I've tried Googling this but the stakes are quite high and I don't want to be wrong, so I'm asking here.

Wildfires are raging in California where we live, and the Air Quality is nearing the 'hazardous' AQI level. I just bought a 6x16" 400cfm active carbon air filter/fan, and I have 1 other air purifier running in different rooms.

The problem is it's extremely cold in our house as I have the heater off. I am wondering if it's safe to turn the heater on, or if it's going to pull in air from outside?

I'll try to give you details which could help figure it out.

  • Our heater box is in the closet under the stairs (photo below).
  • Also, there is 14x14x1 sized air filter behind our couch which I feel it sucking air from when the heater is on. Is this the only location the air comes from?
  • The thermostat offers heat/cool and is a digital box on the wall
  • We have a gas stove so I assume the furnance is also gas?
  • We live with someone with COPD and I also have asthma, so it's important to me not to just rely on some old ass filter which is supposed to filter air from the outside.
  • important side question does it look like my quatro heater is dangerous? In trying to research this i found that some were recalled and now I'm scared because our house has this furnace...

enter image description here

  • 4
    Regardless of all the advice in the answers so far, the only way to know the furnace's source of air (for combustion) is to trace the ductwork. Commented Sep 11, 2020 at 15:22
  • 6
    Given your general concern, it should also be mentioned that clothes dryers, of almost all varieties, suck air from inside the area in which they are located and blow it outside, after using that air to dry the clothes. This will result in negative pressure in the area where the clothes dryer is located, causing air to be sucked into that area at the same rate that it's exhausted to the outside. In other words, using a clothes dryer which is inside your house will result in sucking outside air into your house.
    – Makyen
    Commented Sep 11, 2020 at 18:04
  • 3
    Oh shit... I didnt know that. thank you!
    – Tallboy
    Commented Sep 11, 2020 at 18:06
  • 4
    This furnace is definitely a mid-efficiency furnace - it will be combusting room air and exhausting that outside. Makeup air will be entering the home from outside to compensate, most likely, through a makeup air duct. Combustion will only draw about 10-15CFM, though, so it's not as big a draw as something like a clothes dryer, range hood, bathroom fan, etc (which can be 80-300CFM+). All of those things will exhaust indoor air and will cause outdoor air to be pulled into the house to replace it.
    – J...
    Commented Sep 11, 2020 at 18:36
  • 10
    If the air outside your house is not breathable you need to evacuate.
    – jesse_b
    Commented Sep 11, 2020 at 19:01

4 Answers 4


I think some of the answers here are confusing because they don't really address that your furnace has two different air intake/outlets.

One is for the combustion system. Air is pulled in from somewhere (usually outside, probably the metal duct on the right), mixed with natural gas, and burned. All of the outside air and combustion products go out of the chimney to somewhere definitely outside. Very hot air from the burning of gas heats a metal piece called the heat exchanger.

The "makeup air" mentioned elsewhere would be if the combustion loop sucks air from inside the house instead of through a duct from the outside. In this case some air from the house is blown out the chimney and is replaced by air seeping in through the cracks. This would not be code compliant where I live, for example.

The other air loop is through the ducts in the house. Typically a furnace will suck air from inside of the house (probably the vent behind your couch), pass it over the heat exchanger, and blow the now hot air out of the vents in your rooms. This typically does not pull a significant amount of air from outside because that would waste a lot of $$$ by displacing heated air to outside. Houses are not airtight though.

Your furnace is obviously gas because of the supply pipe on the lower right, and the presence of a chimney on the top of the unit.

There is really no way for anyone here to tell you whether it is safe or not. Age and presence of recalls are good places to start though. Adding a CO detector and making sure you have enough smoke detectors would help significantly.

We live with someone with COPD and I also have asthma,

If this is a matter of LIFE AND DEATH, there is not enough information in your post to provide an absolute answer. YOUR SYSTEM MAY NOT BE NORMAL.

Have you ever changed the furnace air filter?

