Problem Zone HVAC Heating Rate when Tubing Length is doubled

Dear all

I'm modelling TABS in EnergyPlus using 'ZoneHVAC:LowTemperatureRadiant:ConstantFlow'. The problem is when I double the the hydronic tubing length, the output 'Zone HVAC Heating Rate' of a certain zone stays the same. For example, I have a zone of 26,1m² with a hydronic tubing length of 313m. The Zone HVAC Heating Rate peak is 1585 W. When I double the hydronic tubing length to 626m, the Zone HVAC Heating Rate graph stays exactly the same while I expect this to rise.

The number of circuits is 'CalculateFromCircuitLength' and the circuit length is 100m. The rated flow rate is 0,0001148 m³/s and the temperature of the water in the tubes is 27,5°C.

Does anyone know what is going wrong here? Thanks in advance!

I couldn't find a "Zone HVAC Heating Rate" output variable in EnergyPlus documentation, but I did find a "Zone RADIANT HVAC Heating Rate" output variable reported by the same ZoneHVAC:LowTemperatureRadiant:ConstantFlow object that you are using. This output variable is the heating rate from the hot water loop into the surface, depending upon the zone conditions (how much heating the zone requires) and your controls (when flow is allowed to happen) -- changing your hydronic tubing length won't affect those parameters. What the hydronic tubing length WILL affect is the temperature and flow rate of the water entering the TABS, since EnergyPlus will try to apply the same heating rate that the zone requires with longer tubing (more heat transfer surface area). This will alter the overall energy use of pumping and water heating. Output variables that would link to these two factors are "Pump Electric Power" and "Water Heater Heating Rate" or "Boiler Heating Rate" (depending upon if the hot water the TABS receives comes from a water heater or a boiler).