraction, 9 of 0.3. Corresponding reductions in thermal energy delivered

to fixed- volume demands are shown in Figure B2. (The energy delivered to

fixed-temperature demands is constant). Recalling that thermal energy is

converted at approximately the water heater recovery efficiency, it can be seen that for a ICrC thermostat setback, these volume-related energy

savings account for about a 4 percent savings in total energy consumption

(Table 1), or roughly 1/3 of the total energy savings obtained through

thermostat setback (Table 3).

-39-
Reduction

in

Thermal

Energy

Delivered

to

Fixed-Volume

Demands

(kWh/day)

Hot

Water

Usage

Rate

— u'

(L/day)

FIGURE B1 - HOT WATER USAGE RATE VERSUS THERTOSTAT SETBACK

FIGURE B2 - REDUCTION IN THERMAL ENERGY DELIVERED TO FIXED-VOLUME

DEMANDS VERSUS THERMOSTAT SETBACK

-40-
The daily energy required to supply U L/day of hot water is given as

where the first term on the right of this equation is the amount of energy

required per day to heat the required amount of water U

1

, and the second

term represents the amount of energy required per day to make up the

thermal losses from the stored water.

Specifically:

E = total input energy consumed per day by water heater

(energy content of unburned fuel and electric input), (J/day),

p = density of water — taken to be 0.989 kg/L,

• » P c

p

VS A T

2

100

(B2)

C = specific heat of water — taken to be 4180 J/kg°C,

P

E = standard recovery efficiency (from Equation 1 in text)

r

q

= energy input rate to main burner or heating element (kW),

V = tank volume (L)

S = water heater standby loss (from Equation 2 or 3 in text), and

AT

2

= difference between stored water temperature and ambient air

temperature after thermostat setback (°C).

Equation B1 can be substituted into Equation B2 with the following

result,

E = [A-BCAT,la-AT ) ] [9 AT,+(1-9) AT, ] + C (AT,- AT ) 1 1 la (B3)

where

PC U

D

r

( PC ) 2

UVS

C =0,24 PC VS, and

P

’ AT

a

= difference between ambient air temperature and inlet

water temperature (°C).

at

x - at

2

-41-
Equation B3 lends itself to calculating the daily energy requirements

for any condition of inlet water temperature, ambient air temperature,

temperature rise through the water heater, water heater characteristics,

and volume flow fraction.

Through a survey it has been determined by the Product Systems

Analysis Division at NBS that for a temperature rise AT^ = 50°C, and a fixed ambient to inlet water temperature difference, At^, of 0°C, the

average daily hot water use, U, is equal to 243 liters per day or 1700

liters per week. In a separate survey by the same group it was determined

that the fixed volume flow fraction, 4> , of the base flow U was on the

average equal to 0.3.

For actual water heater operation, the cold water inlet temperature

has been geographically, demographicall^ and seasonally averaged to a value of 13 C. When combined with a 50

U

C temperature rise, an average tank

water temperature, of 63°C is obtained. This temperature corresponds to

the average thermostat setting of electric, gas-, and oil-fired water

heaters as normally installed in most households.

-42-
NBS-114A (REV. 9-78)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR79-1783

2. Gov't Accession No. 3. Recipient' s Access ion No.

4. TITLE AND SUBTITLE

Evaluation of Energy Conserving Modifications for

Water Heaters

5. Publication Date

6. Performing Organization Code

7. AUTHOR(S)

Robert L. Palla, Jr.

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, City, st*te, ZIP)

National Bureau of Standards

Washington, D.C. 20234

13. Type of Report & Period Covered

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

I i Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or less factual summary of moat si gni ficant information. If document includes a significant bibliography or

literature survey, mention it here.) The effects of various energy-conserving modifications on water heating energy

consumption were evaluated based on laboratory tests. Nine storage-type residential

water heaters, representative of standard and "energy-saving" electric, gas, and

oil fueled models currently on the market, were obtained for testing. Federallypromulgated water heater test procedures were used to measure the energy

consumption of each unit before and after modifications. Energy-conserving

modifications and corresponding projected reductions in energy consumption included:

reduced thermostat settings (10°C) , 12 percent savings for standard electric, gas,

and oil fueled water heaters (where appliance performance does not degrade below an

acceptable level, and water heater capacity is still sufficient to meet hot water

needs) ; and improved insulation, 9 percent savings for all water heater fuel types.

Also considered for gas-fired units were reduced pilot input rate from 220 W to 60 W

(750 Btu/h to 200 Btu/h) , less than 2 percent savings; use of thermal dampers,

3 percent savings; use of intermittent ignition, 5 percent savings; and use of

intermittent ignition and mechanical flue dampers, 11 to 16 percent savings.

^ Modifications to energy-saving models resulted in somewhat smaller reductions.

Multiple modifications were found to offer energy savings slightly less than the

sum of the individual savings.

17. KEY WORDS (six to twelve entries ; alphabetical order; capitalize only the first letter of the first Key word unless a proper name;

separated by semicolons)

Energy conservation; energy consumption; heat loss; insulation; modifications;

gas pilot; recovery efficiency; residential; standby loss; test procedures;

water heaters

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