ductwork condensation

[alupien]

Hi Guys,

We want to use an existing uninsulated fresh air ductwork hidden in ceiling to cool a post office by adding a DX coil at the supply. Should we be concerned about the condensation on the ductwork exterior surface hidden in ceiling which is not a return air plenum (no air circulation).
The new DX would be 4000 cfm / 2 stages.
Most of the cooling load is sensible (light and sun) but we are located in Montreal where summer is humid.

Thanks
Alex

[genduct]

Is your fresh air duct really big enough for 4000 CFMs ?
Certainly would be prudent to insulate the new supply duct in fact I am surprised that you would have an uninsulated outside air duct in the first place.

[alupien]

Thanks for the reply

The ductwork is big enough for 6000 cfm. Actually, it is not only used for fresh air but also to clean the air with the filters in the AHU. The fresh air is preheated to 65F.
I did'nt open the sheetrock ceiling but I can see that the ductwork connected at the AHU is not insulated. I don't have any as built drawing.

I wonder if we should take a chance and add a DX coil in the AHU since most of the load is sensible (basically humidity comes only from outside air in this place). Could the condensation become a major problem?

[swoosh172]

Have you checked to see if it is internally lined, in which case you wouldn't "see" insulation on the outside of the duct. I would not install a DX coil in this duct without insulating it. The dewpoint of the air will be higher than the duct temperature so it will condense and drip down onto your sheetrock ceiling and start to creat a mold problem.

[Southern Mech]

if it's in the attic, or above the envelope, it's gonna sweat

[alupien]

I agree with the psychometric chart and the theoritical dew point temperature, but in fact, it is unlikely to see uninslated ductwork sweating in open ceiling altough supply air is at 55F and room temperature (or return air) at 75F, 55% r.h.

I am thinking now to create an air return in the closed ceiling by adding transfer grills and return ducts. This would bring the air around the ductwork at the same temperature than the return air (75F, 55%)

[jnsrose]

If it's not lined insulate it. If you don't you will regret it later.

[genduct]

With the ceiling plenum also acting as return then you will probably only be really vulnerable on Monday morning start up.

Good Luck