Duct Insulation

[antonio09]

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.

 

[DLANDISSR]

antonio09:


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.

 

[MintJulep]

It will at some point have condensation problems.

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.

 

[quark]

Dlandissr is right to the point. First check the dew point of air with respect to return plenum conditions and check if the supply air temperature is more than this or not.

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,

 

[MintJulep]

Attempting to prove by calculation that the air conditions in the plenum will never result in condensation on the duct is, in my opinion, a waste of time and effort, as well as a practical impossibility. You need to make too many assumptions.

At some point, some unanticipated condition will occur, and you will get condensation. Count on it.

 

[lilliput1]

The supply duct should be insulated. You will have condensation at start up. There will be considerable heat loss from the uninsulated supply duct such that the condition spaces will not get the designed cooling effect. Usual supply air temperature off the cooling coil is 55°F. Fan heat adds about 1.5°F. Normal supply duct heat gain could be 0.5°F. Thus design supply air to the room would be 55 + 1.5 + 0.5 = 57°F. Room temperature rise at 75°D design is 75 - 57 = 18°F. If however the supply duct heat gain is say 1.5°F instead of 0.5°F, supply air to space is 58°F and using the same temperature rise as design, room temperature = 58+18 = 76°F.

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.

 

[antonio09]

quark:

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.

 

[MintJulep]

There is heat transfer anytime there is a temperature gradient.

 

[friartuck]

Not just condensation problems but 'cross talk' of noise, heat transfer and lower overall system efficiency etc.

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

 

[ChasBean1]

I like the science but for reference, two sources (applicable in US):

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

 

[quark]

antonio,

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,

 

[vannic]

ASHRAE Standard 90.1 (the energy standard) has a convenient table that shows when and under what conditions ductwork needs to be insulated. I do know that a cooling only duct, such as a VAV system, installed in a return air plenum does not require insulation. I have seen many installations where this was the case and, if the entire plenum is a return air plenum, there should be no condensation.

 

[KenRad]

As was stated earlier, you WILL have condensation whenever the return air dewpoint is significantly higher than the supply air temperature. The question that needs to be answered is whether or not this will ever occur in a give system, and a given climate.

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

 

[Sam9461]

Just a thought, Is it possible the supply air duct your
looking at is internally lined?