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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.

enter image description here enter image description here

  • Thanks everyone for the comments and responses! I've had CO sensors in the house since we moved in and I will keep this open to prevent any dangerous CO buildup. As suggested, I will also insulate and seal this room off from the rest of the basement to isolate the cold air. – vizyourdata Jan 9 '18 at 19:17
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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.

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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.

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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.

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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.

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I have my air intake connected directly into the air intake duct on the furnace with flexible insulated piping similar to attached photo.

enter image description here

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First off, a properly operating gas furnace or water heater should not leak carbon monoxide at all. You should purchase a carbon monoxide detector and install it in your utility room. Mine is mounted right next to the furnace and water heater.

Second, it appears they meant to use that as a fresh air intake and someone has used it as a cable TV chase. Your furnace and water heater don't need that much draft air for combustion. Even if your house is super sealed a much smaller opening would suffice. My house uses natural gas for both as yours does and there are no vents for combustion air. Houses in northern climates have never had vents until recently and things have worked just fine since the house usually leaks enough to provide the fresh air. Only recently they have started building supersealed homes that might need a fresh air intake for combustion air.

Get the CO detector and then cover the open vent and see if you have any problems. If everything continues to work without any CO detector alarms then you are good and should save some money on your heating bill. Even if you have issues you should install a proper fresh air heat exchanger and stop wasting heating dollars.

  • CO is heavier than air, so the detector should be mounted at floor level. – Yehuda_NYC Jan 7 '18 at 12:47
  • @Yehuda_NYC Preferably yes but you won't find many floor level receptacles in unfinished basements. Even in a finished basement they will be at least 12" off the ground. I suppose he could use an extension cord and lay it in the floor. Anything is better than nothing. – ArchonOSX Jan 7 '18 at 12:54
  • Depending on the size and type of furnace / waterheater this duct may be quite small. Open vents if not supplied with sufficient air a hazardous condition will result unless these are modern sealed combustion chambers on both units. My last home prior to updating the furnace required 3 square feet fresh air. – Ed Beal Jan 7 '18 at 13:03
  • @ArchonOSX. The alarm company I used wanted to install the battery operated CO sensor at the ceiling. That is just the wrong level. By the time the alarm sounds, the people would be dead in their beds. This is another example of the need to do your homework when buying goods or services. – Yehuda_NYC Jan 7 '18 at 15:37

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