APPENDIX 4­A. ANNUAL EFFICIENCY CALCULATION FOR CONDENSING

BOILERS

TABLE OF CONTENTS

4­A.1 Introduction..........................................................................................................................2

4­A.2 Analysis................................................................................................................................2

LIST OF TABLES

Table 4­A. 1 Operational data for calculation of seasonal thermal efficiency for

condensing boiler (Boston climate) .........................................................................4

LIST OF FIGURES

Figure 4­A. 1 Simplified Condensing Boiler Steady­State Thermal Efficiency as a

Function of Return Water Temperature...................................................................3

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4­A.1 Introduction

Appendix 4­A provides calculations used to estimate the seasonal energy performance of

a condensing boiler when applied to a typical existing building. As discussed in chapter 4 of the

TSD, the performance, couched in terms of operating thermal efficiency, of a condensing boiler

is a function both of boiler design as well as boiler operation—the latter a function of the

hydronic system design and controls. While this is to some extent true of all boilers, it is

particularly true of condensing boilers. For high­efficiency condensing hot water boilers, where

the rated efficiency depends on the return­water temperature, DOE developed a simplified model

of the annual efficiency of a condensing boiler used to serve a typical hydronic coil supplying

heat to a building heating, ventilation, and air­conditioning (HVAC) air distribution system.

This type of distribution system is believed to be the most typical of that served by buildings

using commercial packaged hot water boilers. The underlying assumption in this analysis is that

a simple replacement of the existing boiler (or proposed boiler in a new construction) is what is

being analyzed with no fundamental changes to the underlying system design other than

programming of reset controls. While more optimum combinations of condensing boilers and

modified hydronic systems (e.g. oversizing of hydronic coils in forced air systems, or utilizing

radiant slab heating) potentially could provide additional energy savings when using condensing

boilers, they would incur additional costs and would require additional speculation about what a

purchaser may or may not do when confronted with a new standard for commercial boilers

which effectively required the use of condensing equipment.

4­A.2 Analysis

The analysis shown here estimates the seasonal performance of a condensing boiler

serving hydronic coils in a forced air delivery system. The analysis was carried out based on an

example for condensing boiler calculations for a single climate (Boston, MA). The analysis is

based on a slide presentation made to the Puget Sound chapter of ASHRAE by Jim Cooke of

1 Mechanical Solutions NW in 2005 .

Underlying assumptions for the simplified analysis were that:

1) The building heating load is proportional to the temperature difference between the

outdoor air and a 50oF assumed space­heating balance pointa.

2) That the building design heating load was at an outdoor temperature of 7oF.

3) That the heating season for this climate consisted of the months from September through

May

a The space­heating balance point is the outside air temperature above which no heating is required in the building.

Generally a balance point is a statistical average based either on analysis of building heating use data or based on a

mathematical model of building performance. A 50oF is typical of many commercial buildings and reflects that

these buildings have internal heat generation sources (lights, appliance and other plug loads) that offset building

sensible heat losses through the envelope, infiltration, and ventilation down to the heating balance point temperature.

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4) That the hydronic coils were sized such that the design heating load on the building was

such that it could be met with 160oF supply water temperature.

5) That the boiler thermal efficiency varied as shown in Figure 4­A.1. From a return water

temperature of 180oF to 133oF, the thermal efficiency varied from 84% to 87%

respectively. From a return water temperature of 133oF to 60oF, the boiler thermal

efficiency varied from 87% to 96% respectively. This efficiency profile is a simplified

version of the typical condensing boiler efficiency profile as a function of return water

1 temperature.

6) That condensing boiler efficiency at part load and under low fire conditions was not

addressed explicitly, but was built in to the boiler performance curve in Figure 4­A.1.

Because DOE’s thermal efficiency rating is at full load, a DOE standard for condensing

boiler would set only minimum full load efficiency, but would not address performance

requirements under part load conditions.

7) The boiler operates on a reset supply water temperature, assuming 160oF at or below the

heating design ambient dry bulb temperature (7oF), and 86oF supply water temperature

above 54 oF design ambient dry bulb temperature. The reset water temperature is

designed to meet the anticipated heat load in the space assuming continuous flow through

the hydronic coils under all load conditions.

Based on these assumptions, Table 4­A.1 shows the following operational data for the

boiler for each of 6 heating temperature bins: Hours in Bin, Average Ambient Temperature in

Bin, fraction of peak heating load based on average temperature in bin, the fraction of the annual

heating load based on the product of the average heating load in a bin and the number of hours in

the bin, Supply Water Temperature, Return Water Temperature, whether the boiler is operating

in condensing mode (i.e. Return Water less than 133 F), and the average boiler efficiency in the

bin based on the simplified efficiency profile shown in Figure 4­A. 1 4­A.1.

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Steady State Boiler Thermal Efficiency (%)

98%

96%

94%

92%

90%

88%

86%

84%

82%

50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200

Return Water Temperature (F)

Figure 4­A. 1 Simplified Condensing Boiler Steady­State Thermal Efficiency as a Function

of Return Water Temperature

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Table 4­A. 1 Operational data for calculation of seasonal thermal efficiency for condensing

boiler (Boston climate)

Temperature

Bin

Hours

in Bin

Avg.

Ambient

T in Bin

Fraction

of Peak

Heating

Load

Fraction

of

Annual

Heating

Load

Supply

Water

T (F)

Return

Water

T (F)

Condensing

Mode

Boiler

Thermal

Efficiency

54­68 F 1497 61 0% 0% 86 84 Yes 92.9%

41­54 F 1675 47.5 6% 7% 110 112 Yes 89.2%

28­41 F 2258 35 35% 55% 133 116 ~50% 88.1%

14­28 F 627 21 67% 29% 147 129 No 86.8%

0­14 F 124 7.5 99% 8% 160 140 No 86.1%

(13)­ 0 F 11 ­7.5 134% 1% 160 140 No 86.1%

Average Boiler Thermal Efficiency 87.6%

Using this example DOE estimated that a representative seasonal thermal efficiency for a

condensing hot water boiler would be approximately 88% when installed in this application.

Again, DOE recognizes that other system designs and potentially other assumptions about the

building load profile and the operation of the boiler under part load conditions could result in

different estimates for the seasonal efficiency. In particular, for hydronic systems for which the

existing or modified design could meet the load with average supply and thus return water

temperature lower than shown in this example, DOE would expect higher seasonal thermal

efficiency ratings.

REFERENCES

1 Cooke 2005. “Condensing Boiler Technology”. Presentation made by Jim Cooke of

Mechanical Systems NorthWest to the Puget Sound ASHRAE Chapter in Nov 2005.

Available at < www.pugetsoundashrae.org/PDF_files/AshraeCondensingtechnology.ppt>.

Accessed November 22, 2009

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