In a previous question we established that there probably should be a vent between my utility room (which opens into the kitchen and living space in general) and the outdoors (in my case the attic, which is a separate issue)

This seems hopelessly inefficient. Is there a better way to provide combustion air to the furnace and water heater, without having a direct vent between my living space and the outside air?

  • is there cold air coming in through the vent, or are you worried about warm air going out the vent?
    – Tester101
    Commented Jan 7, 2011 at 19:58
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    This type of setup seems like a code loop hole. I think there is a code against a furnace/water heater getting combustion air from a living space, this way the installer can say the combustion air is coming from the attic. I would think unless the utility room is air tight, it's probably not drawing too much air from the vent.
    – Tester101
    Commented Jan 7, 2011 at 20:02
  • Previous question pointed to inspectapedia.com/Energy/Combustion_Air_Details.htm which seems to justify it - I'm just not happy with the solution. Commented Jan 8, 2011 at 20:02
  • Given that the registers didn't even show up in my energy audit, I'm assuming it's mostly warm air going out. Commented Jan 8, 2011 at 20:12

4 Answers 4


I am a little concerned that you are overthinking this situation. Naill C gave you some good advise in your last question. There are very few houses not built to be virtually air tight with an air exchange system that do not have enough natural air flow to support open combustion appliances.The simple act of opening a door to enter or exit the house allows large amounts of fresh air in during cold weather when windows are closed. Unless you have made extreme attempt to block all air infiltration, there is probably pleanty of combustion air passing through your louvered/slat door. I totally understand your desire to minimize heat loss from the rest of the house. Before any external venting will improve the situation, the utility room will have to be somewhat sealed from air flow from the living space of the house. In new construction a vent of a calculated size is brought in from the out of doors, screened and baffled, and strategically located just above the floor by the appliance. The mechanical room is almost air tight, insulated and lined in a fire rated material. Since this space is indoors and only calculated amount of outside air is allowed in, freezing is not normally a concern if water utilities are also in the room. Drawing air from an attic can be risky. Obviously some think it a good idea, however there are drawbacks. If the attic is not well ventilated the air may not be sufficient. I simple vent register in the ceiling to the attic will let as much heat up as it pulls air down. In fact, any open path into the attic can be dangerous. A register, vent pipe etc. can act as an easy path or chimney for fire or natural gas to spread quickly into the attic. If you decide to proceed in isolating and venting your open combustion appliances, pick one of the proven techniques and run it by your local building code officer for local compliance. Better safe than sorry.

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    I talked to two HVAC professionals, and they both suggested closing off the vent and letting the appliances use house air. Commented Jan 18, 2011 at 14:58

Any gas burning appliance (water heater in your case) needs to get fresh air from somewhere, and vent the exhaust somewhere outside (venting into the house is deadly).

Since it is in a utility room with a door that is closed most of the time, you need some kind of vent to get fresh air. You can pull that fresh air from either the (1) insulated living areas of the house, (2) uninsulated/unoccupied areas of the house (attic), or (3) from the outside.

From an efficiency standpoint, (1) seems like a bad idea, because it takes heated/cooled living air to vent outside. It also seems potentially unsafe, as it allows an easy path for exhaust to get back in to occupied areas if the exhaust vent has issues. (Not sure what the building codes say.)

Based on your other post, you seem to have (2), which seems to be a pretty good approach. I'm no expert, but the biggest pitfall I see with (2) is that if your attic is too tightly sealed (stuffy), the water heater could potentially not get enough air to breathe. In that case, (3) would be the best approach. But (3) allows uninsulated air into the house, which compromises heating/cooling efficiency.

Basically, I don't think your current setup is particularly inefficient. If you want to make your house more energy efficient, there are probably other things around the house that need more attention (windows, ducts, insulating hot water pipes, etc).

The only potential efficiency problem I can see with your current setup is heated/cooled living air leaking around/under the door into the utility room, and getting "wasted" by the water heater's combustion. You can fix this by making the utility room door seal better. Put something under the door to close off the gap. Weatherstripping in the door frame. Basically, treat it as if it were an exterior door. (Caution: If the air provided by the vent is not sufficient on its own, you may have issues.)

Oh, and since you have at least one gas-burning appliance, make sure you have adequate Carbon Monoxide detectors. They could save your life! You should have one inside the utility room as a first line of defense, and then one in the occupied part of that floor, plus at least one more on each of the other occupied floors of the house (particularly near your bedroom). And remember that they go bad and need to be replaced every few years!

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    I am as much concerned about heating the attic (ice dams, icicles) as using conditioned air (which of course points to (3)) Other concerns: utility room door is slated; I suppose I could replace it. Room also has my incoming water pipe, so I wouldn't want the room to get too cold. Commented Jan 8, 2011 at 20:08
  • I don't see it heating the attic much, because the air will be flowing IN from the attic, not OUT. I also wouldn't expect the utility room to get too cold, because there is a burner in there making heat. Commented Jan 8, 2011 at 21:39
  • The pipe is open on top. Warm air rises, and we expect the house to be warmer than the attic. So, especially when furnace isn't running (drawing air), house air will be rising into the attic. Commented Jan 10, 2011 at 4:03
  • When the burner for the water heater is running, the warm air of the burner will be rising out out the exhaust flue. So, air will be getting pulled into the room from any open gaps (e.g. your attic vent). Commented Jan 10, 2011 at 15:52
  • @Justin, you're correct that warm air does rise. That's partly why the vent is at the bottom of the wall in your utility room. The coldest air in that room is the only air that can rise to the attic. But since the amount of heat lost will depend on the difference between the temperature in both places, it's better to vent from the attic than the much more extreme (either hot or cold) outdoors.
    – BQ.
    Commented Jan 10, 2011 at 17:47

There's a style of ducting where the exhaust and intake are run through the same piece, one inside the other. This allows some heat exchange, as the intake air is warmed by the exhaust. The benefit is likely based on the length of the run.

Concentric vent kit:

enter image description here

Picture source: prowaterheatersupply.com

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    This type of exhaust/vent is only available on air tight units such as a Rinai closed combustion system. It is referred to as a direct vent unit. Commented Jan 9, 2011 at 15:46

You need to have a fresh air vent near the appliance otherwise you will be sucking in cold air in another room like your front door. To mitigate any heat loss, you can use the 5 gallon bucket trick. The outside air vent is connected to a 4" flexible insulated duct. The bucket sits next to the water heater with the duct in the bucket. So cold fresh air for combustion fills the bucket and is available to the appliance, but pretty much stays in the bucket and doesn't fill the whole room up with cold air.

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    That bucket isn't going to do a thing. Air is going to flow every which direction, as long as there's a high exhaust and a low air intake, until the space equalizes with the outside temperature.
    – BMitch
    Commented Feb 10, 2013 at 14:09

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