The zone heating load appears to increase when switching from four-pipe fan coils to water loop heat pumps

asked 2018-05-07 12:54:04 -0600

updated 2018-05-07 14:39:25 -0600

I have created two models with different HVAC systems which should be otherwise identical. One is a four-pipe fan coil, and the other is a water loop heat pump. The FPFC model uses condensing boilers for heating and a chiller/cooling tower for cooling. The WLHP model using condensing boilers for heating and a cooling tower for heat rejection. The WLHP appears to be using 20-30% more natural gas for heating than the FPFC model. Given that the WSHP model should transfer some of the heating load to compressor heat, as well has to heat rejection from zones in cooling mode, one would expect the natural gas consumption to decrease.

In order to understand this mystery better, I have isolated three zones and examined the hourly reports for these zones in detail on two cold days in January. Each of the zones has a higher heating energy output from the heating coils in the WSHP model than in the FPFC model. My question is, why does the WSHP model supply additional heat? This happens even at night when there is no ventilation air and the DOAS is off. In this case, the zone terminal is responding only to envelope heat losses. Hourly reports and examination of outputs show that the envelope thermal performance is identical with the same U-values. Hourly reports show that lighting and equipment loads are identical and follow the same schedule, that the ventilation flow rate is identical, and infiltration is identical. Zone air temperature is identical as well.

Given that the heat gains and losses are identical, how is it that the heating coil heating energy is higher in the WSHP model by 20%? The FPFC model appears to size a larger fan, but the difference in fan energy is much smaller than the difference in heating energy.

I'm completely stuck and wondering if someone with greater experience than me might be able to offer some insight.

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