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#4601307 - 12/12/17 08:15 AM HVAC Combustion Air Intake - Worth Adding?
TWG1572 Offline


Registered: 12/18/07
Posts: 558
Loc: Madison, Wisconsin
We have a house that was built in the early 2000's. It seems to be fairly tightly built for the time - spray foamed, sheathed with foam insulation, etc.

It's always bothered me that the furnace is sucking in combustion air from the inside of the house, vs. an outside air intake like you see in newer houses. We do have a duct to the outside air in the return on the HVAC unit, which I'm assuming helps to provide any make-up air from the combustion. But the furnace is still burning conditioned air, since our basement is finished.

I've not found much online that would indicate whether it's worthwhile to add an air intake for the furnace. It's something I could easily do if it would make sense financially. Well, I'd swallow hard drilling a hole in the side of my house for the pipe. But the rest is simple cutting and gluing of pipe. I can handle that.

Any thoughts? Or is it one of those things that isn't going to make much of a difference either way?

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#4601316 - 12/12/17 08:25 AM Re: HVAC Combustion Air Intake - Worth Adding? [Re: TWG1572]
Leo99 Online   content


Registered: 03/30/14
Posts: 3051
Loc: NJ
Why does it bother you that you're recirculating your air?
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#4601328 - 12/12/17 08:30 AM Re: HVAC Combustion Air Intake - Worth Adding? [Re: Leo99]
JohnnyJohnson Offline


Registered: 10/22/09
Posts: 2750
Loc: Wet side WA
Originally Posted By: Leo99
Why does it bother you that you're recirculating your air?


No you're missing the point he's using his heated air in the house for combustion rather than sucking in cold air from outside to feed the fire. So right now he's pulling in heated air and sending it up the chimney. He's losing a few percentage points in efficiency. Lot of the newer woodstoves also do this.


Edited by JohnnyJohnson (12/12/17 08:33 AM)
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#4601339 - 12/12/17 08:42 AM Re: HVAC Combustion Air Intake - Worth Adding? [Re: JohnnyJohnson]
TWG1572 Offline


Registered: 12/18/07
Posts: 558
Loc: Madison, Wisconsin
Originally Posted By: JohnnyJohnson
Originally Posted By: Leo99
Why does it bother you that you're recirculating your air?


No you're missing the point he's using his heated air in the house for combustion rather than sucking in cold air from outside to feed the fire. So right now he's pulling in heated air and sending it up the chimney. He's losing a few percentage points in efficiency. Lot of the newer woodstoves also do this.


Correct. Sorry if I wasn't clear in my OP. The question is whether the efficiency gains would be meaningful enough to make it worthwhile to install a outside air intake.


Edited by TWG1572 (12/12/17 08:43 AM)

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#4601370 - 12/12/17 09:03 AM Re: HVAC Combustion Air Intake - Worth Adding? [Re: TWG1572]
LoneRanger Offline


Registered: 07/02/07
Posts: 4058
Loc: Midwest USA
Did just that on our last house on account of it had a fireplace on one end and the furnace on opposite end of house in laundry/utility room. If the fireplace (traditional wood burning) was going, no matter the flue setting it would backdraft the exhaust gas duct pipe on the furnace and whenever the furnace would light the CO fumes would infiltrate into the utility room and start heading for the living spaces if the utility room door was left open. Had a CO detector plugged into outlet in hallway to utility room and it would set it off if the interior door to utility room wasn't shut.

My dad and I put in a combustion air source duct pipe. Above the utility room was soffit vented attic above garage, we just ran a section of 4" galvanized duct pipe through ceiling up into the attic and put a threshold on the floor to meet the utility room door, some stick-on weather stripping along the door frame to make the utility room's interior door "sort of" air tight to ensure furnace would draw through the combustion air duct we'd installed. Worked out well.
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#4601389 - 12/12/17 09:12 AM Re: HVAC Combustion Air Intake - Worth Adding? [Re: TWG1572]
bepperb Offline


Registered: 01/10/08
Posts: 4985
Loc: Milwaukee, WI
Here in WI I think it is worth running it. Our current and last houses had one, I would add one if they did not. It does not change the efficiency of the furnace, but it does increase the efficiency of the home. But also, while you said you have an outside air intake to your home the make-up air from the furnace will still come in other unwanted places (windows, doors... etc).
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#4601409 - 12/12/17 09:21 AM Re: HVAC Combustion Air Intake - Worth Adding? [Re: TWG1572]
emmett442 Offline


Registered: 12/07/16
Posts: 177
Loc: Wisconsin
This method isn't always a bad thing.

These newer tightly build homes don't breathe very well like the old leaky ones did. In the winter months, this can turn into a condensation problem as humidity from daily living activities builds up, increases the dew point, and condenses on colder parts of the house (windows, for instance). Now that some of these problems have been identified, newer leak free homes now have a whole house ventilator to maintain indoor air quality. That wasn't the case in standard homes build in the 90's and 2000's.

