Field Measurements of Off-Cycle Losses and Seasonal Efficiency for Major Classes of Multifamily Boilers

For multifamily buildings, space heating improvements offer one of the best potentials for saving energy. However, the energy savings and cost-effectiveness of space heating retrofits and boiler replacements depend critically on the efficiency of the existing boiler. Unfortunately, there is only limited information on the performance of space heating boilers under typical on-off cycling conditions, and there is no uniform test method for measuring the seasonal efficiency of boilers larger than 300,000 Btu/h (88 kW) input. Through funding from the Legislative Committee on Minnesota Resources, the Center for Energy and the Urban Environment (CEUE) initiated this study to help fill this information gap. The purposes of this study were:

  1. To investigate possible methods for measuring multifamily boiler efficiency by collecting information on actual boiler performance and by developing and testing boiler diagnostic techniques.
  2. To investigate energy savings opportunities in multifamily boiler systems by collecting information about the energy losses and efficiencies of boilers, as well as the impacts of specific retrofit measures.

A total of 15 typical multifamily boilers were investigated in this project, three of which were instrumented for intensive, long-term monitoring and 12 of which were instrumented for short-term monitoring. These boilers represented a cross-section of boiler types including four power and 11 atmospheric burners, six steam and nine hot water boilers, and six steel¬-fire-tube and 11 cast-iron heat exchangers. The majority of these boilers were packaged, although one was a brickset boiler and two were locomotive style. Rated inputs on these boilers ranged from 300,000 Btu/h (88 kW) to 3,500,000 Btu/h (1,025 kW), with a median rated input of 606,000 Btu/h (177 kW).

For the three intensive sites (which included one steam and two hot water boilers), the part-load efficiencies were directly measured through energy input and output measurements carried out over a wide range of outside temperatures and heating loads. In addition, for the two hot water boilers measurements were taken in two control modes, one in which boiler water temperature was maintained at a constant temperature and the other in which boiler water temperature was adjusted according to outside temperature based on a fixed reset ratio. This yielded five data sets for the three sites monitored. Various theoretical and empirical steady-state models were applied to this data to determine how the boiler losses vary with operating conditions and to find a model that could be used to predict part-load and seasonal efficiency based on shorter term diagnostic tests that measure the off-cycle energy loss rates. All models were based on the simplified assumption that burner on- and off-cycle jacket loss rates are the same. However, it was estimated that the actual difference between burner on-and off-cycle jacket losses could lead to errors as large as 37% of the loss rate magnitude and 66% of the modeled relative seasonal variations. The efficiency regression models suggest that there is a strong dependence of off-cycle energy loss rate on outside temperature, with loss rate decreasing as outside temperatures increased. Between outside temperatures of 60°F (16°C) and 0°F (-18°C), changes for the steam boiler and the hot water boilers were 86%, 88%, and 32% of the value at the seasonal average temperature of 31.1°F (-0.5°C) (with temperature variation in the steam boiler corrected for and the two hot water boilers operating in constant temperature mode). For one out of the five data sets, 84% of this variation was correlated to changes in boiler cycling rate; however, all of the other data sets had 12% or less of the seasonal change in loss rate correlated to cycling rate changes. Direct measurements of the steam boiler's flue loss rate showed that variations in the steady-state off-cycle flue loss rate were 16% to 29% over the same range of outside temperature.

Examination of several models indicated that there is a trade-off between model accuracy and practicality for widespread use. The constant off-cycle loss rate model is the easiest to use, but proved to be the least accurate, while the most accurate models required an excessive amount of data collection. A model based on the assumption of linear variation of boiler off-cycle loss rate with outside temperature provided accuracy and ease of application between these two extremes, and was judged to be the most promising to use with short-term boiler diagnostic tests.

Short-term diagnostic tests that measure off-cycle energy loss rates were also performed at the intensive sites in order to measure the results against the direct efficiency measurements and to study the variation of off-cycle loss rates with boiler operating conditions. The boiler diagnostic methods used were the heat up/cool down (often referred to as cool down or decay) test, the stop-loss test, and the time-to-make-steam test (which is only applicable to steam boilers). The heat up/cool down and stop-loss test results were used to calculate off-cycle loss rates and part-load (as well as seasonal) efficiencies using the assumption of off-cycle loss rate varying linearly with outside temperature. Although these results (as well as those for the time-to-make-steam method applied to the steam boiler) were in fairly close agreement with each other, the loss rates were much lower, and the efficiencies were much higher, than the direct efficiency measurements indicated. Based on these discrepancies, a simplified seasonal efficiency estimation procedure that incorporates a large correction factor was developed for use with the cool down and stop-loss test methods.

