HVAC Combustion Supply from the Attic

Hi, I’m new here and to inspecting. I’m actually working on my practice inspections right now and just completed one on my own home.

Our furnace is located in the basement below grade in a closet with double bi-folding louvered doors. I noticed some insulated flex ducting squeezed through into the ceiling above the furnace that’s just open into the furnace closet, but couldn’t find where it was pulling from. I found it pulling from the attic near the soffit, but it’s just the end of the flex tube zip tied to the truss and the opening is angled down toward the loose fill fiberglass insulation. but it looks like they just dig a hole around in the insulation where the tube is located.

I’m assuming this is not correct, but I’m having difficulty finding anything to verify this. To me, It looks like this could be an issue because you have no idea if the air is restrained anywhere in between the furnace room and the attic, limiting the available combustion air supply. Also, where they dug a hole in the insulation (about 18-24" across from what I could tell) is directly above an upstairs bedroom. So that tells me that room is not going to be insulated as well making it too hot in the summer and too cold in the winter.

Am I correct in this being a problem, or am I missing something that makes this ok?

it looks like it is already failing…I would want that relocated up and away from the insulation…


Absolutely agree with Jim.

Yeah, get the end up out of the insulation. Got a couple of other questions.
Is the basement finished (habitable)? You had mentioned bi-fold louvered doors. Those louvered doors are most likely your combustion air and the flex duct is basically make up air.
Also, it looks like the flue set up is possibly wrong, from what I can see from Okla. Is there another gas appliance to the left of the furnace? Is the vertical section of the furnace flue just a single wall pipe? One other thing, in my area the walls of the furnace closet must be sheetrocked.


Yes, the basement is finished. This is a closet in the main living area of the basement.

Yes there is a mid-efficiency Gas water heater to the left of the furnace. They both vent into the Y-pipe and they’re both single wall until just around the corner of the Y pipe, where they connect to a B pipe that goes up through the second floor and the attic, and then vents through the roof.

I’m in Montana. I’m not sure about the sheetrock, I’ll have to look into that. But every home I’ve been in here so far hasn’t had sheetrock in the maintenance closet/room around here. Then again, that doesn’t necessarily mean it’s correct either.

As far as the flex in the attic is concerned, relocate it away from the insulation and fill the hole back in. Combustion air is allowed to be drawn from the attic space, provided the attic is vented adequately. If you want to test if it’s drawing air, hold a BIC lighter near the end when the furnace is running. If it’s pulling air it will suck the flame in. Just don’t catch your house on fire :fire:.
In picture #1 is that the single walled exhaust vent you mentioned for the furnace? If so, the refrigerant lines for the A/C are too close. That pipe insulation should be at least 6" away from the vent pipe.

That is one of the stupidest ideas I have ever heard…Please don’t hold a lit flame to the end of that dust filled hose while the furnace is running…there have to be a thousand safer ways to check for draft…

Now that I have somebody’s attention, actually if you just use the striker a tiny bit of smoke will come off of it that can be seen and doesn’t carry any odor. Doesn’t take much. I would have figured somebody would have made at least some sort of suggestions for the OP on how they would test for draw. Right wrong or indifferent. I don’t think a lick test would work in this situation…

and now a spark…that has to be the second stupidest thing I’ve heard all day…Maybe You could try holding a piece of paper or tissue at the end of the hose to check for draft…of course I’m sure that is nowhere near as exciting as flames, but Your insurance co and the fire department would appreciate it …


Single wall flue pipes are not allowed on furnaces after the flue leaves the furnace cabinet. It appears that if the water heater is to the left of the furnace, then the furnace flue is attached “above” the water heater flue. The smaller gas appliance flue should join ‘above’ (or beyond) the larger appliance flue.

Yes, that is the AC lines coming in. They are only about 3" away from the single wall vent piping. So that’s a good catch. I didn’t think of that.


According to the installation instructions for the furnace (Rheem Criterion 2 manufactured in 08/2004), it states that single wall is ok with a 6" clearance. But if using Type B vent pipe, clearance could be reduced to as little as 1". But either way, there is a lot in there that is much closer to that vent pipe than the required 6" minimum.

You guys need one of these.
Cirrus Wind Indicator

Almost as dumb as checking for NG leaks with a lighter!

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Those dumbos! I use the long matches. :wink:


Or this?