Retrofitting with condensing boilers

Boilers typically are replaced after they have been in service for 20 to 30 years, or more. This relatively long service life, as well as the significant amount of energy a boiler uses, makes all decisions related to boiler replacement critical from financial, operational, and sustainability standpoints.




    Boilers typically are replaced after they have been in service for 20 to 30 years, or more. This relatively long service life, as well as the significant amount of energy a boiler uses, makes all decisions related to boiler replacement critical from financial, operational, and sustainability standpoints.


    Hot water condensing boilers offer an excellent replacement solution in many applications, and their theory of operation, advantages, and disadvantages are well documented (see Further Reading). Although the efficiency improvement associated with hot water condensing boilers is significant, in many cases they are not chosen to replace conventional boilers because of a concern that they will not operate efficiently in an existing hot water system.



    Condensing boilers are more efficient than noncondensing boilers only when they operate in the condensing mode. In that mode the thermal efficiency increases from the typical noncondensing efficiency (80% and 87%) to a condensing efficiency that ranges between 86% and 99% or more, depending on the operating parameters and the boiler technology.


    The efficiency of a condensing boiler decreases as the temperature of the hot water return to the boiler increases. When the return water temperature to the boiler exceeds approximately 138 F, which corresponds to the dew point of the flue gases, the boiler operates at approximately 87% efficiency. Chapter 31 in the 2008 ASHRAE Handbook, “HVAC Systems and Equipment,” discusses this operation. The thermal efficiency of a condensing boiler is always higher when such a boiler operates at part load. While the manufacturers of conventional hot water boilers and good engineering practice require that the temperature of the hot water return is not lower than the temperature that would cause flue gases to condense, in condensing boilers such practice is recommended to optimize the efficiency of the boiler. Because of that, the automatic control of condensing hot water boilers can allow for the full modulation of the burners to meet space heating load.


    Existing hot water boilers are often replaced, while the existing hot water pipes and radiators are reused. The selection of replacement type boilers is further complicated by the fact that the initial installation costs of condensing boilers exceed that of the conventional type. Maintaining condensing boilers is more costly as well because of the maintenance required for heat exchangers and condensate drain systems. Unlike conventional boilers, ASHRAE has not yet published any data regarding the useful service life of condensing boilers. Is replacing an old conventional boiler with condensing boiler worth considering? The following approach can help answer this question.



    We will calculate the required hot water average temperature in a typical building located in different climates. The results will demonstrate that condensing boilers would operate efficiently when connected to an existing hot water system.


    The building considered is a single-story office building with hot water radiators installed along the perimeter. Existing boilers will be replaced, while existing radiators will be reused. Ventilation is provided via a separate system and is not considered in this analysis. Indoor temperature is maintained at 72 F. The analysis will be performed at three different locations: New York, Chicago, and Los Angeles.


    Heat loss calculations are performed using ASHRAE outdoor design conditions during winter and building characteristics (see “Calculation summary slide” sidebar). The results of building heat loss calculations at ASHRAE outdoor design conditions are summarized in Table 1.


    The lengths and heat output of existing radiators at each location are summarized in Table 2. Note that the heat output of existing radiators is sufficient to offset the building heating load during a design day at an average hot water temperature of 170 F. That average temperature corresponds to the widely used hot water supply and return temperatures of 180 F and 160 F, respectively.


    Temperature requirements

    In the next step, we will calculate the required hot water supply temperature during each hour of the heating season at each location. Heat loss calculations are repeated, following the same method described in “Calculation summary slide” sidebar during each hour of the heating season. Hourly temperatures are obtained from the U.S. Dept. of Energy Web page (see Further Reading).


    We will assume first that the indoor temperature of 72 F will be maintained 24/7. The results of the hourly heat loss calculations are plotted in Figure 1, reflecting the building heat loss during each hour of the heating season. Note that the hourly heating loads for all locations as indicated in Figure 1 are lower than peak heat loss calculated at ASHRAE outdoor design conditions as reflected in Table 1. The reason is that the hourly outside air temperatures are higher than ASHRAE design conditions, resulting in heating loads lower than peak design conditions.


    Existing conventional boilers may be replaced with fully modulating condensing boilers with controls that would adjust the hot water supply temperature to offset building heat loss, but can the heat output from existing radiators complement this sequence of operation?


    Manufacturers of hot water radiators publish the radiator heat output data corresponding to various average hot water temperatures. An example of this data is indicated in Figure 2. Each curve in the chart reflects the performance of a different radiator. There is an almost linear relationship between the radiator's heat output and the average hot water temperature; in other words, reducing the average hot water temperature would result in an approximately proportional reduction in a radiator's heat output. For example, in Figure 2, a radiator heat output at 120 F average hot water temperature is approximately 40% of its heat output at 170 F average hot water temperature. This reduction in the heat output from existing radiators will prevent a condensing boiler from operating at its peak efficiency when outdoor conditions are at or close to ASHRAE outdoor design conditions.


