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Our home has two furnaces (oil fired hot air) and an electric water heater in an enclosed room (approx 6'6" x 9'). Three walls of the room divide it from the remainder of a finished basement, the fourth is an outside wall that includes a vent for combustion air intake.

In cold weather, especially when both furnaces run frequently, the room becomes quite warm -- into the 90s (F) at times. I'd like to be able to divert some of this heat into the house without leaving the door open (which is quite effective, but inconvenient, as it opens into a spot that we often walk through). I've already sealed as many air leaks in the ductwork as I can find (a total of four zones are served by the two furnaces, so there is a lot of sheet metal in there), I think a lot of remaining heat is being radiated from the furnaces themselves and the flue pipes (they are single-walled until they exit the room).

I'm thinking of putting a pair of vents -- one high, with a thermostatic fan to blow the warm air into the room, and a low one, to balance the airflow -- in the wall shared with the office. Outside of heating season, the vents could be closed to prevent outside air (from the combustion air intake vents) from circulating into the office.

The only specific risk I can see would be the risk of CO being pulled into the room if there is a furnace failure. We have working CO detectors in the house, and I suspect it would be circulated throughout the house anyway due to leaks in the return ducts (around the filters, for example), so I'm not sure if this adds risk or not. I know that furnaces in many homes I've seen are not in enclosed rooms, so perhaps I'm overthinking this.

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Do NOT power-vent that room into living space. You never want to vent a room containing combustion devices into living spaces...as you state, that's asking for CO problems. It's not an issue of the furnace being defective, it's an issue of you sucking exhaust out of the normal path (the chimney) due to negative pressure.

You could get around this by using a direct vent/intake system where it becomes a completely closed circuit (furnace draws directly from outside).

I'm not an HVAC specialist, but a 90 degree room seems to me that there's something else amiss here...like maybe your circulating fan is underpowered?

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    Our furnace room was like that due to bad insulation on the hot water pipes. Another thing to check.
    – Alex Feinman
    Commented Feb 22, 2012 at 15:01
  • On the temp in the room, I think it's a combination of the flue pipes and numerous places where small amounts of air leak from the ductwork. Also, each of the three zone dampers has a slot where the actuator ahem goes into the duct; I think they account for a fair amount.
    – TomG
    Commented Feb 23, 2012 at 18:26
  • @AlexFeinman - Hot water pipes are insulated and the tank is fairly new, so reasonably well insulated, I think.
    – TomG
    Commented Feb 23, 2012 at 18:27
  • If you have leaky ductwork...fix that. Get some proper metal tape (not duct tape) and seal all the seems/holes.
    – DA01
    Commented Feb 23, 2012 at 19:14
  • @DA01 - I've sealed all that I can get to. Biggest leaks were around the condensate drains from the A/C and the duct-mounted humidifier. What remains now are relatively small, except for the slots the actuator arms for the zone dampers use. Because the two furnaces are side-by-side and have a wall directly behind them, there's no access to a lot of the sheet metal to check for possible leaks. The metal tape is great, btw.
    – TomG
    Commented Feb 24, 2012 at 3:45
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Another alternative would be to install a door with louvers in it to allow the air to circulate. It could be a simple door with just one vent, or a full louver door like the one below.enter image description here.

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  • The problem is that when the furnace isn't running, the room would be filled with cold combustion air.
    – DA01
    Commented Feb 21, 2012 at 21:53
  • I don't think I understand your comment. What is the difference between having a furnace in a room with a vented door compared to an unfinished basement?
    – SchwartzE
    Commented Feb 22, 2012 at 19:47
  • The difference is if you're using the basement as living space or not.
    – DA01
    Commented Feb 22, 2012 at 20:06
  • Where I live this happens all the time.
    – SchwartzE
    Commented Feb 22, 2012 at 20:17
  • I'm not following you. A furnace need combustion air. In a basement, it can draw that from inside the house...where you'd want the louvered doors like that. Alternatively, you can add a vent to the mechanical room and it will use outside air for that...in which case you don't want a louvered door.
    – DA01
    Commented Feb 22, 2012 at 21:22
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I would exchange heat passively, with something like the louvered door suggestion above.

Any use of a fan could, as suggested, introduce CO into your living space, or worse damage your furnaces if the exhaust is not allowed to exit properly.

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