Design guidelines for hybrid boiler systems.

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1 Design guidelines for hybrid boiler systems. The first element of designing a hybrid boiler system is to employ a multiple boiler control that incorporates both an outdoor reset function and the ability to prioritize boilers as dedicated first-on and dedicated standby. Without these functions, the hybrid boiler system will not function as desired. General heating season data The process of designing a hybrid boiler system begins with an analysis of the area s heating season characteristics. One source of this information is ASHRAE, which provides this information for numerous cities in the United States. This information is known as ASHRAE Bin Data. An example of ASHRAE bin data is provided in the table below. In this case, this is the data for the New York City. This information has been collected over several decades and is averaged. ASHRAE Bin Data - New York City of Total Hours % 14.0% % 25.7% % 36.1% % 44.9% % 55.2% % 66.9% % 79.7% % 89.4% % 95.0% % 98.2% % 99.6% % 99.9% % 100.0% The first column contains temperature bins; hence the name bin data. These are merely ranges of outdoor temperature. The lowest range is the anticipated coldest temperature for the New York City during the heating season. Naturally, the low temperature will be different in different cities. Column two is the number of hours during the heating season where the outdoor temperature is within the range indicated in column one. For example, over the entire heating season, there are on average 690 hours when the outdoor temperature is between 45 and 49 F. Totaling all of the individual bin hours provides the total number of hours in the heating season; in this case,. Columns three and four are mathematically deduced from column two. Column three indicates the percentage of total hours in the heating season that are represented by the hours per bin in column two. Column four is cumulative hours of all temperature bins starting with the warmest outdoor temperature and ending with the coldest. For example, for 55.2% of the heating season, the outdoor temperature is 45 F or warmer. Likewise, for 95% of the heating season, outdoor temperature is 25 F or warmer. It is clear that the vast majority of heating season hours occur when it is considerably warmer than design day. Design guidelines for hybrid boiler systems 1 of 6

2 Extrapolating boiler requirements from ASHRAE bin data To begin designing a hybrid boiler system, building heat loss characteristics are added to the heating season data. This example assumes a building with 8 million Btu of heat loss on design day. As it is warmer outside, building heat loss drops in a direct and linear relationship with outdoor air temperature. This in turn directly impacts required output from the boiler system, as illustrated in the last column. New York City Heating Season Data of Total Hours Building Heat Loss in Btu Percent Building Heat Loss/ Boiler Output % 14.0% 615, % % 25.7% 1,230, % % 36.1% 1,845, % % 44.9% 2,460, % % 55.2% 3,076, % % 66.9% 3,691, % % 79.7% 4,306, % % 89.4% 4,921, % % 95.0% 5,536, % % 98.2% 6,152, % % 99.6% 6,767, % % 99.9% 7,382, % % 100.0% 8,000, % Modifying boiler loop temperature to meet building heat loss In the example, assume that all air heating coils in the building are properly sized for design day with F entering water temperature and 160 F leaving water temperature. While that is required for design day, the supply loop temperature does not need to remain at F throughout the heating season in order to maintain the desired building temperature. With milder outdoor temperatures, the supply loop temperature can be reduced. For this example, by calculation and information provided by heating air coil manufacturers, it is known that a 140 F entering water temperature reduces the output of an ventilation makeup air coil to approximately 63% with a 30 F outside air temperature and reduces the output of an interior heating coil to 65%. As long as the coil heat output exceeds the building heat loss, the loop temperature will be sufficient to keep the building at the desired temperature. In our example, with a 30 F outdoor temperature, the building heat loss is 61.5% of the design day heat loss. Therefore, a 140 F boiler water supply loop provides sufficient coil output at a 30 F outdoor temperature. Greater detail about the mathematics involved with outdoor reset and the effect on coil output with differing entering water temperatures is in the technical paper Principles of Reset. Design guidelines for hybrid boiler systems 2 of 6

3 From the information provided about air coil performance with different entering boiler water temperatures, a boiler supply loop temperature schedule can be added to the heating season data table, as provided below. These boiler water temperatures will adequately heat the building during the corresponding outdoor temperatures. New York City Heating Season Data Building Heat Loss in Btu Percent Building Heat Loss/ Boiler Output % 615, % % 1,230, % % 1,845, % % 2,460, % % 3,076, % % 3,691, % % 4,306, % % 4,921, % % 5,536, % % 6,152, % % 6,767, % % 7,382, % % 8,000, % Boiler Supply Loop Temperature Schedule Condensing operation possible Applying condensing boilers to the system A condensing boiler will begin to condense when water entering the boiler (loop return temperature) is less than 130 F. From the above loop temperature schedule, condensing operation will be possible when outdoor temperature is 30 F or warmer. This means that the first 4.9 million Btu of boiler system output can utilize condensing boilers. (Actually the entire boiler system can utilize condensing boilers, but when outdoor temperature is lower than 30 F, the boilers will no longer condense because loop temperature will be too high.) The boiler input corresponding to an output of 4.9 million Btu is approximately 5.4 million Btu. Therefore, three 1.8 million Btu condensing boilers will satisfy the load. Completing the hybrid boiler system The remaining boiler output of 3.1 million Btu (8,000,000-4,900,000) will be supplied by non-condensing boilers. The output would be satisfied by two 2 million Btu input boilers. Design guidelines for hybrid boiler systems 3 of 6