The same three short-term diagnostic tests were conducted on 12 additional boilers to provide a comparison of the off-cycle energy loss rates for a number of boiler types and to provide seasonal efficiency estimates using the simplified calculation procedure. For the 12 boilers which exclusively underwent the short-term diagnostics, the computed normalized off-cycle loss rate ranged from 0.94% to 5.29% of burner input for all tests performed on all boilers and assuming a standard boiler temperature of 180°F (82°C). Comparatively, for the three intensively monitored boilers, the modeled off-cycle loss rates at 30°F (-1°C) ranged from 3.1% to 5.0% of burner input. Not surprisingly, comparisons of distinct off-cycle loss rate estimates for the same boiler indicate that greater differences in these estimates occurred between test methods rather than between the results of the same test procedure. In spite of this, the cool down and stop-loss test methods resulted in off-cycle loss rates which were in fairly good agreement, with relative differences of less than 35% for most cases. However, the time-to-make-steam method typically resulted in estimates of off-cycle loss rate which were considerably lower than estimates by the other two methods. Results of a multiple regression analysis on the results from just the 12 short-term test sites indicate that the outside temperature at which the test is performed, the draft type (i.e. power versus atmospheric) and the boiler medium (i.e. steam versus hot water) all have a significant effect on the estimated value of off-cycle loss rate. However, the analysis also showed that other building or boiler characteristics which were not taken into account may play an even larger role than any of the effects considered.

The measured seasonal efficiencies of the boilers which were intensively monitored were 54.7% for the steam boiler and 66.5% and 62.8% respectively for the two hot water boilers operated in constant temperature mode. Comparable seasonal efficiencies for the hot water boilers when operated by a reset control were slightly higher: 69.5% and 64.3% respectively. The steady-state stack efficiency values for these three boilers are 73%, 81.5% and 72.5%. The estimated seasonal efficiencies for the four steam boilers which underwent short-term diagnostics exclusively ranged from 53.7% to 66.7%, at an assumed runtime of 25%, with an average of 58.2%. Comparative stack efficiencies for the same cases varied from 70.2% to 80.0%, with an average of 75.9%. For the seven hot water boilers which only underwent diagnostic tests estimated seasonal efficiencies ranged from 40.0% to 76.0% for an assumed runtime of 25%, which yields an average of 65.6%. Corresponding stack efficiencies for these seven cases ranged from 76.8% to 84.9%, with an average of 80.4%. In all 15 cases the seasonal efficiency value is much lower than the measured steady-state stack efficiency, suggesting the substantial advantages of establishing a commercial boiler part-load efficiency standard which would provide much more accurate information on the actual operating performance of larger boilers.

The seasonal efficiency estimates observed in this study have implications for boiler replacement, including the fact that boilers equipped with power burners (and possibly those equipped with atmospheric burners and motorized secondary air) are considerably more efficient (70.3% on average) than boilers equipped with atmospheric burners and fixed secondary air (57.2% on average). The results of this study can also be used in general to more accurately predict potential savings from new boiler technologies. In addition other information was obtained in this study that has relevance to energy savings opportunities besides boiler selection or replacement. Space heating energy savings from the use of an outdoor reset control was found to be 18.4% and 20.1% for two cast-iron boilers. Although total energy savings was not quantified for any other measure, information about the impact of a boiler vent damper and reduction of boiler cycling rate was obtained. Estimates of the relative magnitudes of off-cycle flue and jacket loss rates indicate that these components of the total loss rate are on the same order of magnitude for the three boilers measured, and suggest that retrofits which reduce either jacket or flue loss can have a significant impact on efficiency, but that both must be addressed in order to achieve a very large reduction in the total off-cycle loss rate.

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Field Measurements of Off-Cycle Losses and Seasonal Efficiency for Major Classes of Multifamily Boilers