Problem Zone HVAC Heating Rate when Tubing Length is doubled

edit

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).