4 Graphical representation of hybrid boiler system operation from the example building Hours NYC ASHRAE Bin Data Condensing Operation Temperature F Non-condensing Operation For 90% of heating season hours, condensing boilers will operate with loop temperature low enough for boilers to condense. This also represents 95.5% of the boiler system output over the heating season. The remaining 10% of heating season hours represents 4.5% of the boiler system output over the heating season. This output will be provided by non-condensing boilers. reset schedule for this example Building Loop Temp F Temp F Building Loop Temp F Design guidelines for hybrid boiler systems 4 of 6

5 Justifying the hybrid approach With this system, for nearly 90% of the heating season hours, the building heat demand will be satisfied by operating only the condensing boilers. When at low fire and a supply temperature, the efficiency of these boilers could reach the mid 90 percentiles. The non-condensing boilers would operate for only the last 10% of heating season hours, totaling on average only 713 hours annually and consuming about 9000 therms. At $1.20 a therm, cost to operate the boilers would be approximately $10,800. If condensing boiler were used as the supplemental boilers instead of the non-condensing boilers, they would at best be 7% more efficient at the higher water temperatures and therefore save about $760 a year. Considering that there can be a $20,000 to $30,000 difference in the installed cost of two 2- million Btu condensing boilers versus two non-condensing boilers, it could take 26 years for the condensing boilers to pay back. New York City Heating Season Data Building Heat Loss in Btu Percent Building Heat Loss/ Boiler Output % 615, % % 1,230, % % 1,845, % % 2,460, % % 3,076, % % 3,691, % % 4,306, % % 4,921, % % 5,536, % % 6,152, % % 6,767, % % 7,382, % % 8,000, % Boiler Supply Loop Temperature Schedule This example did not contemplate any redundancy in the boiler system. If the system required 50% redundancy for example, all of the redundant boilers would also be non-condensing. In this example, two additional 2-million Btu non-condensing boilers would be added to obtain the 4 million Btu in redundancy. All four of the non-condensing boilers could rotate to share the load during the coldest days of the heating season. Design guidelines for hybrid boiler systems 5 of 6

6 Applying the hybrid boiler system Hybrid Piping Diagram 1 Hybrid Piping Diagram 2 Two examples of piping diagrams are provided for the hybrid boiler system. Diagram 1 indicates a system composed entirely of medium-mass boilers and uses a reverse-return piping arrangement with flow through all boilers provided by the building circulator. Diagram 2 combines medium-mass condensing boilers piped reverse-return with instantaneous non-condensing boilers piped primarysecondary. The instantaneous boilers require their own circulating pumps. The main difference between the two approaches is that instantaneous non-condensing boilers will have a lower initial equipment cost, offset by somewhat higher installation cost and more details required in the piping design. In both diagrams, boilers 1 through 3 are condensing and boilers 4 and 5 are non-condensing. The condensing boilers (B1, B2 and B3) will be established as first-on boilers. The multiple boiler control should allow rotation through the boilers. In addition, the control should establish load sharing through a parallel operating approach. This enables more than one condensing boiler to operate at a lower firing rate to meet a particular load as opposed to a single boiler firing at a higher rate. Because the efficiency of condensing boilers improves when at a lower firing rate at a given temperature, this slightly improves the overall system efficiency. The non-condensing boilers (B4 and B5) will be established as dedicated standby boilers, which means that the non-condensing boilers will not fire until all condensing boilers are firing at full rate and cannot maintain the boiler loop set point. The multiple boiler control should enable rotation through these boilers to equalize runtime as well. Their pumps, if instantaneous boilers are used (diagram 2) will be controlled by the boiler firing cycle. The boilers will be brought on and offline simply by attempting to maintain the loop temperature set point established by the outdoor reset control. If the building heat loss calculations are correct, the process of targeting and maintaining appropriate loop temperatures for the different types of boilers will occur automatically. If necessary, the multiple boiler control allows the outdoor reset schedule to adjusted. Greater details and assistance in properly designing a hybrid boiler system, including payback and other financial information are available from Riverside Hydronics. Design guidelines for hybrid boiler systems 6 of 6