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#4627064 - 01/07/18 03:11 PM Furnace venting
OVERKILL Online   content


Registered: 04/28/08
Posts: 36451
Loc: Ontario, Canada
So these last few weeks, with the insanely cold temperatures we've been seeing, I've had two instances where the inlet vent on my furnace has frosted over, causing my furnace to shut off. In both cases this happened over night and we woke up to a house that was quite cold (13 degrees C). Took me a bit to figure out what was wrong the first time, but the warning LED on the furnace gave me a clue and I figured it out.

I have a power-vented natural gas furnace and a power-vented natural gas hot water heater. All exit the house on the same run of concrete, as does the inlet vent for the furnace. A quick glance at the manual for the furnace indicated that mine was configured incorrectly and some internet searching shows that this is uncomfortably common, despite furnace manufacturers giving some rather detailed instructions on how vent and intake are supposed to be oriented. The intake is supposed to be BELOW by 12" the exhaust so that when the hot exhaust exits, it doesn't cause condensate to frost-over the inlet screen, which is exactly what was happening. Here is what my install looked like:



Left-to-right: Furnace inlet, furnace exhaust, hot water heater exhaust. You can see how little snow there is around the pipes from the hot exhaust melting it.


Given the snow load we get, I opted for the "Alternate Horizontal Vent Termination (Dual Pipe)" configuration as indicated in my furnace manual:



With a 2nd vent added for the hot water heater.

The finished product:



However, I'm concerned about rain getting into the vents when the aren't in use and have considered putting small extensions on them with a 45-degree slash-cut, what do you folks think about that? Nothing long, maybe 2" or so?

Something like this, but not as hokey:
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#4627071 - 01/07/18 03:21 PM Re: Furnace venting [Re: OVERKILL]
JHZR2 Offline



Registered: 12/14/02
Posts: 41784
Loc: New Jersey
We run a mod-con boiler so all the vapor gets knocked out inside the home. Our intake and exhaust are also concentric perhaps to avoid some of this, and assure some level of warmth at the exterior intake.

Not sure why you wouldnt just use a street 45 onto the vents to help cover it a bit...


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#4627075 - 01/07/18 03:28 PM Re: Furnace venting [Re: OVERKILL]
alarmguy Offline


Registered: 07/10/12
Posts: 2152
Loc: South Carolina
Amazing and you were able to correct the plumbers installation because of the Internet.

I assume everyone who has a home that uses any type of fossil fuel for any purpose has a working CO detector.


Edited by alarmguy (01/07/18 03:28 PM)
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#4627079 - 01/07/18 03:29 PM Re: Furnace venting [Re: OVERKILL]
irv Offline


Registered: 10/08/06
Posts: 1063
Loc: Oshawa, Ont. Canada
I was going to comment, before I seen your fix, that your intake should be pointed down. Your other 2 fixes also look fine to me as our's has been like that for over 17+ years now without an issue.
Our's exits out the back of the house, or on the south side, but I do see what you are talking about as far as rain goes. Personal choice I suppose, but I think you'll be alright the way you have it currently configured.


Edited by irv (01/07/18 03:34 PM)
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#4627085 - 01/07/18 03:30 PM Re: Furnace venting [Re: alarmguy]
supton Online   content


Registered: 11/09/08
Posts: 11963
Loc: NH
Originally Posted By: alarmguy
Amazing and you were able to correct the plumbers installation because of the Internet.

I assume everyone who has a home that uses any type of fossil fuel for any purpose has a working CO detector.


Ironically the only time I've seen my CO detector move off zero was when I was using my wood stove. [I immediately stopped using said stove.] I don't think "fossil" is the right descriptor, I think anything requiring combustion is--any fuel source can make CO.
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#4627130 - 01/07/18 03:59 PM Re: Furnace venting [Re: JHZR2]
OVERKILL Online   content


Registered: 04/28/08
Posts: 36451
Loc: Ontario, Canada
Originally Posted By: JHZR2
We run a mod-con boiler so all the vapor gets knocked out inside the home. Our intake and exhaust are also concentric perhaps to avoid some of this, and assure some level of warmth at the exterior intake.

Not sure why you wouldnt just use a street 45 onto the vents to help cover it a bit...



I would, I just don't want to direct the exhaust down at all, which is why I thought a small slash-cut piece on the elbow that's there might be more appropriate shrug
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#4627139 - 01/07/18 04:09 PM Re: Furnace venting [Re: OVERKILL]
doitmyself Offline


Registered: 06/03/02
Posts: 6218
Loc: MI
I like the less restrictive very short angle cut pipe idea. Any concern about moist air wafting into the gray telephone connection box and oxidizing the contacts?

Last week I serviced our "broken" greenhouse heater where a bird got into the exhaust vent and made its way down to the exhaust blower assembly. Just a thought.


EDIT: Also, I always scratch my head a bit when I trust/pay for certified union trades people and then discover stuff like this.


Edited by doitmyself (01/07/18 04:14 PM)

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#4627140 - 01/07/18 04:13 PM Re: Furnace venting [Re: doitmyself]
OVERKILL Online   content


Registered: 04/28/08
Posts: 36451
Loc: Ontario, Canada
Originally Posted By: doitmyself
I like the very short angle cut pipe idea. Any concern about moist air wafting into the gray telephone connection box and oxidizing the contacts?

