Setpoint mismatch in case of RadiantwithDOAS system


It seems that the setpoint for the zone is not matching with the setpoint for the radiant system.

I would like to use the RadiantwithDOAS system for my building performance simulation and looking into the EnergyPlus documentation, I have found that the control of the radiant system is based solely on the heating control temperature schedule, the cooling control temperature schedule, and the control temperature type set in the ZoneHVAC:LowTemperatureRadiant:VariableFlow inputs.
The radiant system does not use any zone thermostat, therefore it will not use the same heating/ cooling setpoint as the heating_schedule and cooling_schedule input defined in the Setpoint() module.

Could you help what would be the way to use the same zone temperature setpoint both in the Air system and in the radiant heating/ cooling system? There is only a limited input option when we specify the RadiantwithDOAS system.

Hi @Helga ,

This is a good observation and the issue you raise is one that the developers of the OpenStudio Standards gem wrestled with a lot. It is their code that is being used to generate the radiant floor and ceiling systems when you select the “Radiant with DOAS” system in Honeybee.

The issue comes down to the fact that real radiant systems that cast pipes with chilled/hot water into concrete slabs have such a lag between the flow of water through the slab and any effect sensed by a thermostat in the space that these systems tend to do away with a zone thermostat altogether. Instead, they usually cast a temperature sensor into the slab to determine the time at which the flow of hot/chilled water should be stopped. This results in much more stable temperatures in the space and lessens the risk of overshooting the setpoint. However, with this setup, the thermal lag tends to eliminate most benefits of setting back the temperature sensor setpoint, meaning that you’re effectively just aiming to keep the conditions in the zone stable 24/7. So any scheduled setpoint temperature is ignored.

So that’s the explanation for why the Zone thermostat is not impacting the default radiant system and why it is instead set in the ZoneHVAC:LowTemperatureRadiant:VariableFlow inputs, which mimic a temperature sensor that is cast into the slab.

Now, not all radiant systems have such a large thermal lag and, if you use the HB Radiant HVAC Properties component to change the radiant_type_ to “CeilingMetalPanel” or “FloorWithHardwood”, these types of radiant systems actually will respect the Zone thermostat setpoints. This is because there is much less thermal mass in these radiant systems and therefore much less lag between when the hot/chilled water starts flowing and when the conditions change in the space.

So, if you want to experiment with radiant systems and setpoint schedules, I recommend using one of those two radiant system types.

Hi @chris,

Thank you for the explanation and for the suggestion.

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FYI, if you want the primary source reference for the temperature controls of the radiant floor and ceiling slab, you can find that it is documented here in the openstudio-standards source code: