We've had thick smoke outside our home for several days now due to the fires on the west coast. We're doing what we can to filter the inside air and keep excess outside air from coming in (covered fresh air intakes, the dryer vent, etc).

You usually can't smell any smoke indoors even though it smells awful outside all day. However, every morning and every evening the smoke smell comes in for a few hours. I'm curious what causes it.

My best guess is that the air pressure outside increases during those times and that causes the exchange of air to increase. But that's just a wild guess.

Can anyone shed on light on what's happening?

  • Do you have central heat or air?
    – Ed Beal
    Sep 14, 2020 at 18:57
  • Yes, I have a central HVAC system.
    – Terence
    Sep 14, 2020 at 19:53

2 Answers 2


The most common issue when you have make up air on a hvac systems is when the kitchen exhaust or bathroom exhaust fans are turned on they usually push 80-1000 cuft and that air is sucked in vis the central heat or air ductwork. If an older home is is pulled in through all kinds of leaky air spaces.

The most noticed time when the bathrooms and kitchen exhaust fans are running pushing house air out and bringing fresh / Smokey air in.

  • That makes sense. I'll make sure we don't use kitchen/bathroom exhaust fans until the air clears up. Hopefully that stops our twice-daily smokey smell.
    – Terence
    Sep 14, 2020 at 23:03

Most central HVAC systems have an “outside air intake vent”. This vent is used to provide fresh air into the home and is usually adjustable.

Depending on the size of your home (and thus the size of your unit) the outside air vent can be from a 4” diameter metal vent to 8” diameter. Most have an adjustable blade that is set with a wingnut. You can adjust the vent so it’s closed and you’ll bring in very little outside air. (They are usually set at about 10% - 15% outside air.)

The outside air intake vent is connected into your HVAC unit on the “return” side and will extend to an exterior wall (and will have a grill covering the opening).

If you close it now due to the smoke, be sure to reset it when the fires stop.

This will allow you to just reuse the indoor air without bring in outside air.

  • 2
    Actually I have seen them as small as 1” I don’t see them over 4 for residential and if they are they are heavily throttled bu a damper. They can draw air through a penetration nut more common is from the attic air space and even the crawl space under the home but not as often there due to dampness. But how is this answering the question?
    – Ed Beal
    Sep 14, 2020 at 21:42
  • 1
    This is good advice, of course, but a bit different than my question. FWIW, I did determine that we do have a fresh air intake and closed as much as I could a few days ago.
    – Terence
    Sep 14, 2020 at 23:01

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