Thread: Appearance of exhaust fumes

Hi, and sorry to post an ignorant question, but there ya are, I'm just a homeowner.
I have just started up my hi-eff. furnace for the first time in a new house. Furnace vent and cold-air return are direct-vented to outside wall directly next to the furnace room (but vertical to the furnace itself).
Note: I just had a contractor break up the floor of the furnace room to install a sump pit, and, in an adjoining room, a new backwater valve; so I suppose that kicked up a bit of construction dust. That job was done 3 days ago.
QUESTION: Upon examining the hot air venting from the flue, I was starled to see its color. It was a fairly dense white vapor, a bit like white smoke, but seemed more water vapor than anything else.
My CO detector (1/2 a storey below in family room) has not sounded. Is all this normal??

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When a unit is new, it is not unusual to burn off any residual contamination in/on the system.

Having said that, I recommend having a new unit started up and go through at least a few cycles with the installer present; just to make sure that everything is doing what it should. You should be present so you can be shown how to operate/maintain the system.

I suggest you have the installer check it out asap, so there is no question. It may very well be fine, but if you are going to err, err on the side of caution.

As far as the detector upstairs not setting off; carbon monoxide is heavier than air, so it will settle. In other words it is possible for there to be an accumulation of CO in the basement that hasn't reached the detector. Becase of this and the fact that you have combustion going on; you should have smoke and carbon monoxide detectors in the basement too.

Hi Steve, and thanks for answering.
One thing might not be clear from my original post. The furnace is not new. It has had a new air-conditioning unit installed on it, along with re-installing of the air intake/exhaust pipes.
When all that was done, I believe the furnace was run-- just to test a few cycles of the a/c. Obviously, though, I will need to do the annual cleaning/inspection.
I have moved my CO detector to the furnace room, just to be safe, and will also be installing one in the room above.

Hi Walter,

OK, furnace not new, but if there is any question regarding its performance, be safe an have it checked out.

"...carbon monoxide is heavier than air..."


Correction, if I may, CO is about the same as air
Carbon monoxide - Wikipedia, the free encyclopedia

My bad, but I would still install a CO detector in the basement.

Hello Walter. If you still have smoke or vapors coming out of your high efficiency furnace vent, you should have it inspected and serviced.

You didn't say what part of Canada (big country). If you are above the Arctic Circle, then maybe a bit of vapour would be normal.

High efficiency furnaces will normally have exhaust air that has water vapor suspended in the exhaust, much like seeing your breath on a cold day when you exhale but more pronounced. A photo of the system and the exhaust would be helpful.