Heating Rate greater than Heating Design Capacity in a Low Temp Rad Var Flow

Hi, in a simple model of one zone heated with a LowTemperature Radiant Variable Flow by an AWHP it is strange for me to see values of the Radiant Heating Rate [W] higher than the Heating Design Capacity. This happens, both in case of hardsized and autosized values of the radiant system, even for higher than design outside air temperature. Regarding the Radiant Mass Flow Rate, it follow the set limit of the Design Size Maximum Hot Water Flow as expected. I'd like to understand why.

There could be a couple of reasons for this: the water delta T may be larger than design, at the design flow rate. Is there a zone thermostat setback? This could cause the heating rate to increase above capacity due to a cooler zone temperature, which would drive a larger delta T between the radiant surface and the zone air.

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Greg Estep

answered 2024-09-27 18:34:06 -0500

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Thanks for the answer. Yes, the water delta T (inlet-outlet) of the radiant appears larger than the Loop Design Temperature Difference. Modelling the AWHP with a HeatPumpWaterHeater, a zone equipment, I therefore also have a zone thermostat (Heating Thermostat Schedule). This is a constant schedule set to 20°C.

The radiant system is controlled using a HalfFlowPower control type based on the Mean Radiant Temperature (MRT), with a setpoint of 19.5°C and a throttling range of 1°C.

Could the mismatch between the zone thermostat and the radiant setpoint be causing the issue?

lollokai (2024-09-30 04:07:39 -0500) edit

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Heating Rate greater than Heating Design Capacity in a Low Temp Rad Var Flow

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Heating Rate greater than Heating Design Capacity in a Low Temp Rad Var Flow