Hot water coil in main vav air handler?

[swoosh172]

I'm reviewing a design done by an outside engineer for a new 2 story office building. The engineer has specified a vav w/ reheat system. The main cooling cfm is 30,000 and heating cfm is 15,000. The OA% is high enough that on the coldest day the mixed temp is 40 degrees. The engineer specified an additional heating coil in the ahu itself, which is understandable, but he specified the LAT from the hw coil to be 82 degrees, instead of 55 degrees. I can't think of a reason for this, but i'm sure there is a logical explanation. the reheat terminals are hw and sized for 55-90 degree F. Any ideas?

 

[remp]

It must be a miss print.

 

[DrRTU]

Morning warm-up.

 

[ChrisConley]

Safety factor? People oversize heating compontents on a regular basis.

Is there a chance the system would go to 100% outdoor air? If so, the 'oversized' coil could be intended to bring the air from outdoor temp to 55 deg F.

Or... could be a misprint.

 

[CptHk]

Is it the smallest single row coil available for the particular scheduled RTU? Maybe that was the lowest capacity of the coil with the minimum gpm before laminar flow begins.

Otherwise, the designer is guilty of oversizing, which I see too much of in my own office.

Morning warmup should not need the OA, so a coil capable of 40-82 will only need to heat 60 (worst case return air temp) to 70 or 80 or whatever the warm up sequence calls for.

 

[swoosh172]

Thanks everyone. For my own future knowledge, is it acceptable to design a LAT for a warm up sequence that is below 90-100F? I've always been told to design coil LAT greater than 90.

 

[ChrisConley]

LAT greater than 90? That sounds backwards. Stratification increases dramatically at supply air temperatures over 85F, this makes the morning warm-up less effective.