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Boilers: Prescriptive or Performance — That is the Question?

Examining why boiler part-load efficiencies should be taken into account in all energy-related ventures.

By Raimundo A. Ramos

In 1973, an energy crisis led to the development of ASHRAE Standard 90. This event served as a catalyst for the creation of building design efficiency requirements, which eventually led to the creation of what is now known as the energy code for buildings. These requirements are crucial in America because 39 percent of all energy consumption in the U.S. takes place in the residential and commercial sectors.

In the commercial sector, space heating consumes 25 percent of all consumption, with boilers being among the most popular heating equipment. For this reason, policies for energy efficiency invariably target boiler efficiency.

Prescriptive or Performance:

Part-Load vs. Full-Load Efficiency
Startup and shutdown losses
Radiation and convective losses
Excess air losses
Boiler Efficiency Standardized Tests
Conclusion
References

In the U.S., the energy code mechanism is established by individual states with few municipalities having separate and seldom more stringent energy policies than their state counterparts. Figures 1 and 2 show the status of statewide energy code adoption for the commercial and residential sectors. Two avenues of energy code compliance are used: prescriptive and performance. The prescriptive compliance path itemizes the minimum requirements for individual building components and systems. The performance path sets the target to be achieved by the building as a whole. In both options, boiler equipment efficiencies are used, either directly or indirectly, to attain compliance.

> FIGURE 1 and 2. Status of commercial building energy code adoption (U.S. DOE 2018 a & b).

For the prescriptive procedure, boiler efficiency rating is used directly in order to show compliance by meeting its minimum efficiency requirement. Adhering to the performance path requires building energy simulation and comparison to a baseline building set by the code. In the latter scenario, boiler efficiency ratings are used indirectly as an input in the energy simulator to achieve compliance. In either case, part-load boiler efficiencies are not required for compliance except in units with a rated input below 300 MBH.

The problem is that boiler efficiency ratings seldom correspond to real-world boiler operations. Boiler systems generally operate at their rated efficiency at peak capacity, and a majority of systems are designed for the worst case winter conditions, which happen only a few days in the year. This results in oversized boilers running in part-load capacities, producing less than optimum efficiencies. In addition, the standardized boiler efficiency test procedures used today hardly portray the operation of boilers in practice, and in some cases, the boiler will never run at the rated efficiency, even if it’s running at full load.

This complication may lead to overestimated energy consumption reductions due to more stringent energy codes, aggravation of already existing inherent inaccuracies in building energy modeling simulations, and cost savings misrepresentation of energy efficiency improvement projects that is more often than not highly exaggerated.

Part-Load vs. Full-Load Efficiency

It is an industry practice to design boiler systems to meet the worst case winter conditions with an added safety factor to account for unforeseen occurrences and errors that are intrinsic to the assumptions and calculations made. The building heating load constantly fluctuates depending not only on weather conditions but also the facility’s occupancy rate and internal gains. This often results in oversized boilers that rarely operate at their full capacity.

Most boiler equipment today — with few exceptions, such as condensing boilers — yield decreased efficiencies at part-load operation. Both in practice and theoretically, boiler efficiency is highest when running at full load.

There are three simple reasons for the reduced efficiency at part-load/seasonal operation: startup and shutdown losses, radiation and convective losses, and losses related to excess air used for combustion at lower firing rates.

Startup and shutdown losses

For safety reasons, startup and shutdown losses are required for all boilers. The burners are programed to provide pre-purge and post-purge in all cycles. This proceeding ensures that no residual exhaust flues are present inside the boiler on startup and shutdown. Part-load and seasonal conditions cause boilers to modulate to lower firing rates. If the heating demand drops below a boiler’s modulating capabilities, the boiler will start to cycle on and off. This condition aggravates the startup and shutdown losses causing the overall efficiency of the boiler to drop.

Radiation and convective losses

Losses due to radiation and convection happen to all boilers regardless of insulation levels. Older boilers with damaged or worn insulation may experience heat losses of up to 10 percent of their energy input. The radiation emitted and the convective heat loss through airflow across the boiler are essentially constant. A 40°F variation in the ambient temperature can affect efficiency by 1 percent or more. Since the efficiency is expressed as a fraction of the boiler’s input rate, these losses will have a greater impact at low firing rates.