    However, during the much more frequent part load/mild outdoor conditions, the lower heat output from the radiators would be sufficient. This will be demonstrated in the following sections.


    Using data from manufacturers' catalogs, the performance of the radiators at the three locations considered is plotted in Figure 3. Figure 3 reflects the total radiator heat output at different average hot water temperatures at each location. Applying the linear relationship approximation to Figure 3, the average hot water temperature required during each hour to offset the heat loss is determined. The results are summarized in Figure 4. A numerical example is provided in the sidebar “Heating system operation example.”



    The heat loss and the required average hot water temperature curves indicate that during most of the operating hours, the boiler will operate in the condensing mode, allowing for the higher thermal efficiencies of the boiler to be realized. For example, in Chicago, the entire boiler operation will be within the condensing mode during five months or 71% of the heating season.


    One can also conclude that the application of condensing boilers would be more beneficial at locations where the difference between hourly outdoor temperature and ASHRAE winter outdoor design temperature is great. This is because the existing radiators were originally sized based on design outdoor conditions, which allows for relatively high-heat output at reduced hot water supply temperature. In our analysis, the application of condensing boilers in Chicago would yield better efficiency improvement than their application in Los Angeles.


    Night setback

    The above analysis was based on hot water systems that maintain an indoor air temperature of 72 F, 24 hours per day, seven days a week. Assuming that the building will be maintained at 72 F between the hours of 6 a.m. and 6 p.m. and will be maintained at night setback temperature of 55 F otherwise, the corresponding heat loss and required average hot water temperatures are generated following the same procedures described above for all locations. The required hourly average hot water temperatures are indicated in Figure 5.


    The results illustrate that night setback operation would further increase the number of hours at which boilers would operate under the condensing mode, even during the coldest months of the year. For example, at the New York location, the boilers would operate at the condensing mode more than 90% of the time.


    Although an office building was considered for this analysis, similar energy improvement can be expected at other occupancies; especially where buildings remain unoccupied for extended period of times and where night setback settings are used, such as places of worship and schools.


    Fuel cost savings

    Less fuel is consumed as the boiler efficiency increases. Fuel cost comparison per million Btu output at various efficiencies and at different fuel prices is summarized in Table 3. When a condensing boiler is applied on an existing hot water system, it would operate in both condensing and noncondensing modes, and therefore a range of operating efficiencies would be expected rather than a single operating efficiency figure. Any improvement to the building envelope would further increase the number of hours of efficient boiler operation and hence reduce fuel consumption.



    Fully modulating condensing boilers should be reviewed for the feasibility of their use in existing hot water system. The operating efficiency is expected to be the highest in the following cases:


    • The difference between ASHRAE outdoor design condition and hourly temperature is great.


    • The building uses a night setback setting.


    • Condensing boilers' automatic controls are provided to enable the boilers to fully modulate based on outside air or building thermal conditions and to orchestrate the operation of multiple boilers, maximizing the number of hours at which boilers operate at part load.


    Heat output from existing radiators at lower hot water temperature should be reviewed. This information is available from radiator manufacturers. During part load conditions, the lower average hot water from the boiler, which corresponds to lower heat output from existing radiators, could be sufficient to maintain desired indoor conditions. During peak outdoor conditions, the boiler would operate in the noncondensing mode, but this scenario is less frequent.


    Table 1: Heat loss at ASHRAE 2009 outdoor winter design conditions


    New York Chicago Los Angeles
    All figures and tables: Ramez Afify
    ASHRAE heating dry bulb (99%)17.2 F2.2 F46.5 F
    Building heat loss (Btu/hr)381,318485,694177,438

    Table 2: Existing radiator heat output at ASHRAE outdoor design conditions


    New York Chicago Los Angeles
    Radiator length (ft)405375340
    Radiator heat output (Btu/hr/ft)9461,299553
    Total radiator output (Btu/hr)383,130487,125188,020

    Table 3: Fuel cost per million Btu output at different efficiencies and different fuel prices.


    Natural gas price per therm Fuel cost / 1 million Btu at different efficiencies
    *One therm = 100,000 Btu


    Footnotes and references


    • James B. Rishel and Benny L. Kincaid. 2007. “Reducing Energy Costs with Condensing Boilers & Heat Recovery Chillers” ASHRAE Journal .

    • 2008 ASHRAE Handbook, HVAC Systems and Equipment - Chapter 31.

    • EnergyPlus Energy Simulation Software


    Further Reading
    2008 ASHRAE Handbook HVAC Systems and Equipment
    2009 ASHRAE Handbook Fundamentals



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