Covering an air return

I was at the local hardware store and saw a box of cut-to-fit magnetic material used to cover unused HVAC vents. The box said that this stuff can be used to block air returns to keep cool air in a room and not pulled back through the return system. Is this practical and/or safe to do. I have covered vents before, but never thought of blocking returns. It sounds strange to me. I would think it would impact the efficiency of the system. Would it? I'm no HVAC expert and it just does not sound right to me.
Les
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You should NEVER close off a return, as shutting the air flow to the unit can cause damage, serious damage depending on how the system is designed. These things that are offered like you see are simply junk.
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That's pretty much what I figured. Thanks.
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"Les and Gina"
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If you have central air, NO, don't do that!
We plug ours (with carpet pieces), BUT ... we don't have central air, and the vents aren't part of the cooling system. We have strategically placed window/wall units, so the heating system isn't used, meaning the vents are also unused. To keep the cold air from going down into the basement thru the vents, we cover them. No sense cooling the basement when we don't live down there!
If you have UNUSED vents, vents that aren't part of the ac system, those you could cover. But never cover a vent that's either a supply or a return for the ac system! You'll defeat the whole setup. In particular, if you close off a return air supply, you could run into some problems, and the cooling just wouldn't happen. Cover supply vents and the cool ari's gone from that area.
Does that help?
Pop
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can
back
system.
If you have a closed system where is the blower going to get the air to recalculate? Try this--put a 20" or so window fan in the window, on high, blowing out. Then shut the door to the room. Watch the fan speed slow down and almost come to a stop. Not too good! MLD
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MLD wrote:

Um, I've never see a room that tight nor a window fan that fit so closely that that the fan would nearly stop. BS about ruining fans by closing vents is fairly common. I have completely closed off the exit of a squirrel cage fan and that does not stop the fan and even additionally closing off the entrance to the fan will not stop the fan. The motor will overheat because there is no cooling flow. And a box fan? I'm not sure it would even slow down.
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On Mon 01 Aug 2005 07:12:38p, MLD wrote in alt.home.repair:

If you have multiple air returns (some homes have one in every room where there is a delivery vent), then blocking one or a couple of returns will not hinder overall operation in the least. My dad was an HVAC engineer and routinely did this in his own home, depending on season. He wouldn't have done it if it had caused a problem.
In my case, I have only one rather large air return. I could not possibly do this without many causing problems.
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Wayne Boatwright **
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In alt.home.repair on 2 Aug 2005 06:29:34 +0200 Wayne Boatwright

For one thing, air will be sucked into the open returns in other rooms from the room with the closed return.
Another thing to consider, if one could really keep the cold air from going back to central unit, by not sucking it out of the room, that air would warm up from the people in it and the ceiling and the windows, and if no air was leaving, no new cold air could get in.

That's what I've got too.
Meirman -- If emailing, please let me know whether or not you are posting the same letter. Change domain to erols.com, if necessary.
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On Tue 02 Aug 2005 12:20:42a, meirman wrote in alt.home.repair:

That's true, but often leaving the room more slowly. Depends on volume of air being sucked back by that individual return being blocked.

Oh, the air definitely leaves since more air is being pumped into the room, but depending on vent location, more cool air may remain in the room for a longer period of time.

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Wayne Boatwright **
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Les and Gina wrote:

You are right, it doesn't make sense. If you block a return how does one expect the air to get to the delivery vent. You can adjust delivery and return vents for various purposes, but you have to be reasonable and retain a good flow. For example, I blocked off about 1/3 of one vent so that more hot air would be pulled from a vent closer to a wood stove. It worked well, but I didn't attempt to block more than that much flow and in fact added a return vent to a room that previously had no return vent.
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if you turn off the heating/cooling unit, it wont blow air from room to room either, and its free...
randy
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Its been quite a few years, and I never lived there, but my sister and BIL had a system that was designed to have some returns blocked. When they switched from heating to cooling, they moved the magnetic covers from one set of returns to another.
Les and Gina wrote:

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