HVAC filter being changed from a standard receptacle

  • 2
    Yes. In a perfect system, the breathable air is drawn from inside your house, heated (via a heat-exchanger) and then released inside your house again, still clean and breathable. The air from outside burns the gas and vents outside as carbon dioxide/monoxide. Never the twain should meet. If the two air flows do meet then you are in danger of CO poisoning. NOTE: @trognanders - Sorry, I think I've just repeated what you said. Never mind - I think it's worth saying again. Commented Sep 11, 2020 at 11:06
  • 2
    @chasly-reinstateMonica Only high efficiency appliances draw combustion air from outside. Mid-efficiency open combustion appliances simply burn using air from inside the house, creating negative pressure.
    – J...
    Commented Sep 11, 2020 at 12:15
  • @J... - Much like an old-fashioned coal or log fire does I suppose. I remember my father putting a sheet of newspaper over the opening "to create a draught". The pressure difference was very noticeable. The problem of course was that, if the wind was in the wrong direction, we could get smoke coming into the room while the fire was burning normally. We had no idea about Carbon Monoxide at that time. Commented Sep 11, 2020 at 12:32
  • 6
    In Texas, and I assume in other warm climates, its common for the furnace to be in the attic, so combustion air is taken directly from the unconditioned attic space and then vented out the roof. The attic is heavily vented, so there's never worry about negative pressure or pulling air from inside the house. Just so other folks with an attic-based system understand whats going on in their house... It is common to talk about "make up air" when dealing with a gas dryer tho.
    – JPhi1618
    Commented Sep 11, 2020 at 14:22
  • Thanks so much for your explanation, it was a lot simpler than the other answers which are talking about things I have no idea about. Eeverything you said makes sense, thank you!
    – Tallboy
    Commented Sep 11, 2020 at 17:48

The answer is that it should have a fresh air source! This is an absolute for gas furnaces.

Some systems use the “leaks throughout the house” to get the air. The filter in the furnace is for dust to keep that out of the heat exchanger it dose nothing with the gasses that a open combustion fire box generate.

The most important thing you can do is to install carbon monoxide detectors just out side your furnace room and at each level of the home.

I did not look up the recall, but you should. Is it a cracked heat exchanger issue? Is it a fire hazard? Detectors are your first safety after verifying it is in good shape.

I can’t tell but your supply could be coming in on the unfinished part of that closet (that would be a code violation today in my jurisdiction). If the stud bays or ceiling joists bays are used they require a metal grate--one close to the ceiling and one close to the floor. This may not be required in your location but getting a pro to inspect and clean the system may provide all the answers you are asking for.

  • 3
    The last line is the important one - OP really should have the unit profesionally inspected. Especially if it hasn't been looked at in a while, it could probably do with some service anyway.
    – J...
    Commented Sep 11, 2020 at 13:05
  • I had a guy come about a month ago because the furnace wouldn't turn on. he "fixed" it but it was someone hired by our landlord, so I have no idea if he also inspected it. I would imagine he would have said something if anything seemed very wrong about it.
    – Tallboy
    Commented Sep 11, 2020 at 17:49

Your furnace probably exhausts more air than it intakes, creating negative pressure in the house which will draw air in. You can try to control is by "encouraging" the makeup air to come in through a route where it hits a filter ASAP.

High efficiency furnaces have a (typically PVC) combustion air intake and exhaust and don't result in net air movement. This is in addition to the air return (entering furnace from bottom right, coming in from the top) and main supply duct (insulated thing going out the back, possibly through an heat exchanger for AC). In your case it doesn't look like there's air intake, so the furnace just ejects combustion gas out the main outlet (middle going left), and intakes air just from the stairwell. Because more air is going out the house than in, the air leaks in the house will just pull air in from outdoors.

Depending on how tightly airsealed your house is, if you're very concerned about letting in any unfiltered air from outdoors, you may try to control your input by intentionally opening a window and putting a filter right there, possibly with a fan to "encourage" makeup air to come through there instead of through unfiltered cracks in the house. In general though, the rate of negative pressure generated by combustion gases should be pretty slow, and I'd expect that just by continuously running your main fan or a few air purifiers, you'd achieve mostly the same result.

  • 2
    Open combustion appliances like OP's furnace require that the house also have a makeup air duct near the unit to allow outside air to be drawn in to replace the combustion air. Unless the unit was installed illegally, or the duct was removed illegally, OP should already have a vent with a duct supplying fresh air from outdoors. Running an air purifier cannot increase the rate of air changes per hour of a dwelling - it can only mix the air that is already in the house. In a modern air-tight house an HRV or ERV is what provides the ventilation.
    – J...
    Commented Sep 11, 2020 at 12:09

I'm also an HVAC novice, but this article is a good explanation of how air may or may not be drawn in, depending on the design of your HVAC system: https://pharoheating.com/hvac/is-it-bad-to-run-my-furnace-fan-all-the-time/

I would also recommend getting an air quality sensor (e.g. Purple Air, etc.) so that you can directly see whether wildfire smoke is coming into the house. We've been running our furnace fan (without running the heater), and noticed that it significantly increased AQI on our air quality sensor.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.