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The radiant fraction of an electric baseboard heater does not get taken into account in the surface inside face heat balance calculation when using the conduction finite difference (CondFD) as the heat balance algorithm.

I was tying to model a simple cubic box (single zone) with an electric, radiant/convective baseboard heater (ZoneHVAC:Baseboard:RadiantConvective:Electric) using EnergyPlus version 23.1. Furthermore, I used the CondFD heat balance algorithm, which allows me to get a temperature profile over my wall constructions, instead of the default CTF algorithm.

When checking the energy balance at the inside surface of the walls, the heating system contribution to the long wave radiation (Surface Inside Face System Radiation Heat Gain Rate) was always zero. At the same time the radiative fraction of the baseboard heater (Baseboard Radiant Heating Rate), which should be taken up by the surfaces was not zero.

I tried multiple things to search for the error:

1. Use HVACTemplates to let EnergyPlus create all required objects for the IDF and to be sure no required input was missing, by using the HVACTemplate:Thermostat and HVACTemplate:Zone:BaseboardHeat. Besides creating only a convective baseboard heating system, all objects matched with the ones in my IDF.
2. Use other radiative input objects (People, ElectricEquipment) to see if their radiation gets included in the surface inside face heat balance calculation. This turned out to be true.
3. Use the an EnergyPlus example file with an electric, radiant/convective baseboard heater (5ZoneElectricBaseboard.idf - the only one) to see if the radiation of the baseboard gets included in the heat balance calculation in this file. This was true, which means in general it should work.
4. Compare the example IDF to my IDF file. By doing so, I noticed a few differences, of which the heat balance algorithm showed to be the problem. Changing the default CTF algorithm in the example IDF to CondFD I obtained the same surprising result: no/zero radiation from HVAC systems to the surface.
5. Check source code of EnergyPlus. Since I am not familiar to C++ and not very familiar with software packages I had struggles to understand the code and where to look exactly.

Since I am interested in the results of the CondFD algorithm, I do not want to change to CTF. In general EnergyPlus should also work with this combination.

Does anyone have experienced that issue before or could provide some help? Would highly appreciate it!! :D

Example IDF: //EnergyPlus-23-1-0/ExampleFiles/5ZoneElectricBaseboard.idf

Corresponding weather file: //EnergyPlus-23-1-0/WeatherData/USA_IL_Chicago-OHare.Intl.AP.725300_TMY3.epw

Altered example IDF file: Altered example IDF file

• SPACE2-1 Baseboard: Efficiency = 1, Radiant Fraction = 1, Fraction radiant energy on people = 0, all radiation on one surface: RIGHT-1.
• Heat Balance Algorithm/Settings: ConductionFiniteDifference. -> requires minimum timestep of 20.
• Sequence of heating system in Equipment list for Space2-1 Eq. Baseboard: 1, AirDistributionUnit: 2
• Additional outputs for energy balance.