Should we use monthly average outside temperature instead of monthly average ground temperature for FC-Factor method

[Nigusse]

Issue overview

ASHRAE Standard 90.1 baseline buildings uses F-factor method to present slab-on-grade floor construction. This makes EnergyPlus's FC-Factor Method still relevant despite we have more robust ground heat transfer models. However, I have a concern that monthly average ground temperature is used when approximating F-factor when using Ueff method. The Engineering reference V9.3 and older versions for the derivation of the Ueff starts with the assumption of Equation 3.193 here.

The Ueff is defined as follows:

Q = \text{Area} \cdot U_{eff} \cdot (T_{air,out} - T_{air,in}) = (T_{air,out} - T_{air,in}) \cdot (P_{exp} \cdot \text{F-factor})

The above assumption to hold true the temperature difference used should remain the same. That is we have to use monthly average outside air temperature, instead of the ground surface monthly average temperature currently used in FC-Factor Method; otherwise, the assumption of Equation 3.193 is violated.

Here are a couple of relevant comments ad references from unmet hours 092_Baylon paper.

I tried to examined and compared the use of Site:GroundTemperature:FCfactorMethod and monthly average outside temperature starting from example file (1ZoneUncontrolled_FCfactor_Slab_UGWall.idf) and found out different results. Then, I tested the same example file using a detailed slab-on-grade heat transfer model in EnergyPlus (expected to be more accurate) to verify which boundary temperature is more representative of the heat transfer.

Details

Some additional details for this issue (if relevant):

• All Platforms
• EnergyPlus V9.3 and older
• Unmethours link here

Checklist

Add to this list or remove from it as applicable. This is a simple templated set of guidelines.

• [ ] Defect file added (list location of defect file here)
• [ ] Ticket added to Pivotal for defect (development team task)
• [ ] Pull request created (the pull request will have additional tasks related to reviewing changes that fix this defect)

[Nigusse]

Ground Floor Inside and outside face temperature comparison plot is shown below. The monthly average air temperature use with F-Factor method matches better with the detailed ground heat transfer model. Note that the F-factor were developed to represent the heating season, but not the cooling season. So, the large deviation during the cooling season is not surprising.

[Nigusse]

Zone mean Air temperature plot for this example file using three methods. This example file has uncontrolled zone. See also the monthly average ground temperature and monthly average outside air temperature plots used for F-factor method. The monthly average ground temperature are the monthly specified values in Site:GroundTemperature:FCfactorMethod for this example file while the monthly monthly average outside air temperature were taken from "USA_CO_Denver.Intl.AP.725650_TMY3.stat" file.

[Nigusse]

uploaded the test files