Gymnasium Duct Insulation Recommendation?????

[LeeEng]

I am requesting some assistance/direction in the design of an air-conditioned gymnasium. The system (DX rooftop) will provide cooling during occupied hours for the summer months. My concern is that there may be a condensation problem on the uninsulated ductwork currently designed. The lead engineer has noted that insulating duct in this space in "not typical". However, it is probable that the space dew point will be higher than the duct temperature and cause condensation. Can someone indicate if the insulation is typical and possibly a resource to reference? Thank you. Brian

The facility is in the Metro NYC area.

 

[trashcanman]

At start-up, there will probably be condensation dripping from the ductwork. If the floor is hardwood, the water can damage it. Also, if someone slips & gets hurt on a wet spot, they might sue the school & engineer. I would insulate the duct, min. 1" thick armaflex sheet. Don't use fiberglass, it attracts and holds moisture & the vapor barrier is likely to be punctured.

My $.02 worth.

 

[mizzoueng]

Is the roof insulated or is it a corrugated roof with a air plenum between the two layers? The reason I ask is that we had a similar situation at an arena we built.

 

[LeeEng]

The roof is a corrugated metal deck that is supported on an open joist system. As far as I can tell, there is no plenum. The return air is pulled from the floor level and is sent up two shafts to the roof top units.

In the arena you identified, did you have condensation problems?

 

[KiwiMace]

I'd lean towards speccing the insulation, but you need to work out your design worst case dewpoint temperature and compare to your supply air temp. If the gym is purged with outdoor air, or sits unconditioned for lengthy periods, then condensation is highly likely.

 

[LeeEng]

Thanks. That is my concern. The unit is currently designed to be shutdown during the "unoccupied hours". In the humid summer months, I think this could cause a concern with moisture during system start up.

The RTU has a preheat and DX coil. Has anyone every seen a the coils reversed? If these coils are reversed, this could provide the reheat to the space (as required).

 

[mizzoueng]

We had problems with the duct work running in the plenum condensating and dripping down through the corragated deck. We eventually forced the insulator through an opening to insulate it and stop the condensation. We also had problems with the CHWS & R lines going through said decking. They were insulated, but they used fiberglass insulation and this caused the sweat to run down the pipe and pool in the insulation until it burst onto the newly laid (and not sealed) hardwood BBall court below.

I would reccomend any piping going through the roof space to the RTU be covered in Armaflex and penetrations foam sealed BEFORE insulating. As for the duct work, the main trunks connecting directly to the RTU should be insulated, the branches are okay. And you only need to insulate about the first 12' or so of the duct.

As for your last post, yes I have seen this reversed. Sometimes it has worked and sometimes it caused the cooling coil to be frozen and break. If you need re-heat, it might be more economical to add reheat coils throughout the building.

 

[walkes]

Do the preheat coil first then the cooling coil, with a dx system you can add a hot-gas reheat section.

 

[LeeEng]

Thanks for all the assistance.

I do have a question regarding the recommended insulation length - 12'. Will the remainder of the duct surface main (about 50') be greater than the space dew point temperature. I figured the insulation would maintain the metal duct temperature further downsteam.

I am also going to look into the hot-gas reheat. This seems like a good idea. My initial thought was that the climate in NYC would not cause freezing concerns on the DX coil. However, I will take a look at other options.

Thanks again.

 

[cme]

line with 1" thick duct liner

 

[mizzoueng]

The remainder of the duct should be fine. Just as you thought, the insulation will maintain the duct temperature down-stream. As for freezing, I didn't know you were in NYC. What is the average winter temp there, and what is the coldest temp you typically see during winter times? Remember to add in wind chill.

 

[walkes]

mizzoueng, when you say wind chill, are you meaning infiltration due to wind pressure?

 

[MAragorn]

I disagree with the suggestion to use duct liner. I almost never use ductliner.eventually it will break down and cause other problems. It can support mold growth, lots of negatives.

I would also insulate the entire length. When this is started up after sitting for a long time, hot and humid, I think there is a real potential to have condensation anywhere the duct is cool.

 

[mizzoueng]

Nevermind, not sure what I was trying to say there.

 

[cme]

johns manville has some good duct liner

i wouldn't be concerned about it

 

[mizzoueng]

I've seen duct lining work extremly well, and I've also seen it fail miserably (in this case it tore and went into the fans).

I think it all depends on the attachment method. Simple fastners seem not to work for prolonged running times. The resin method seems to hold the best.