Excess air losses

Similar to the purging cycles, excess air above the required theoretical air for combustion is provided as a safety measure to avoid a potentially dangerous condition of fuel-rich operation. The air that is not used for combustion is still being heated and exhausted, thus, losing usable energy in the process.

When the boiler begins to lower the fire rate and the fuel valve begins to close, the combustion air controlling mechanism reduces its flow at a lower rate than the fuel valve decreases its fuel input to the boiler. The non-linearity of the linkage causes an increase in excess air with decreased firing rate, which in turn causes higher energy losses.

This is especially true for single-point positioning control systems. Better air-to-fuel ratios and control over excess air can be achieved with linkageless burner controls and digital oxygen monitoring controls. However, even with advanced mechanisms, excess air is still maintained at higher rates at part-load operations to ensure a clean and stable combustion, hence, losing even more energy than when running at full load.

Boiler Efficiency Standardized Tests

The authority having jurisdiction (AHJ) enacts a standard into a code. In New York City, for example, the 2016 NYC Energy Conservation Code (NYCECC) is based in part on the ASHRAE Standard 90.1-2013. As previously explained, the energy code will provide minimum criteria to be met by the boiler equipment. Table 1 shows the commercial building minimum efficiency requirements for the gas- and oil-fired boilers section of the 2016 NYCECC.

> TABLE 1. 2016 NYCECC minimum efficiency requirements for gas- and oil-fired boilers (NYC DOB 2016).

In order to maximize compatibility and repeatability, standardized test procedures are necessary to determine a boiler’s operational efficiency. Ideally, these test methods should equate to the real operating conditions encountered by the equipment; however, the complexities involved with the wide range of field conditions make simplifications indispensable. These varying field conditions are differences in patterns of use, local climates, control mechanisms, fluctuating building loads, etc.

The concern is that simplifications inevitably lead to the unreliability of results. Parameters chosen for the normalized tests often do not represent realistic operational scenarios. Moreover, some test parameters allow for a manufacturer to choose between a range of values. Since the equipment manufacturer has a vehement interest in expressing efficiency in the scenario that yields the highest efficiency possible, no test specifications should be left for the manufacturer to decide.

As an example, the boiler efficiency test method used for the 2016 NYCECC requirement is from the U.S. Department of Energy’s Title 10, Code of Federal Regulations (CFR), Parts 431. It’s used for boilers with inputs rated equal or above 300 MBH. As part of its requirements, the ambient room air temperature has to be maintained between 65º and 100º at all times during the test. As stated earlier, a 40° variation in ambient temperature can affect efficiency by 1 percent or more. Furthermore, the Cleaver-Brooks Boiler Efficiency Guide mentions that ambient air conditions above 80° are not consistent with engineering practice, as these higher temperatures are favorable to higher efficiency ratings. However, it must be pointed out that the Part 430 of the CFR measures efficiency of boilers below the rated input of 300 MBH with the annual fuel utilization efficiency (AFUE) rating, which is based on ASHRAE Standard 103-1993. The procedure does take into account part-load efficiency as well as including varying outdoor temperatures and an oversizing factor for the boiler. Even though the simplification assumptions used may not be applicable to a particular situation, the fact that the boiler is subject to a more realistic scenario is a great step toward accurate and representable efficiencies.

More recently, ASHRAE released standard 155P, which provides a method for determining efficiency at full, part, and seasonal loads for units with input ratings equal to or above 300 MBH.

Conclusion

From energy savings calculations, where full-load efficiencies are forced at all part-load conditions and standby/startup energy losses are completely disregarded, to state and citywide energy conservation code equipment enforcement policies, it’s evident that part-load boiler efficiencies should be taken into account in all energy-related ventures.

Even when energy codes do account for part-load conditions in one way or another, ultimately, the ideal concept to adopt should be post-occupancy measurement and verification of the equipment’s energy consumption; however, building codes are designed to verify compliance only up until a certificate of occupancy is issued. Beyond that, a post-occupancy energy enforcement measure rarely exists.

But a comforting prospect exists. There is a trending recognition of the importance of the performance path of compliance over its prescriptive counterpart. The performance path, as a holistic approach, offers better overall efficiency results.

In addition, the green building movement can help diminish the lack of realistic boiler efficiency ratings by advocating for policies that require test measurements to be unbiased in favor of higher published efficiencies.

References

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