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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?
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.
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!
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.
Picture source: prowaterheatersupply.com
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.
