Uninsulated exposed duct - where is it OK

[carlosgw]

Speaking of regular HVAC systems. Rule of thumb is that insulated ducts are OK if exposed in the conditions space.
What is your experience?
The concern is condensation.

I am in northern USA.
I would never use them in restaurants and bars.
I had an issue one in a retail space. It was a building out of an old barn and one of the theories was that the wood had not dried out yet when the HVAC systems were started up.

 

[Ziggypump]

I'm not too sure on what the question is but here are my thoughts on exposed ductwork. Exposed is fine where required by architect, owner, or building conditions. I prefer spiral duct, flat oval if absolutely necessary, but I try to avoid this at all costs due to cost. And size for a maximum of 800 fpm. This will result in larger duct, but will reduce any possibility of wind sounds or whistling. Return duct in an exposed area is a little different, as usually you can get away with just an opening (bell mouth, opening on top of rectangular duct).

I personally hate the look of rectangular duct in an exposed space. Restaurant and bars I don't see what the big deal would be. Worst case you internally line the duct.

As far as your barn, I have no idea on what you are talking about regarding the wood drying out. Isn't your ductwork sheet metal? Is there a wood duct out there I'm unaware of? I have heard of duct board.

 

[Drazen]

you should differ "exposed" from "insulated" - there are fine prefabricated insulated ducts made of sandwich panels liked by some architects as they are smooth.

usual insulation, no matter how good tap-wrapping job done is not nice to be seen outside technical spaces. however, currently some manufacturers offer paints. ordinary insulated ducts painted with the same color as background (ceiling) look very modern and attractive.

one thing is certain - as a hvac engineer you cannot allow uninsulated ducts wherever there is possibility of condensation, no matter what artists ask.

 

[carlosgw]

Just looking for examples of where un-insulated duct has been installed exposed in a conditioned space (not concealed above a ceiling) and there was not a condensation problem. Or alternatively examples where it was installed and there were condensation problems.


The situation in the barn is that the thought was that there was more moisture in the air from the old barn wood.

 

[RossABQ]

Pretty easy to figure when condensation will occur, any time the supply air temp is less than dewpoint. In the midwest or southeast, that's likely all summer. Return ductwork is a different story. I would think that the energy codes aren't going to allow it anyway.

 

[MTES93]

All based on room RH and supply air temperature. In the dry mountain west we do it all of the time, our space RH rarely gets above 30 to 35% unless actively humidified or has a ton of people.

 

[ChasBean1]

It’s usually fine (and normal) to have uninsulated supply ducts in conditioned spaces. All supply ducts should be insulated in unconditioned spaces.

The reason for not insulating supply ducts in conditioned spaces is that to dehumidify, the AHU has a discharge temperature of about 55°F during summer, so supply air dew point (depending on coil bypass and ADP) is about 50-53°F. The 55°F supply air might warm on the path to the space, making the duct surface temperature ~58-60°F. So the space dew point would have to be over 58-60°F before condensation occurs on the duct.