Duct Insulation
Duct Insulation
(OP)
We have a building with supply duct without insulation, the duct is galvanized steel. Also the duct are in the return plenum, Therefore the air is return to the mechanical room by the space between the celing and the slab called return plenum, they didnt use return duct. We understood that the supply duct exposed to the conditioning space dont require insultation, but in this case the duct is in the interstisial space between the ceiling and slab. Question: Will be problem with duct condensation?. Somebody recommend use insulation or is not necessary.





RE: Duct Insulation
The fact is, if the temperature of the surface of the duct is at or below the dewpoint temperature of the air in the space where the duct is, moisture will condense on the surface of the duct.
If you can determine that the duct surface temperature will never be at or below the dewpoint of the air in the space where the duct is, then you will not need insulation.
This would be true if sensible heat only is added in the conditioned space. This would mean that the dewpoint of the supply air would be the same as the dewpoint of the return air. Since normally both sensible and latent loads are added in a conditioned space, the temperature of the supply air and consequently the duct surface temperature will normally be below dewpoint of the return air.
In conclusion: Duct insulation will most always be needed.
RE: Duct Insulation
It will be noisier than an insulated duct.
It will cost more to operate because you are dumping energy back into the return wihtout it having done any useful work.
Insulate.
RE: Duct Insulation
Secondly, as there will be heat exchange between the supply air and return air in the plenum and you will have problems with the precise control of space conditioning.
Better insulate it.
Regards,
RE: Duct Insulation
At some point, some unanticipated condition will occur, and you will get condensation. Count on it.
RE: Duct Insulation
At 55°F db and say 54.8°F wb, dewpoint is about 54.7°F. Thus anytime humidity is above this like above 75°F and 45%RH (same dewpoint per psychrometric chart)then there will be condensation. Definitely at each start up during summer there will be condensation.
RE: Duct Insulation
What happen when we installed the supply duct exposed to space conditioning, per example: department store, etc.
Where will be heat exchange between the supply air and return air in this case.
RE: Duct Insulation
RE: Duct Insulation
We had a similar situation only the other week where the client complained that one of his systems had never operated very well. The reason, ...well the return air duct passed through a plenum (which in this case was an underground concrete supply plenum duct)The heat picked up by the exhaust air affected the return air stat and gave very poor control results.
Friar Tuck of Sherwood
RE: Duct Insulation
1. International Mechanical Code (2000): (paraphrased) Ducts with content that can be over 140°F need to be reduced to 120°F or below.
2. Energy Code (in MA State Building Code): "All supply and return air ducts and plenums shall be insulated with a minimum of R-5 insulation when located in unconditioned spaces..."
There are four exceptions, two of which are:
- When design temperature difference between the interior and exterior of the duct or plenum does not exceed 15°F
- Exhaust air ducts
RE: Duct Insulation
When your uninsulated supply duct in the return plenum exchanges heat with the return air, there will be an increase in the supply air temperature and decrease in the return air temperature. There will be more heat transfer as the dT is relatively larger. The control becomes difficult as lowering the room temperature further lowers return air temperature.
When the return ducts are placed in a conditioned area, the dT will be lower dT as the air starts picking up heat as soon as it gets diffused. Further as the return ducts are placed near to ceiling, the temperature difference my not be significant.
Regards,
RE: Duct Insulation
RE: Duct Insulation
In the southern US, it is not uncommon to have these conditions during the summer months. System startup is another time when you can see this; before any dehumidification has taken place, the return dewpoint can easily be 5 or 10 degrees F higher than the supply air temperature.
---KenRad
RE: Duct Insulation
looking at is internally lined?