Fairly new to this house and I have a large open vent in our basement that is letting a lot of cold air in. The diameter is 8-9 inches. The inside duct is connected directly to the outside vent screen in the second image.
It is in the furnace/utility room. The furnace and water heater both consume gas. Will blocking it impact carbon monoxide levels in the basement?
Here is what it looks like inside and out.
As others have mentioned, that's almost certainly a combustion air vent. Your appliances consume air from the utility room, and that air must be replaced, otherwise safety problems can develop.
There's a couple different things you can do about it.
Switch to all sealed combustion or electric appliances. This is what I did, 90% AFUE furnace has its own intake snorkel, and an electric water heater.
It's possible, though I don't know if it's allowed, to put a damper on the combustion air intake that opens whenever an appliance fires up. However this mechanism could fail, or pilot lights may consume too much air to close the intake.
Seal off the utility room. You're allowed to box the appliances into a tiny utility closet, if you follow certain rules (I think you need two intakes, one high and one low) and if you air seal it and insulate it, cold air won't get in to the rest of the house.
Combustion air vents are pretty terrible for home efficiency, my old 6" vent was passively flowing 100-200cfm at all times, including the dead of winter.
I believe the vent is for combustion air. If there is no fresh air to the furnace and water heater this could create serious problems both with carbon monoxide and inefficient operation. If you block the large vent air may be pulled in from the vents on the water heater and furnace causing a buildup of carbon monoxide and because of the reduced fresh air the gas may not be completely burn. Plugging the fresh air vent could be quite hazardous and cost in inefficiency.
We had a similar 6” hole providing fresh air to our old boiler. We were able to pipe it directly into the new boiler and create a closed fresh air system for the boiler so that it no longer uses air from the boiler room but instead gets all the air directly from an intake pipe which thus sealed the hole.
Another possibility is that this used to be connected to the furnace or hot water heater and one or both systems were replaced with the combustion output vented through a different duct. If the furnace & hot water heater ductwork are currently combined then that is very possibly what happened. It is also possible it was the dryer vent. If your washer/dryer is currently located elsewhere but there is indication (electrical and/or plumbing) that it used to be near this vent then that is another possibility. If either of these scenarios is what happened then this would definitely not be needed for fresh air.
The last time my plumber worked on my furnace, he made it clear to me that I should have multiple CO detectors for safety (I installed them right away) - so I second that recommendation made by others already.
The furnace and water heater both consume gas. Will blocking it impact carbon monoxide levels in the basement?
Definitely it will impact carbon monoxide and not only that - the vent is REQUIRED by code.
Don't block it!
Buy an electric or heat pump water heater and furnace if it is too unbearable.
I have my air intake connected directly into the air intake duct on the furnace with flexible insulated piping similar to attached photo.
Be careful. CO is lighter than air at 70 F, but heavier than air at 32 F. Where you place the CO monitor will depend on the room temperature. Maybe use two monitors, one located at an outlet 12" off the floor, and one at the ceiling level.
