Simulation of "Packaged Single-Zone System with Central ASHP" Fails when Heating Design Load is Zero

[chriswmackey]

This is a relatively minor issue but it still feels like a bug since practically all of the HVAC systems supported in the Create Typical Building measure are able to handle the case of zero heating design load except for "PSZ-AC with central air source heat pump".

When this template is used and the heating design load of the zone is zero on the winter design day, EnergyPlus will throw the following error and exit:

   ** Severe  ** Autosizing of heating coil UA failed for Coil:Heating:Water "ROOM_2DB19777 PSZ-AC WATER HTG COIL"
   **   ~~~   **   Bad starting values for UA
   **   ~~~   **   Lower UA estimate = 2.238170 W/m2-K (1% of Design Coil Load)
   **   ~~~   **   Upper UA estimate = 2238.170804 W/m2-K (100% of Design Coil Load)
   **   ~~~   **   AirloopHVAC "ROOM_2DB19777 PSZ-AC" coil sizing conditions (may be different than Sizing inputs):
   **   ~~~   **   Coil inlet air temperature     = -8.979 C
   **   ~~~   **   Coil inlet air humidity ratio  = 1.503E-003 kgWater/kgDryAir
   **   ~~~   **   Coil inlet air mass flow rate  = 3.776498E-002 kg/s
   **   ~~~   **   Design Coil Capacity           = 0.000 W
   **   ~~~   **   Design Coil Load               = 2238.170 W
   **   ~~~   **   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 = 48.888 C
   **   ~~~   **   Plant design loop delta T          = 11.111 C
   ** Severe  ** Developer Error: sizing of U-Factor Times Area Value [W/K] failed.
   **   ~~~   ** Occurs in Coil:Heating:Water ROOM_2DB19777 PSZ-AC WATER HTG COIL
   **  Fatal  ** Preceding water coil sizing errors cause program termination
   ...Summary of Errors that led to program termination:
   ..... Reference severe error count=2
   ..... Last severe error=Developer Error: sizing of U-Factor Times Area Value [W/K] failed.

Here is a simple single-zone OSM file with this HVAC template that can recreate the error:
https://drive.google.com/file/d/1JdDjeIXbLofVypGqFeWRhGjyMPVqpi4l/view?usp=sharing

Given that the zone in the sample above is composed of adiabatic surfaces, it is possible to recreate the error using EPW files from cold climates like New York, though the error is easiest to recreate in warmer climates.

I realize that there are several workarounds for this issue but it's enough of a problem that some users have brought it up and they were not able to figure out what went wrong because of EnergyPlus's cryptic error message. So having some means for the ASHP custom object to handle this case without simulation failure would be very helpful.