What would be some potential reasons that the heating coil in an air loop would be incorrectly autosized? It's a simple air loop with an outdoor air system, two-stage dx cooling coil, one-stage gas furnace, and a variable volume supply fan. It serves ~25 zones with SingleDuct:VAV:Reheat terminal units.
The dx cooling coil gets correctly sized. The following graph taken from one of the cooling design days shows that the supply temperature follows the supply temperature setpoint and fluctuating cooling energy.
Now we look at the heating design day and see something completely different. Everything is constant and the supply air temperature is nowhere near the setpoint.
I'm stumped. I can increase the Fraction of Autosized Heating Design Capacity and get it to behave, but I don't see why it's sizing the coil so much lower than what it needs for heating.
If I'm not mistaken it should be grabbing the heating flow rate from the zone terminals, then it's just a matter of btu = Q * deltaT to get the btu/h required for the coil. deltaT would be based off the OAT for the design day and the central heating temperature found in the zone sizing object. Based on the *.eio the calculated heating design flow rate is 9.55613 m3/s or 20239.88 cfm, so a quick calculation yields 20239.88 * (57 - 5.9) * 1.08 = 1,116,999 btu. This is compared to the 562,513.3 btu that is being calculated for the capacity by E+.
If I hardsize it using my calculated value, it again responds appropriately adhering to the setpoint. Maybe this is a convergence issue since it's such a high flow rate loop or is there something else I'm missing?
Sizing Object...
Sizing:System,
Lab RTU, !- AirLoop Name
Sensible, !- Type of Load to Size On
Autosize, !- Design Outdoor Air Flow Rate {m3/s}
0.3, !- Central Heating Maximum System Air Flow Ratio
-17.7777777777777, !- Preheat Design Temperature {C}
0.008, !- Preheat Design Humidity Ratio {kgWater/kgDryAir}
-17.7777777777777, !- Precool Design Temperature {C}
0.008, !- Precool Design Humidity Ratio {kgWater/kgDryAir}
11.1111111111111, !- Central Cooling Design Supply Air Temperature {C}
13.8888888888889, !- Central Heating Design Supply Air Temperature {C}
NonCoincident, !- Type of Zone Sum to Use
Yes, !- 100% Outdoor Air in Cooling
Yes, !- 100% Outdoor Air in Heating
0.0085, !- Central Cooling Design Supply Air Humidity Ratio {kgWater/kgDryAir}
0.008, !- Central Heating Design Supply Air Humidity Ratio {kgWater/kgDryAir}
DesignDay, !- Cooling Supply Air Flow Rate Method
10.88, !- Cooling Supply Air Flow Rate {m3/s}
0.01016, !- Cooling Supply Air Flow Rate Per Floor Area {m3/s-m2}
1, !- Cooling Fraction of Autosized Cooling Supply Air Flow Rate
3.94754559999994e-005, !- Cooling Supply Air Flow Rate Per Unit Cooling Capacity {m3/s-W}
DesignDay, !- Heating Supply Air Flow Rate Method
10.88, !- Heating Supply Air Flow Rate {m3/s}
0.01016, !- Heating Supply Air Flow Rate Per Floor Area {m3/s-m2}
1, !- Heating Fraction of Autosized Heating Supply Air Flow Rate
1, !- Heating Fraction of Autosized Cooling Supply Air Flow Rate
3.1588213e-005, !- Heating Supply Air Flow Rate Per Unit Heating Capacity {m3/s-W}
ZoneSum, !- System Outdoor Air Method
1, !- Zone Maximum Outdoor Air Fraction {dimensionless}
CoolingDesignCapacity, !- Cooling Design Capacity Method
Autosize, !- Cooling Design Capacity {W}
234.7, !- Cooling Design Capacity Per Floor Area {W/m2}
1, !- Fraction of Autosized Cooling Design Capacity
FractionOfAutosizedHeatingCapacity, !- Heating Design Capacity Method
Autosize, !- Heating Design Capacity {W}
157.000000000001, !- Heating Design Capacity Per Floor Area {W/m2}
1, !- Fraction of Autosized Heating Design Capacity
OnOff; !- Central Cooling Capacity Control Method
