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Hi I have some vents I'm wondering about their functions/significance.

There are these outdoor vents, that I see called 'dryer vents' online. But I have only one of these connected to the dryer. There's an extra one in the front and another in the backyard.

As pictured there is one of these in the backyard that just looks like the grate is clogged up but very thinly. The one in the front that's NOT a dryer exhaust has a flap on the inside.

Is that flap important or should it be removed? What is the function of these? Do they let air in or are they just exhaust vents of different kinds? (Is there one from the fridge or something?)

Then there are also these square vents against the bottom of the walls. I think they are called 'return air vents'. I've had a few of these blocked by furniture on the main floor but at least one open. How much of a difference would that make?

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RyanD
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    Do you have access to the inside of the building to see where they run inside? First guesses would be dryer vent(s) and bathroom vent fan vents. The flaps are important to help keep outside air from coming in when you're not blowing air out, and also help keep critters (birds/mice/rats) from getting into your vent lines. – Milwrdfan Sep 28 '22 at 13:52
  • "Return air vents" are vents used to supply air to a forced-air HVAC system for its intake. Since the system is sending conditioned air throughout the house, it needs a source of replacement to take in. Older leaky houses just relied on outside air leaking in around windows, doors etc, but more air-tight houses need vents to let in air. That prevents pressure differences that as Fresh Codemonger notes can lead to poisonous combustion gases getting sucked in instead of exhausting out of the house. You need to figure out the airflow for all the systems in your house. – Armand Sep 28 '22 at 19:07
  • Yes! about the AIR TIGHT HOUSE you mentioned, I do ask about all this in the context of some air flow problems. One vent on the main floor has good output but the others are noticeably weaker, on the top floor even worse. I was wondering if the blocked registers or clogged exhaust vents would be behind this OR it would more likely be due to another underlying issue. (I thought the air came from outside the house not from re-circulating from the registers) I got a High efficency furnace and do think I need an HRV but wanted to check if other vent issues could be behind it? @Fresh Codemonger – RyanD Sep 29 '22 at 14:33

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The dryer vent does need to be cleaned, and the lint there is an indicator that there's probably a fair bit of lint in the dryer duct that needs to be removed, too.

For the other duct, go to the basement/crawl space and see what's on the inside of the wall at that point. Follow the duct to see where it leads. If nowhere, then you'll want to cap it off inside to keep cold air out. If there's ducting attached follow it and you'll know what it's for.

FreeMan
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    Before you go sealing any vents it is important to make sure you have makeup air for any combustion devices. The last thing you want is to negatively pressurize the house and backdraft cabon monoxide into your living areas. – Fresh Codemonger Sep 28 '22 at 16:55
  • Good point, @FreshCodemonger. However, aren't combustion air vents usually ducted directly into the appliance that needs them? – FreeMan Sep 29 '22 at 12:25
  • A lot of older houses just rely on the fact that the house is poorly sealed and they do not have direct makeup air to the mechanical room or to the appliance. I have a 1995 boiler and it pulls air from the mechanical room. There are newer combustion appliances as you say have both makeup air and exhaust air inside a single vent pipe - is it a b-vent? – Fresh Codemonger Sep 29 '22 at 18:09
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    Looks over his shoulder Are you looking at my house? :) – FreeMan Sep 29 '22 at 18:10
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Typically bathroom exhaust fan and kitchen cooktop exhaust fan vents. The flap keeps hot/cold air from outside getting in, and also helps keep unwanted wildlife out. Typically an exhaust fan is required for the kitchen and a bathroom exhaust if the bathroom does not have an operable window.

manassehkatz-Moving 2 Codidact
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    Those are typical applications for vents, but that low to the ground? Unless there's a basement kitchen & bathroom, that seems odd... – FreeMan Sep 28 '22 at 17:20