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Figure 3-5 gas-fired modulating / condensing boiler boiler model MAXIMUM HEAT OUTPUT (Btu/hr) MINIMUM HEAT OUTPUT (Btu/hr) model 55 51,000 7,700 model 85 79,000 7,900 model 110 102,000 10,200 model 155 144,000 14,400 model 200 185,000 18,400 model 285 264,000 26,400 model 399 377,000 75,600 Figure 3-6 a design heating load of only about 20,000 Btu/hr. This is about 1/3 the design load of a typical 2,000 square foot house constructed during the 1980s. 5. Lack of suitable combustion-based heat sources: As building design heating loads continue to drop, it becomes difficult (or impossible) to find combustion- based heat sources that would not be significantly oversized for those loads. The smallest gas-fired mod/con boiler currently available has a rated output of approximately 50,000 Btu/hr and a turndown ratio of about 6.7 to 1. Even at its lowest stable firing rate, the heat output from this boiler would provide 39% of a 20,000 Btu/hr design heating load. Oversized combustion- type heat sources tend to short cycle which lowers their net efficiency. By comparison, there are more selections for electrically operated heat pumps that can better match lower heating loads. 6. Basic service charges & pricing for natural gas: For houses with natural gas service, the monthly basic service charge associated with having the gas meter becomes an increasingly larger percentage of the gas invoice as the building’s design heating load decreases. For modest homes, constructed to ENERGY STAR® or other contemporary energy conservation standards, the total monthly service charges may even exceed the cost of the gas consumed. Many gas utilities also charge a higher rate per CCF (100 cubic feet) for some initial quantity of natural gas, with a decreasing rate as usage increases. This tends to increase the average price per CCF of gas required for homes with low heating loads. Both of these factors can be seen in the residential gas service invoice in Figure 3-6. 22PDF Image | Heat Pump Systems 2020
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