I'm trying to figure out the right questions to ask. I've not had good luck with a HVAC service company when we first purchased the house. As I don't think I have any urgent risks given the 3x Nest sensors are working (i.e. they would have to fail on different floors), I'd like to try and get an understanding of potential root-causes before trying my luck with a service company. Appreciate all advice.


  1. Our Nest Protect CO alarm has gone off twice since it got cold - 90 days apart, in the basement only. Opening the windows cleared the gas and the issue did not re-occur.

  2. Separately, near the air-filter area, a strange smell of post-processed gas(if that makes sense) is lingering in the air. Again, opening a window clears this right up.

While I think the air-filter might be old(?) and changed it for issue #2, could I have a more generic issue of an exhaust valve being clogged that might be causing things to linger? I'm not aware of any physical trauma to the HVAC system between the last two winters - so it's perhaps some natural build-up?

Thank you.

HVAC units

  • 3
    "As I don't think I have any urgent risks given the Nest sensors are working..." VERY WRONG! If you know you have Carbon Monoxide, especially if you the alarm has gone off multiple times, it is not ok to assume the alarm will save you. Alarms can fail at any time. – Moshe Katz Jan 5 at 1:30

Where is the make up air for the boiler?

I think I see a vent to the top left but it appears blocked. If that is a fresh air inlet and it is blocked I hope your last will and testament is in order!

I lost a friend to carbon monoxide poisoning years back it is nothing to mess with. Your system needs fresh air for the CO to escape with no fresh air it escapes into the living space.

The fact that it is detected on another floor means it is an IMMEDIATE HAZARD !

Make sure there is fresh air this is probably the issue. The small amount of cold air being drawn in by your boiler having to be heated is nothing compared to cost of a casket, no I am not kidding!

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