I am currently affected by the US West smoke, so I bought some good filters for my furnace. I also bought extra filters and taped them in my ceilings where the air from the HVAC comes, for some extra filtering. I heard that I'm supposed to put my HVAC system to "fan on" (not heating nor cooling) so the air keeps passing through the filter and its quality improves. On the other hand, I also heard that if my system has "external air intake" I should turn it off completely.

The problem is that I don't know how to verify if I have external air intake. I watched some youtube videos and couldn't find anything similar to what I saw there. I looked for on/off switches everywhere and couldn't find any, except for one described below.

I took a picture of my furnace. I am not sure, but it seems to me the air flows in the direction of the arrow I painted in the picture, because the filters are right below the arrow and dust is on the up side. I also drew a black circle on the right side of the picture. There is a hole in the cover and it says "open" and "close", but given the size of that tube and where it is attached, I am not sure whether it is for external air intake or whether it is to fully close the heating system.

I should also note that all this is inside the garage, and the garage now has a much stronger smoke smell than the rest of my house, so I'm not sure if it my system is also taking air from my garage (instead of air from outside).

And a last detail, the filters don't really rightly fit my system, so air definitely goes through the side forming a path of least resistance. That is why I also taped some filters inside my house where the air comes.

What would be your guesses? Where should I be looking for things?

Thanks in advance.

here is the HVAC picture

  • external air intakes are typically associated with newer, high-efficiency HVAC systems, and those (ideally) go through a heat exchanger. Not to be mean, but your system is older and the equipment is on the cheaper end, so I would be shocked if you're (intentionally) pulling in outside air.
    – dandavis
    Sep 15, 2020 at 20:42
  • STOP! Call a pro, blocking things can damage your furnace, and create unhealthy conditions! Really I just had a friend call be because his co monitors went off after blocking his combustion chamber intake! We are all dealing with it no heat is needed don’t kill yourself or a 10k heat exchanger with out knowing what you are doing!!!
    – Ed Beal
    Sep 16, 2020 at 0:54
  • I have to say this again! It is not safe to block air intakes. Turn the furnace /ac off if you don’t have a electrostatic air cleaner it’s is not that hot or cold to block vents that may provide the legally required square inches of air flow for open combustion systems including water heaters! . Never block the vents for a high efficiency furnace there are 2 required in this case one intake and one exhaust. If you have make up air and no gas in the home this can be reduced but expect headaches and other issue with newer homes as they reach higher Co2 levels that you exhale with each breath
    – Ed Beal
    Sep 16, 2020 at 2:39

2 Answers 2


They're not very common in residential, but an outside air duct would connect to the return air duct. Trace out the return air duct and see if there are any smaller ducts attached to it. Or walk around your house and look for the opening the duct would connect to. It would most likely be near the furnace. As for your filter, you can get some foil tape from the hardware store to tape over the filter opening so no air is bypassing the filter.

  • Make up air are in a high % of residential air systems it can be a small opening 1” usually not more than 4 “ and a damper , close to the furnace or on the return side in the attic or crawl space.
    – Ed Beal
    Sep 16, 2020 at 0:59

Please look at the comments. Your system looks like a down flow system. Possibly an early sealed combustion because of the condensate pump but that’s is probably for just the ac not sure. This means the combustion system draws fresh air when it is heating. The 12” maybe larger round duct looks like an addition and the damper (slot with open close) is to adjust the flow in that duct and balance the flow in the vents. Ok that’s the basics air is drawn in and is pushed out.

Adding additional filters at the intakes is actually counter productive because that increases the vacuum level on the duct work in the attic in your case. Your soffit vents or gable vents are open to the outside in the attic the increased vacuum because of extra intake filters sucks air through the seams of the systems of the return trunk line. Based on age & in the PNW may have a thin insulation blanket without taped seams very common in older homes. With the double wall gas vent if you have a make up air vent it would normally be in the attic. Possibly on the duct above the furnace but not likely in a garage.

Quite honestly I would suggest getting an electrostatic fan for use in the living area.

There are powered electrostatic filters that can be installed on the intake small systems like yours 700$ maybe a bit higher. Another possibility is a passive electrostatic filter you put in the intake these create a static charge and trap the smoke not as effective but you may be able to get one on amazon or local hvac distributor. I am not sure of the cost but know they don’t creat the pressure drop that higher MERV filters will.

The smoke in the garage may be from any heating and your water heater as both draw air in while running.

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