Does furnace need outdoor air supply?
bergsteiger1
| Posted in Energy, Heating & Insulation on
I have a 2600 square foot house with two floors above ground and an unfinished basement. A new forced air furnace with A/C was recently installed. I noticed there is a duct running between the cold air return duct near the top of the furnace and the outside of the house. I asked the installer if I should remove it but he said I should keep it. I don’t understand the need for supplying additional outside-temperature air to the furnace as it seems like it would increase the heating and cooling load. Can anyone please shed any light on the purpose of this duct and any reasons I should or should not remove it? BTW, my previous house in the same locality and of the same size had the same furnace but no outside duct. Thank you
Replies
If this is a gas furnace it is most likely combustion air. It shouldn't run to the return air but to the burner. Otherwise I'd see no reason for it.
It is a gas furnace but it is definitely not combustion air in that it is simply a duct from the cold air return to the outside. It just allows outside air to be sucked into the conditioned air passing through the furnace. I just can't understand why you would do this. Thanks for your comment.
It's probably an attempt at make up air for bathroom fans and kitchen fan.
Get rid of it. Unless your home was custom built and an air leakage test was done, it has enough leaks without needing extra "make-up air". Your utility bills will be unacceptably high.
Perhaps the installer assumed that you are going to put in an air exchange unit.
I've seen so many people with such units experience astronomical utility bills because they (and no one) knows where to set the dial on the (completely unscientific) scale of 1-10. One has to know the cubic feet of the home, how many cubic feet to replace per hour and the actual calibration of that scale.--and of course, air replacement depends on how many people live in the house, their activities and even whether they cook with gas, etc.
You don’t mention whether your furnace has an intake for combustion air from the outside. If it does not, it’s getting combustion air from the living space. The process of combustion uses up oxygen, which, it happens, is necessary for humans to not die. If you close off that duct and don’t have another means for outside air to come into the house, the combustion happening in your furnace could cause the oxygen concentration in your indoor air to drop below the level required to sustain human life.
Millions of homes, mine included, don't have combustion air coming from the outside. Nevertheless, it tends to be self-regulating. The more leaky your home the more your furnace will come on and need the air from the leaks. The tighter your home, the more efficiently heated it is and then the furnace doesn't come on as frequently.
The makeup air is coming from outside regardless. The only question is whether it’s coming through leaks in a leaky envelope or through an intentional hole sized to replace the air being used by the appliance for combustion and vented out of the house. The unconditioned outside air is 100% going to come inside when the furnace is running. Would you rather have it mixed with air in your duct system to temper it before it’s introduced to the home, or have it introduced to the home directly via some crack you’re unaware of?
Go to the Energy Vanguard website and ask Allison , he'll know what course you should take.
The duct could be satisfying the need for a whole-house ventilation to address indoor air quality, see Section R303.4 for code requirements:
https://up.codes/viewer/colorado/irc-2021/chapter/3/building-planning#R303.4
The duct to the exterior could also be part of what is typically called an economizer cycle, that introduces exterior air to reduce fuel consumption, during various specific conditions.