ASHRAE PRM - Low HVAC Loads

[Eng-wintersmith]

I’m working on an early phase LEED/ASHRAE PRM model. I work in a mechanical engineering firm, and have 10 yrs experience as an energy modeler, and 5+ years experience as an HVAC designer.

The building loads (both baseline and proposed) calculated by the PRM simulation are too low to be believed.

My, inputs, and the PRM report indicate that the internal loads inputs seem reasonable:

Lighting = 0.94 w/sf
Receptacle = 0.45 w/sf
People = 275 sf/ppl

The load reports indicate the autosized design flow is 0.3 cfm/sf - 0.4 cfm/sf and the cooling load is between 13 and 15 btuh/sf (800 - 920 sf/ton).

These HVAC loads are simply too low. No practicing mechanical engineer is ever going to size his units this low. I do not believe it’s appropriate to size the ASHRAE Baseline Units this low. My (and industry) expectation is about 0.75 – 1.0 cfm/sf and ~400 sf/ton.

In fact, since the min turndown flow for the Baseline VAV system is 0.40 cfm/sf, it is effectively a constant volume unit.

Why are the calculated HVAC loads so low when my inputs seem appropriate?

[clorenz531]

I agree that those are suspiciously low.

It's hard to guess without knowing about your building, and I apologize if these are obvious but these are just the first thoughts that come to mind:

*Did you input your envelope correctly? The numbers you gave look like there is no load from your envelope. I don't know what your climate is or anything, but that's probably not right. You can check that if you ran your loads with the conduction gains breakdown option selected and get a better look at what's screwed up.

*Related to the above. Did you run suncast? If so, did you make sure to check the 'suncast link' box in the

*What variation profiles are assigned to your internal gains? I like to set them to the 'system plant profile' when doing the sizing runs so that they're not being calced at fractional values