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I just moved into a house with a gas fired forced-air furnace. Whenever I'm in the basement and the furnace is running I can hear a whistling noise coming from the mechanical room. I traced this sound to a hot air outlet mounted directly onto the supply duct coming out of the top of the furnace.

If I put some tape over the outlet the whistling stops and the furnace runs fine. There is a fair amount of air coming out of this outlet and given that the mechanical room is in the centre of the house I don't think it needs to be heated. The only other theory I have for why the mechanical room needs air supplied to it is to provide air for combustion however the room already has a 5" duct coming directly from the side of the house for this purpose.

Can I permanently cover this outlet?

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Based on experience it is quite likely that grill is unnecessary, however your local HVAC guy can check if there's any valid functional or code related reason to have it there. Having a large difference between duct restriction to different grills or improperly sized pipes can cause significant imbalance in air delivery between grills, and if it's tapped on to the main duct, it effectively has 0 duct length where as all your other grills are likely fed by 10 or 20 feet of 4-6" duct off the main. This can effectively waste a ton of your air movement capability per energy spent, making your blower motor work harder or longer than necessary in order to deliver heat to the extremities. within reason, this can be compensated by adjustable grills.

I think it's extremely unlikely that a professional would install a unit with a blower so overpowered that it required an extra vent as a massive static pressure relief.

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  • Agree completely with the comment about this grill having 0 duct length. It is no wonder so much air was coming out of it. I think the basement was originally left unfinished so its possible this grill was meant to get some air flow into the otherwise unfinished basement. No other reason I can think to have it there. Jul 13 '21 at 4:41
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If there is whistling then it tells you the ducts are too small ie the air is travelling too fast.

Solution provide larger ducts.

Possible way to kill yourself or others is to block existing ducts.

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  • Why would covering a register (or grill) "kill yourself"? If anything, a lot air ENTERING a mechanical room would need a place to go, like into the rest of the basement, possibly carrying combustion gasses with it. The furnace already has combustion air supply from outside the house. If anything I'd think covering the register would make it safer. Jul 11 '21 at 15:10
  • This was my thinking. The other thought I had was that if the basement was originally unfinished then this grill would have provided a minimal amount of heat. Now that the basement has been finished it seems unnecessary. Jul 11 '21 at 19:05
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The register doesn't have an adjustable damper? Then I think it's called a grill.

Perhaps it's there to provide an 'outlet' if someone in the house shuts off the majority of the registers. With the extra register, the blower won't work too hard and eventually burn out.

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  • The register does not have an adjustable damper. I revised the title to say grill. Not a bad hypothesis... Jul 11 '21 at 19:04

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