So, in effect, your furnace is doubling as a ventilator. It's burning and exhausting the humid air and drawing in the fresh, dry air - keeping relative humidity in check when it matter most - wintertime.

I would expect a decent increase in Relative Humidity if you moved the furnace intake to the outdoors. Whether or not it would be a problem isn't really predictable at this point.

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#4601604 - 12/12/17 12:38 PM Re: HVAC Combustion Air Intake - Worth Adding? [Re: emmett442]
TWG1572 Offline


Registered: 12/18/07
Posts: 558
Loc: Madison, Wisconsin
Originally Posted By: emmett442


I would expect a decent increase in Relative Humidity if you moved the furnace intake to the outdoors. Whether or not it would be a problem isn't really predictable at this point.



Good point. We do have a whole house humidifier on the furnace today. My perception is that it runs pretty frequently in the winter, but I can see where the issue could happen. I suppose I could open the damper wider on the return to bring in more dry outside air, but the risk is that it takes me back to square one as far as efficiency gains.

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#4601683 - 12/12/17 01:45 PM Re: HVAC Combustion Air Intake - Worth Adding? [Re: TWG1572]
SHOZ Offline


Registered: 06/28/03
Posts: 5750
Loc: Illinois
You need some constant ventilation to keep pollutants and CO2 down in a house. The more mammals the worse the CO2 will be if the house is all sealed up.

Gas stove or cooktop?
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#4601721 - 12/12/17 02:12 PM Re: HVAC Combustion Air Intake - Worth Adding? [Re: TWG1572]
EricG Offline


Registered: 03/03/09
Posts: 148
Loc: SW Louisiana
What efficency is the gas furnace 80%, 92% or 96%?
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#4601825 - 12/12/17 04:24 PM Re: HVAC Combustion Air Intake - Worth Adding? [Re: TWG1572]
Oldmoparguy1 Offline


Registered: 01/21/05
Posts: 5613
Loc: Charlotte, NC
Quote:
Correct. Sorry if I wasn't clear in my OP. The question is whether the efficiency gains would be meaningful enough to make it worthwhile to install a outside air intake.

When you use inside air, you pull outside air from every crack and cranny in the house. In well sealed houses, you could pull from the dryer vent, the stack for a gas water heater, any external vent, like a bathroom fan.

My first house back in Michigan was old and very leaky. When the furnace ran, the basement would actually get colder. I did many things to seal the house, one of the most noticeable was the external vent for combustion air. I used a modified drier vent, ran a 4" pipe across the ceiling and down right next to the burner. That winter my wife could do the laundry without wearing a coat.
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#4601845 - 12/12/17 04:47 PM Re: HVAC Combustion Air Intake - Worth Adding? [Re: TWG1572]
JustinH Offline


Registered: 04/17/04
Posts: 5177
Loc: Texas
I always thought the 80 percent furnaces recirculate air and exhaust through a chimney exhaust, while the high efficiency ones use two PVC pipes for exhaust and intake that are ran outside the house.
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#4601920 - 12/12/17 06:08 PM Re: HVAC Combustion Air Intake - Worth Adding? [Re: TWG1572]
04SE Offline


Registered: 05/04/07
Posts: 915
Loc: IL
I recently added a combustion air vent to our 95% furnace. The utility room has a floor vent to crawl and ceiling vent to WELL ventilated attic. I had the PVC laying around and my cost was only my time. I have noticed a difference so far. My wife even commented on it. It seems that even with what is deemed 'code approved' ventilation in the utility room it was still creating a negative pressure in the house and pulling in loads of cold air. We have noticed the furnace running less and for shorter periods of time so far this winter.

At first I thought I was crazy but it really has made a difference.

In researching it I read that some high efficiency furnaces have problems if the exhaust and intake see different atmospheric pressures, even small differences because the intake wasn't properly piped outside the home. Ours was just stubbed up like 3 inches and sucking conditioned air while replacing it with cold air from every leak we had.
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#4602172 - 12/12/17 09:38 PM Re: HVAC Combustion Air Intake - Worth Adding? [Re: EricG]
TWG1572 Offline


Registered: 12/18/07
Posts: 558
Loc: Madison, Wisconsin
Originally Posted By: SHOZ
You need some constant ventilation to keep pollutants and CO2 down in a house. The more mammals the worse the CO2 will be if the house is all sealed up.

Gas stove or cooktop?
Gas stove


Originally Posted By: EricG
What efficency is the gas furnace 80%, 92% or 96%?
It's a condensing gas furnace, but I can't find specs on the furnace online. If it's condensing, it looks like it is 90+ from what I read, but I could be wrong.

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