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I created a simulation of an existing building in OpenStudio 1.10.0. The heating in the simulation is provided by a hot water heating coil (which is supplied by a natural gas boiler). I have hourly gas data from the actual building which I am using to verify that my heating system is working correctly. The real building uses 'rail heating' - exposed pipes that sit about 6 inches off the floor. Hot water circulates through the pipes to provide radiant heating. I have tried to model this using the ZoneHVAC:LowTemperatureVariableFlow object in OpenStudio but the gas usage is way off, about 3 to 5 times greater than the usage from the actual building.

I thought I would try using the Coil:Heating:Water object in place of the ZoneHVAC:LowTemperatureVariableFlow object. I made an air loop with just a fan, the coil, an uncontrolled diffuser and the thermal zone that the coil is supplying. I used the SetpointManager:SingleZone:Reheat setpoint manager for the air loop. The gas usage profile shape was nearly an exact match to the profile from the actual building, but the values were several times less than the actual values (peak hourly usage from the simulation was 3.8 GJ vs 16 GJ for the actual data).

I'm wondering if it makes sense that the gas usage from the simulation using a heating coil is much less than the gas usage from the building which is essentially using a radiant floor?

Thank you.

I created a simulation of an existing building in OpenStudio 1.10.0. The heating in the simulation is provided by a hot water heating coil (which is supplied by a natural gas boiler). I have hourly gas data from the actual building which I am using to verify that my heating system is working correctly. The real building uses 'rail heating' - exposed pipes that sit about 6 inches off the floor. Hot water circulates through the pipes to provide radiant heating. I have tried to model this using the ZoneHVAC:LowTemperatureVariableFlow object in OpenStudio but the gas usage is way off, about 3 to 5 times greater than the usage from the actual building.

I thought I would try using the Coil:Heating:Water object in place of the ZoneHVAC:LowTemperatureVariableFlow object. I made an air loop with just a fan, the coil, an uncontrolled diffuser and the thermal zone that the coil is supplying. I used the SetpointManager:SingleZone:Reheat setpoint manager for the air loop. The gas usage profile shape was nearly an exact match to the profile from the actual building, but the values were several times less than the actual values (peak hourly usage from the simulation was 3.8 GJ vs 16 GJ for the actual data).

I'm wondering if it makes sense that the gas usage from the simulation using a heating coil is much less than the gas usage from the building which is essentially using a radiant floor?

Thank you.