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This thread, http://www.diychatroom.com/f17/whirring-noise-return-duct-42268/, addressed fixing a whirring noise, which would indicate low return airflow, by a contractor simply installing a vent on the return plenum in a closet. I realize that the OP did not provide all relavant information but this does not sound like a proper way of solving this problem. Do the HVAC pros ever just arbitrarily add a vent to the return plenum to alleviate lack of return airflow?
 

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No, not arbitrarily, but a long time ago as a Heil dealer the factory advocated cutting a new return up-stream of the CA drop. The idea was to get the new grill away from any other gas appliances to prevent a negative pressure zone that could cause flue back flow.

This would afford some increase in in return air flow and some times cured the problem completely. Nowadays, the purists would a call that a hack's approach. It had one drawback that could affect home comfort and that was the temp of the basement. If a home's cellar is colder than norm, the new RA grill could add to the home heating load by pulling in cold basement air.

I'll still use this method today if there is no other alternative.
 

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Its some what a last ditch effort. When running a larger or second return becomes a problem.
 

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Interesting. I would have never believed this to be a proper way to handle lack of return air. How does placing a vent on the return plenum affect existing return vents airflow?
 

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Depends how bad the return is.

Usually makes them quieter.
Because it lowers the velocity through them.

Its not a cure all. Butm when a customer can't afford a new return system. you have to find a way to get them more return. And a way to do it that is safe.
 

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Interesting. I would have never believed this to be a proper way to handle lack of return air. How does placing a vent on the return plenum affect existing return vents airflow?
I would avoid cutting a new grill into the RA drop (plenum if you prefer).

For the reasons I mentioned in my other post, it could place the furnace area
under a negative pressure. Especially if the furnace is enclosed by a room that cuts it off from the rest of the basement.

Not only do you have issues with possible flue back flow from the water heater and furnace, any floor drains could allow sewer smells to enter the return at a grill cut in the RA drop.
 

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Discussion Starter #7
I would avoid cutting a new grill into the RA drop (plenum if you prefer).

For the reasons I mentioned in my other post, it could place the furnace area
under a negative pressure. Especially if the furnace is enclosed by a room that cuts it off from the rest of the basement.

Not only do you have issues with possible flue back flow from the water heater and furnace, any floor drains could allow sewer smells to enter the return at a grill cut in the RA drop.
Got it. Is it safe to assume this does not apply to a heat pump system where there is no floor drain in the immediate area?
 
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