Impossible to autosize UnitHeater: Autosizing of heating coil UA failed for Coil:Heating:Water

edit

I've got a large model with offices, corridors, and stairwells. The stairwells are only semi-heated to 50°F with Unit Heaters equipped with Hot Water Coils, connected to a boiler loop that serves the rest of the building.

I cannot get the sizing to happen properly. I've isolated one stairwell zone to make debugging easier and I've tried playing on all the setpoints but to no avail.

Here is the error log:

   ** Warning ** Calculated design cooling load for zone=1-STAIR-A ZN is zero.
   **   ~~~   ** Check Sizing:Zone and ZoneControl:Thermostat inputs.
   ************* Beginning Plant Sizing Calculations
   ** Warning ** The design coil load is zero for Coil:Heating:Water 1-STAIR-A ZN UNIT HEATER HEATING COIL
   **   ~~~   ** The autosize value for maximum water flow rate is zero
   **   ~~~   ** To change this, input a value for UA, change the heating design day, or lower
   **   ~~~   **   the system heating design supply air temperature
   ** Severe  ** Autosizing of heating coil UA failed for Coil:Heating:Water "1-STAIR-A ZN UNIT HEATER HEATING COIL"
   **   ~~~   **   Bad starting values for UA
   **   ~~~   **   Lower UA estimate = 2.288029 W/m2-K (0.1% of Design Coil Load)
   **   ~~~   **   Upper UA estimate = 2288.029488 W/m2-K (100% of Design Coil Load)
   **   ~~~   **   Zone "1-STAIR-A ZN" coil sizing conditions (may be different than Sizing inputs):
   **   ~~~   **   Coil inlet air temperature     = 6.617 C
   **   ~~~   **   Coil inlet air humidity ratio  = 4.434E-003 kgWater/kgDryAir
   **   ~~~   **   Coil inlet air mass flow rate  = 6.611502E-002 kg/s
   **   ~~~   **   Design Coil Capacity           = 0.000 W
   **   ~~~   **   Design Coil Load               = 2288.029 W
   **   ~~~   **   Coil outlet air temperature    = 40.555 C
   **   ~~~   **   Coil outlet air humidity ratio = 8.000E-003 kgWater/kgDryAir
   **   ~~~   **   Inadequate water side capacity: in Plant Sizing for this hot water loop
   **   ~~~   **   increase design loop exit temperature and/or decrease design loop delta T
   **   ~~~   **   Plant Sizing object = HOT WATER LOOP
   **   ~~~   **   Plant design loop exit temperature = 82.000 C
   **   ~~~   **   Plant design loop delta T          = 11.111 C
   **  Fatal  ** Preceding water coil sizing errors cause program termination
   ...Summary of Errors that led to program termination:
   ..... Reference severe error count=1
   ..... Last severe error=Autosizing of heating coil UA failed for Coil:Heating:Water "1-STAIR-A ZN UNIT HEATER HEATING COIL"

Here is my isolated model (one stairwell zone and an autosized boiler):

• OpenStudio Format (.OSM)
• EnergyPlus Format (.IDF)

Can someone help please?

EDIT: even after I fix what Chandan said, I can get it to run in my small one-zone model but not in my full model. Note that my hot water plant loop is rated at 180°F with a 40°F deltaT, and that's dictated by the VAV boxes that will represent almost all of my heating anyways. So changing the $\Delta T$ isn't an option

ZoneHVAC:UnitHeater,
  1-Stair-A Zn Unit Heater,               !- Name
  Always On Discrete,                     !- Availability Schedule Name
  Node 1235,                              !- Air Inlet Node Name
  Node 1236,                              !- Air Outlet Node Name
  Fan:ConstantVolume,                     !- Supply Air Fan Object Type
  1-Stair-A Zn Unit Heater Fan,           !- Supply Air Fan Name
  Autosize,                               !- Maximum Supply Air Flow Rate {m3/s}
  Coil:Heating:Water,                     !- Heating Coil Object Type
  1-Stair-A Zn Unit Heater Heating Coil,  !- Heating Coil ...