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Old 09-03-2012, 07:14 PM   #1
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Vent on furnace duct


Why is there a nonclosable vent on the main trunk exiting the furnace? The vent is about 12 inches above the furnace itself. This seems like a complete waste of hot (winter) or cold (summer) air that could be coming to the main living space. The furnace is in the basement where there are other normal vents that open or close.

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Old 09-03-2012, 08:14 PM   #2
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Vent on furnace duct


mine had one that you could close. i took it off and screwed sheet metal over the hole.

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Old 09-03-2012, 08:21 PM   #3
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Vent on furnace duct


On the plenum? Is it possible a humidifier was in that location at one time? Any screw holes around the opening?
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Old 09-03-2012, 10:12 PM   #4
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Vent on furnace duct


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On the plenum? Is it possible a humidifier was in that location at one time? Any screw holes around the opening?

MB - Thanks for your response. It could be the plenum. I am not very well versed in HVAC vocabulary and can't find anything to help me speak more acurately.

The vent is not on the first retangular "box" above the furnace, but the second one. I was mistaken aboout the distance from the furnace. It is maybe 2-3 ft up from the furnace and before the turn where ducts begins to travel between the joists . No screw holes. There is a whole house humidifier installed.
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Old 09-03-2012, 10:18 PM   #5
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Vent on furnace duct


Perhaps it was a mis-cut that was never fixed. Can you post a picture? It may be easy to seal it up with metal and tape up any screws and seams to close it off.
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Old 09-04-2012, 02:04 PM   #6
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Vent on furnace duct


It could be for make up air for combustion. Lots installed that way here. A vent from outside will be piped into the return air, and then a spill grill is installed on the plenum to provide air into the mechanical room. Before sealing it up, ensure you have an adequate air supply or you could end up with serious problems, including carbon monoxide.
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Old 09-06-2012, 09:38 PM   #7
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Vent on furnace duct


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Perhaps it was a mis-cut that was never fixed. Can you post a picture? It may be easy to seal it up with metal and tape up any screws and seams to close it off.

MB - Sorry for the slow response. I have been away for a few days tending to my mother after surgery.
I have attempted to post a picture.
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Old 09-06-2012, 11:03 PM   #8
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Vent on furnace duct


Most likely used to help heat and cool that space. Not a good place for it. Just take it off, get a piece of sheet metal, screw in place, then use Foil tape to seal it. BTW, are there any other ducts for the basement elsewhere down there? Really it is not taking from the upstairs, if the unit was sized properly for both the basement & upstairs.
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Old 09-06-2012, 11:11 PM   #9
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Vent on furnace duct


Combustion air. For many years it was permitted(depending on local codes) to stick a non-closable vent in the furnace room while doing a replacement for combustion air if the total free area of the space met 50% of the required air. Here that vent was required to have a sticker stating "combustion air-do not cover, possible death or illness if covered".

You need 50 cubic feet of open space for every 1000 BTU's of burn. Look at the furnace rating and water heater rating,add them then get the tape measure out and do some figuring. For example a 100K furnace and 50K water heater need 7500 cubic feet of area with no doors to have the required oxygen for the fires to burn correctly. As the oxygen gets consumed the carbon monoxide levels in the flue go up and can quickly exceed the 100ppm legal limit. If the room/rooms are not big enough then the air has to brought in from outside(hot and humid or freezing cold). That exception using indoor air to supply the oxygen, via the furnace blower and non-closeable grill,was just a way to keep people safe without bringing in a bunch of outside air.

Your picture looks exactly like a lake home I was at last year. The finished the basement with too little room for the furnace and WH combustion air. With the door open it ran 9ppm of CO in the flue. Closed the door and the CO got so high the little "run forest, run" symbol came up on my analyzer as the oxygen levels got lower and lower in the room and the CO got higher and higher in the flue. Stuck the grill on like that and she ran nice and steady 9ppm of CO in the flue and the O2 levels in the room remained good.

Last edited by Marty S.; 09-06-2012 at 11:25 PM.
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Old 09-06-2012, 11:26 PM   #10
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Vent on furnace duct


Marty....but isn't that the supply side? Those vents are supposed to be on the return air plenum....and I think that is the supply side.
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Old 09-06-2012, 11:34 PM   #11
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Return would suck the air out of the room. It belongs on the supply like it is.
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Old 09-06-2012, 11:38 PM   #12
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Yeah, you are right. Not sure what I was thinking there....

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