In that case , the effectiveness of the heat exchanger can be calculated at its scheduled air and water flow conditions ( cfm and gpm ). The effectiveness could then be used to determine the capacity of that coil against varying supply water inlet conditions . Figure 4 shows the capacity reduction realized from reducing the inlet water temperature on a 1-row coil for several standard variable air volume ( VAV ) box sizes . The capacities below are based on heating cfm that is 50 % of the recommended maximum ( Taylor & Stein , 2004 ) box cooling cfm . The heating water flow rate was determined by locking in the heating coil heating hot water ( HHW ) ∆T at 30 °.
FIGURE 5 : Tracking variable air volume ( VAV ) reheat percentage capacity versus entering water temperature ( EWT ).
Assessing Existing Buildings
Determining the annual heating load profile for a building ’ s system-level preheat coils and zone-level reheat coils is critical to right-sizing a heat pump as well as determining the minimum allowable water temperature capable of satisfying a substantial amount of annual heating load .
FIGURE 4 : A single-row variable air volume ( VAV ) reheat capacity versus entering water temperature ( EWT ).
Figure 4 illustrates the concept of heat exchanger effectiveness in that the capacity will vary linearly with inlet temperature for a given airflow rate and water flow . Figure 5 shows the percentage of the total heating capacity achieved at 180 ° entering water temperature ( EWT ) verses lower EWT temperatures , assuming the water flowrate remains constant . The graph gives the reader a quick way to determine the capacity of any commonly available VAV box reheat 1-row reheat coil for a given fixed air and water flow rate . The 130 ° EWT still provides 60 % of the original 180 ° heating capacity . To explore the effect of changing the airflow and water flow rates through a heating coil , an engineer would need to determine the details of the construction of the coil and rerun the coil performance using commercially available coil software .
If possible , measure the actual heating load for a heating season ( or year ). Annual historical load and outside air temperature trend data can be used to assess the financial feasibility of a heat pump or HRC . Measured actual heating demand for an existing building is instrumental in right-sizing a supplemental heat pump system . An alternative to measuring actual heating load is to do an 8,760- hour load simulation on the system using energy modeling software . This method is feasible but not as accurate as direct measurement .
On occasion , a building owner may agree to test a building ’ s heating system to determine the minimum heating water supply temperature at various outdoor conditions necessary to satisfy the building ’ s heating loads . This data could be used to generate an HHW reset curve .
While we have illustrated that building heating systems are usually oversized , there may be zones in an existing building that are right-sized or marginally
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