Thread: return air

On an inspection I did today, there were many return vents. 2 story. The 1st floor return vents had dampers on them so you cold shut off and force all return air to come from the 2nd floor. I need to know what the benifit of this is so I can explain it to my clients in the future. Can you explain?

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I don't think there are any benefits to this setup John. First, return air vents help ensure good air flow to the supply vents in the rooms. Closing the returns may reduce the air flow. Second, if the house has AC, you'd want to be able to draw return air from all areas in order to properly dehumidify the house. Your clients would be better served by leaving those return dampers open and adjusting the dampers on the supply vents if they want to balance the air flow.

I wonder if somebody installed the adjustable dampers on the returns thinking they were supply vents?

My guess is that they used 'supply registers' for 'return grilles'.

'Supply registers' typically have some sort of control to allow for balancing the system, while 'return grilles' are simply slotted, louvered, covers over openings which allow the air in as un-restricted as possible with the louvers placed at a position to cut of the line of sight into the duct as best possible.

Another reason why it's not a good idea to choke down the air flow on the returns is that it can lead to higher than normal temperatures at the heat exchanger, which can shorten the exchanger's life. Dirty filters have the same effect on the exchanger.

In theory, supply and return air to each area should be balanced in one way or another to avoid pressurizing/depressurizing zones with all the possible attendant negative effects (which vary by zone within the building and by climate). Diagnosing this situation is probably beyond the scope of a home inspection, but recognizing a possible problem is not.

Sometimes one sees two sets of cold air returns, one high for the A/C season and one low for heating.

Since there is a lot of room for doubt about this configuration, I recommend referral to a qualified technician. One good method of choosing a qualified technician is to see his credentials from National Comfort Institute (NCI), North American Technical Excellence (NATE), or the Building Performance Institute (BPI).

Special care is advised here, since one potentially harmful consequence of duct system imbalance is to drive warm moist air into contact with a cool condensing surface. This often occurs inside wall assemblies where we may not know about it until significant harm has been done.

Too many possible variables to know for sure. Like Jerry said it could be that they put registers on instead of grilles. Happens all the time. On the other hand if the system was designed for it, they may have done it to suck more hot air out of the 2nd floor in the summer to help with the AC.

Originally Posted by Markus Keller

Too many possible variables to know for sure. Like Jerry said it could be that they put registers on instead of grilles. Happens all the time. On the other hand if the system was designed for it, they may have done it to suck more hot air out of the 2nd floor in the summer to help with the AC.

Actually, this is what I was thinking.

I would agree that it sounds like the wrong grills were installed but I could see where a homeowner might install these with the intent to regulate more air flow in certain rooms at certain times of the year. Just a thought.

Originally Posted by Robert Runchey

I would agree that it sounds like the wrong grills were installed but I could see where a homeowner might install these with the intent to regulate more air flow in certain rooms at certain times of the year. Just a thought.

True, but as Brandon pointed out, that could easily cause problems with the return air being restricted.

No arguments here. I would certainly call it out for an HVAC SME evaluation.

If you keep adding returns to a 2nd floor without an established air/pressure boundary, where do you think all of that negative pressure will draw air from? Maybe the ATTIC? Of course, this is into realm of an energy audit but it is nice to be able to tell the clients that they should have the system balanced, preferably by an NCI or NATE trained technician.