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Calculating Heating Loads: EnergyPlus vs. Manual J

aj1950 | Posted in Pretty Good House on


Background: I am trying to model the heating and cooling loads for my future home using EnergyPlus. I want to investigate different building construction methods and window options that would allow us to build a pretty-good house (

But I am an EnergyPlus beginner, so I wanted to verify my results using a CoolCalc Manual J prediction of heating and cooling loads. I tried to match the size and construction in the two approaches as closely as possible.

Parameters: The climate zone is 5A, and the home is 2000 square feet over two floors.  The first floor is a walk-out basement, and about 50% of the walls are below ground.  The estimated R values are: walls R30, roof R66, basement walls R20, basement slab R10.  Assuming 2-pane windows with U 0.34.

Problem: My EnergyPlus model matches the cooling loads of CoolCalc pretty closely (11124 BTU/h from eplus versus 13189 BTU/h from CoolCalc), but the heating loads do not match at all: EnergyPlus predicts 12905 BTU/h versus CoolCalc’s 23890 BTU/h.

Question: Is a manual J prediction expected to over predict heating loads by this much, or should I suspect an issue with my use of EnergyPlus?  More to the point, does 12905 BTU/h seem right for a home of this size and insulation in 5A?

Thank you in advance!

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