[Bldg-sim] Cooling Design Calc Methods

[Hall, Brendan]

Hey all,

I am looking at an energy plus based sim program and one of the major selling points would be doing the load calcs in the same program as the energy modeling. From what I have read, the heat balance method that E+ uses should be more accurate than the transfer function method that a widely used load calc program like HAP would use. However the results are different so I wanted to get people’s thoughts on the subject.

I did a test case, a 20 x 20 office space (384 ft conditioned area) with one southern facing window.

-          10 ft tall

-          4 People

-          1 W/ft2 Lighting

-          0.5 W/ft2 Equipment

-          ASHRAE Office default schedules

-          Basic Walls and Roof, ASHRAE minimum windows (U=.55,SHGC=.4)

-          Design Weather – 95 db / 75 wb

I’m attaching some of the results, but overall the E+ calc has a lower peak load. Occupant, lighting and plug loads seem to follow their schedules. HAP looks like it very heavily weights the delayed load effect (TFM) even though I used a medium weight (70lb/ft2) wall (fyi - changing it to a lightweight wall has some but not a huge effect). HAP also ignores the slab heat loss in its cooling calculations. This shows up most clearly in the unoccupied zone temperature. In HAP the residual loads drive the temperature up to 82 where in the E+ calc the slab loss dominates, driving the temperature into the 60s. I could see leaving it out of the peak to be conservative but then allowing the unocc temperature to jump up overnight and having to deal with that load seems a bit too unrealistic. HAP also seems to over predict the peak roof conduction gain, probably due to the use of the sol-air temperatures, which I have read can over predict gains.

I tend to want to believe the E+ analysis because I know that it’s calculation methods are in general more rigorous but I am very interested what others may think, sim engines are known for being poor design load predictors (I’m looking at you eQuest) and HAP is a very established and trusted program.

Thanks in advance for anyone that feels like diving into this with me.

Brendan Hall
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