oversized for the actual heating load and would not respond as favorably to aggressive hot water reset temperatures . This is analogous to zones that were improperly designed for their cooling loads and end up driving the building ’ s chilled water system . Replacing a few heating coils in a building to help increase the amount of time a heat pump boiler could support the entire building load is a worthwhile investment . Figure 6 compares capacities available from 1- and 2-row heating coils on a 16-inch VAV box . In this example , replacing the 1-row with a 2-row heating coil allows the air terminal to meet the 180 ° design load with 125 ° water . For most commercial VAV boxes , there is no difference in physical size between a 1- and 2-row coil , making the possibility of a coil replacement at a few critical zones feasible .
Conclusion
The decarbonization of existing buildings presents a challenging but vast opportunity to reduce the built environment ’ s operational carbon . The application of heat pumps to supplement traditional heating equipment is an economically viable strategy that can drastically reduce the amount of time combustion-driven heating needs to occur . HHW reset strategies applied to hybrid heating plants allow all-electric heat pumps to serve the building heating load when the required HHW supply temperature set point is within the operating range of a heat pump . In considering any heating system decarbonization project , it is critical to understand the true peak heating load and at what load bins the system emits most of the annual emissions . Designers should keep in mind that a partial upgrade of a heating system ’ s generation equipment and some ancillary components is more likely to occur than a complete heating system replacement . In these situations , the maximum capacity available from existing heating hot water coils operating at reduced water temperatures will be limited by the original design building pipe sizes and the construction of the coil .
FIGURE 6 : A variable air volume ( VAV ) 1-row and 2-row reheat percentage capacity versus entering water temperature ( EWT ).
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