High Eff. Gas Furnace - Intake Air

Did an inspection yesterday on a home with a 1 year old high eff. gas furnace. The fresh are intake was pulling air from directly in the house, not from a fresh air intake outdoors. This is the first time I have seen this before. Obviously there will be some loss in efficiency since the furnace is using indoor air for combustion but I am wondering how many of you have seen this before and what are your thoughts?

hi Erich,

most high efficiency furnaces draw combustion air from the outside, I would look up the manufactures installations instructions online and see what they call for.



Ì see this a lot. You will loose about 1-2% efficiency. We did not do that when I owned my heating and cooling company, but some people are just lazy.

This is what I figured. I made a note of it in my report but didn’t consider it a major defect seeing as how the furnace would still function ok, the efficiency would just be down a bit.

They may be legal, but stupid.
They are pulling inside air and quite possibly setting up a negative house pressure situation.
If the place is newer it may be very air tight and for instance say they have a fireplace , may draw combustion by products back into the home.
Remember that water heater , which may be in the closet also needs combustion air.
Is it in a small utility room.Does it have vent openings at the top and bottom.
Back to that water heater it may have flue gas rising naturaly that could be reversed and pulled into the house.
I just did a place last week where they ran the schedule 40 out the wall.
(good deal right?)
Well it turns out the Polish speaking HVAC guy mis-heard the AHJ tell him the intake from outside was not needed.
Duh,…he went and cut all the intakes at the elboes hours before I arrived.
When I met him I said put them back up, right away please.
Water heater next to it had gas leaking at the same time but thats another story.

BTW, when you see this at new construction and suspect that the furnace may have been used as a “construction heater”, listen carefully for small pieces of construction debris tumbling around in the inducer fan housing.

Hi Mike
Not to mention a dirty blower and filter , which can void the warranty

From this afternoons Inspection
Notice lack of filter.

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Some good replies. Good points Robert, I wasn’t too concerned about a neg. pressure or backdrafting since it was in a large open unfinished basement.

That is where they become radon gas suckers! Any leaks in the return air duct system and it gets pumped throughout the house. A good reason to add $10 worth of PVC pipe.

Depending on the furnace and the location. I will add this to my report.

· **Monitor: **Ideally class IV furnaces with sealed combusting chambers should have combustion air intake vented to the outside for maximum efficiency.

Excellent note to add into a report. I made a similar note in my report.