I have a furnace with an uncovered air intake. I cant imagine it is pulling very much fresh air in through the duct because it is uncovered. I am wondering, is this correct? I believe there should be some grill covering this. Should I cover the outside panel so that most air is being pulled through the duct?

Outside: Outside

Intake: Intake Hole

Duct work: Duct view

Overview: Whole view

  • Was there a cover that you removed in order to take this picture?
    – Matthew
    Aug 24 '20 at 14:50
  • No I did not remove anything.
    – aehyao
    Aug 24 '20 at 14:51
  • Anything you put over that will reduce the airflow (even if minimally), so I'm not sure why you're worried that it's not pulling enough air...
    – FreeMan
    Aug 24 '20 at 14:52
  • Well I am concerned it is not pulling air through the duct, which I believe leads outside.
    – aehyao
    Aug 24 '20 at 14:53

It's hard to tell from your photos, but I believe that duct is a passive makeup air system.

When the furnace goes on it needs to exhaust gasses out the flue. For it to be able to exhaust gasses it needs to recover fresh air.

Most buildings have plenty of little leaks which can let air get back in. But some new homes are built with sealed building envelopes, trying to keep the home bottled up. This presents a problem for gas burning appliances (or any exhaust appliance for that matter).

Ducting in from the roof will allow the furnace to recover air, but it's unlikely to seriously impact your thermal envelope.

If your home were "too air tight" then your furnace wouldn't work.

  • Ok this makes the most sense. So you don't believe it should pull air in, just to balance the pressure.
    – aehyao
    Aug 24 '20 at 16:17
  • Passive? I see it has AC power going to it. Maybe that's to open and close a vent. Aug 26 '20 at 14:10
  • I don't see any power going to the duct but if there were that wouldn't be uncommon either. Sometimes makeup air systems open dampers when their connected appliance turns on.
    – Matthew
    Aug 26 '20 at 14:39

Hate to say it but I don't think anybody got it right yet. From what I see this is just an air handler unit because there appears to be refrigerant lines visible in the upper right of the 2nd picture. I also don't see a flue or gas supply line. So it's almost certainly just an air handler connected to a heat pump (outdoor unit) and the small round duct is for fresh air. There also appears to be a condensate line coming out the bottom of the unit, which you would have for A/C.

Bottom line, I think you're fine as long as the fresh air return is ducted to the outside and not the attic.


When a furnace is setup to use outside air, it is for combustion...the fire. Then the air goes up the chimney.

The air going through the filter is from inside the building...normally. Where does that duct go?

You are missing a panel which means the air is being pulled in adjacent to the furnace and not pulling it in from other parts of the building. So you are not getting good circulation.

  • Sorry, my understanding of this terminology is pretty poor! I may be calling this a furnace but it may be something else. The duct goes to the outside I believe. img
    – aehyao
    Aug 24 '20 at 14:33
  • If it's a heat exchanger then you do want the outside air....which means you are still missing a panel and not sucking outside air in. Aug 24 '20 at 14:39
  • How can I find out what this is? Duct view Whole view By the way I appreciate your advice!
    – aehyao
    Aug 24 '20 at 14:45

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