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As part of a new residential boiler installation, two 6-inch ducts venting to the outside will be installed, one near the ceiling and one near the floor. They won't be attached to the boiler itself, as it's not the type of unit that takes direct venting. Just vents open to the room, but very close to the boiler and hot water heater.

The gas boiler and adjacent gas hot water heater are against one wall of a large space that is to be inside the thermal envelope of the house, but I think that lots of cold air will be entering through the new vents.

I want to build a closet around the furnace and water heater, and my question is: Should I seal it off and insulate it as much as possible, since it has it own source of outside air, or do I need a louvered door for some reason?

I'm pretty sure that with this amount of outside venting the right thing to do is to seal off the furnace. (There will be doors to allow access for maintenance.)

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Given that setup, I can't think of any reason you'd need to draw inside air. On the other hand, I'm not sure you want it entirely at outside temperature, from the point of view of freezing risk when not circulating. (Then again, I'm in a northerly climate; you may not be.)

Did the folks who installed it have any opinion on this approach?

Make sure there's enough space around the unit to (a) meet its specifications for fire safety and (b) allow servicing it and its support plumbing/pumps/whatever without having to rip down and rebuild that closet.

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  • Thanks... Installer still working. Will ask him tomorrow about the closet. Nov 18, 2014 at 4:00
  • Update: Installer confirms that sealing the boiler and water heater in a closet is the right choice. (Note to others: This is true ONLY because there is adequate outside air.) Nov 18, 2014 at 16:22
  • Update #2: Turns out that the unfinished basement gets pretty warm, indicating that the small amount of outside air isn't causing any undue cooling. So I left the boiler exposed to the unfinished basement. Aug 17, 2016 at 19:42

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