Last week I serviced our "broken" greenhouse heater where a bird got into the exhaust vent and made its way down to the exhaust blower assembly. Just a thought.


That grey box is actual the Coax connector for the cable. I don't believe it's even connected to anything, as the cable runs straight in the basement and up to my cable modem. The phone box is on the other side of the house.

Yeah, I liked the slash-cut idea myself (hence the question). Figured I'd put some pipe in the mitre box and make some angle-cuts.
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#4627142 - 01/07/18 04:15 PM Re: Furnace venting [Re: OVERKILL]
KrisZ Online   happy


Registered: 12/23/06
Posts: 7468
Loc: Toronto, Canada
I don't think the exhaust pipe outlet angle is the problem. The distance between them is. With the correct wind direction, when one of the pipes is venting and not the other, the vapors will hit the inactive pipe, condense and freeze.
Like JHZR2 suggested, I would use the 45, but clock the middle one at 6 o'clock and the far right one, down and slightly away, like 5 o'clock. Or use a 45 on one and extend the other one.
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#4627145 - 01/07/18 04:19 PM Re: Furnace venting [Re: KrisZ]
OVERKILL Online   content


Registered: 04/28/08
Posts: 36451
Loc: Ontario, Canada
Originally Posted By: KrisZ
I don't think the exhaust pipe outlet angle is the problem. The distance between them is. With the correct wind direction, when one of the pipes is venting and not the other, the vapors will hit the inactive pipe, condense and freeze.
Like JHZR2 suggested, I would use the 45, but clock the middle one at 6 o'clock and the far right one, down and slightly away, like 5 o'clock. Or use a 45 on one and extend the other one.


I don't think there's much risk of that, the middle pipe is the furnace vent, it runs far, FAR more frequently than the one to the right of it (hot water heater) and the wind generally pushes to the left, which is why the inlet kept getting frosted over in the previous configuration. This is the corner of the house, just to the right of the downspout is the end of the wall if that helps with a visualization?

I believe I've completely rectified the frosting-over problem. I'm just concerned about rain ingress, which is why I asked about the slash-cut.
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#4627154 - 01/07/18 04:24 PM Re: Furnace venting [Re: OVERKILL]
Wolf359 Offline


Registered: 04/27/12
Posts: 4752
Loc: MA
You might also want to check your local code. Around here they raised the minimum height of those vents. Seems that when it snows a lot, the snow blocks the intake/exhaust which can cause carbon monoxide poisoning.

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#4627158 - 01/07/18 04:25 PM Re: Furnace venting [Re: Wolf359]
OVERKILL Online   content


Registered: 04/28/08
Posts: 36451
Loc: Ontario, Canada
Originally Posted By: Wolf359
You might also want to check your local code. Around here they raised the minimum height of those vents. Seems that when it snows a lot, the snow blocks the intake/exhaust which can cause carbon monoxide poisoning.


It was already within code (inspected by Enbridge last year). I've raised them significantly with this modification as you can see.
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#4627167 - 01/07/18 04:35 PM Re: Furnace venting [Re: OVERKILL]
Wolf359 Offline


Registered: 04/27/12
Posts: 4752
Loc: MA
Originally Posted By: OVERKILL
Originally Posted By: Wolf359
You might also want to check your local code. Around here they raised the minimum height of those vents. Seems that when it snows a lot, the snow blocks the intake/exhaust which can cause carbon monoxide poisoning.


It was already within code (inspected by Enbridge last year). I've raised them significantly with this modification as you can see.


I believe ours used to be one feet, then they changed it to two or three feet. But some code says at least 12 inches above the highest anticipated snow level. Then there's all that other code regarding how close to windows etc.

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#4627170 - 01/07/18 04:36 PM Re: Furnace venting [Re: OVERKILL]
MedicRxDoc Offline


Registered: 04/12/08
Posts: 80
Loc: Leesport, PA
Wouldn’t these 2 x 90 bends and the additional vertical pipe need to be added to the calculation to make sure the vent isn’t too restrictive?

For my propane water heater and the type of vent pipe, there was a calculation of maximum of linear footage that could be used. In my case, it was 50 foot. Each 90angle was equivalent to 10 foot. The concentric vent terminus was given a number too. I then had my horizolnal run length. All these need to be less than the max of 50.

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#4627172 - 01/07/18 04:39 PM Re: Furnace venting [Re: MedicRxDoc]
OVERKILL Online   content


Registered: 04/28/08
Posts: 36451
Loc: Ontario, Canada
Originally Posted By: MedicRxDoc
Wouldn’t these 2 x 90 bends and the additional vertical pipe need to be added to the calculation to make sure the vent isn’t too restrictive?

For my propane water heater and the type of vent pipe, there was a calculation of maximum of linear footage that could be used. In my case, it was 50 foot. Each 90angle was equivalent to 10 foot. The concentric vent terminus was given a number too. I then had my horizolnal run length. All these need to be less than the max of 50.


The furnace and water heater are directly behind that wall, there isn't a huge amount of pipe length.
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