Supply air temp reset - ASHRAE 90.1 - Appr G

4 posts / 0 new
Last post

All,
RE: ASHRAE 90.1, App G, Article G.4.2.3.1.2, Supply Air Reset

Addendum A revised this article to say: "The air temperature for cooling shall be rest higher by 5F under the minimum cooling load conditions".
This sounds to me like they are saying the supply air temperature should be reset based on outdoor air temperature. This would be quite a change in concept from the original version of this article which required the supply air temperature to be reset based zone demand.

Are others out there interpretting the revised addendum A version of this article the same way I am....i.e. that the SAT should now be reset based on outdoor air temperature? Or, are people still interpretting it as the SAT should be reset based on zone demand?

Would appreciate any feedback anyone has on this, especially with respect to doing a LEED energy model.
Thanks.
Julia

Julia Beabout's picture
Offline
Joined: 2011-10-01
Reputation: 3

I would interpret that as by zone demand, since you will have both internal and external gains figured in to the "minimum cooling load conditions". There could be instances where the outdoor air temperature is relatively low, but your solar load in conjunction with the internal heat gains may be high, so a straight outdoor air reset wouldn't work.

My thoughts.

Stirling

All,
RE: ASHRAE 90.1, App G, Article G.4.2.3.1.2, Supply Air Reset

Addendum A revised this article to say: "The air temperature for cooling shall be rest higher by 5F under the minimum cooling load conditions".
This sounds to me like they are saying the supply air temperature should be reset based on outdoor air temperature. This would be quite a change in concept from the original version of this article which required the supply air temperature to be reset based zone demand.

Are others out there interpretting the revised addendum A version of this article the same way I am....i.e. that the SAT should now be reset based on outdoor air temperature? Or, are people still interpretting it as the SAT should be reset based on zone demand?

Would appreciate any feedback anyone has on this, especially with respect to doing a LEED energy model.
Thanks.
Julia

Stirling Walkes's picture
Offline
Joined: 2011-09-30
Reputation: 0

I'd interpret it as either. But now with "zone demand," (DOE2 "WARMEST") one can meet the intent of the PRM baseline and model a maximum RA humidity setpoint that resets the SAT back down when sensible gains are low, but OA is moist.

Fred Porter
AEC

Fred Porter's picture
Offline
Joined: 2011-09-30
Reputation: 0

Thanks for the input.
Now for a follow on question. I use Trace. Trace can model zone demand supply air temp reset very easily. However, does anyone know how to put the ASHRAE 90.1, App G, allowable limit of 5F reset into Trace.

What I'm finding is that the supply air temp reset drastically improves the performance of both my proposed and baseline models. The 5F ASHRAE reset criteria would seem to help me limit the improvement in my baseline model which would be good with respect my proposed bldg's performance.

Again, thanks for any feedback or insight out there.

----- Original Message ----

Julia Beabout's picture
Offline
Joined: 2011-10-01
Reputation: 3