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Furnace venting #4627064
01/07/18 03:11 PM
01/07/18 03:11 PM
Joined: Apr 2008
Posts: 37,916
Ontario, Canada
OVERKILL Online content OP
OVERKILL  Online Content OP
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Posts: 37,916
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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Re: Furnace venting [Re: OVERKILL] #4627071
01/07/18 03:21 PM
01/07/18 03:21 PM
Joined: Dec 2002
Posts: 42,293
New Jersey
JHZR2 Offline
JHZR2  Offline
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Posts: 42,293
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...


Re: Furnace venting [Re: OVERKILL] #4627075
01/07/18 03:28 PM
01/07/18 03:28 PM
Joined: Jul 2012
Posts: 2,561
South Carolina
alarmguy Offline
alarmguy  Offline
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Posts: 2,561
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.

Last edited by alarmguy; 01/07/18 03:28 PM.

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Re: Furnace venting [Re: OVERKILL] #4627079
01/07/18 03:29 PM
01/07/18 03:29 PM
Joined: Oct 2006
Posts: 1,881
Oshawa, Ont. Canada
irv Offline
irv  Offline
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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.

Last edited by irv; 01/07/18 03:34 PM.

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Re: Furnace venting [Re: alarmguy] #4627085
01/07/18 03:30 PM
01/07/18 03:30 PM
Joined: Nov 2008
Posts: 12,761
NH
supton Offline
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Posts: 12,761
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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Re: Furnace venting [Re: JHZR2] #4627130
01/07/18 03:59 PM
01/07/18 03:59 PM
Joined: Apr 2008
Posts: 37,916
Ontario, Canada
OVERKILL Online content OP
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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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Re: Furnace venting [Re: OVERKILL] #4627139
01/07/18 04:09 PM
01/07/18 04:09 PM
Joined: Jun 2002
Posts: 6,438
MI
doitmyself Offline
doitmyself  Offline
Joined: Jun 2002
Posts: 6,438
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.

Last edited by doitmyself; 01/07/18 04:14 PM.
Re: Furnace venting [Re: doitmyself] #4627140
01/07/18 04:13 PM
01/07/18 04:13 PM
Joined: Apr 2008
Posts: 37,916
Ontario, Canada
OVERKILL Online content OP
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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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2016 Grand Cherokee SRT
Re: Furnace venting [Re: OVERKILL] #4627142
01/07/18 04:15 PM
01/07/18 04:15 PM
Joined: Dec 2006
Posts: 8,011
Canuck living in California
KrisZ Online content
KrisZ  Online Content
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Posts: 8,011
Canuck living in California
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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Re: Furnace venting [Re: KrisZ] #4627145
01/07/18 04:19 PM
01/07/18 04:19 PM
Joined: Apr 2008
Posts: 37,916
Ontario, Canada
OVERKILL Online content OP
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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.


2018 RAM 1500 Big Horn EcoDiesel
2016 Grand Cherokee SRT
Re: Furnace venting [Re: OVERKILL] #4627154
01/07/18 04:24 PM
01/07/18 04:24 PM
Joined: Apr 2012
Posts: 5,576
MA
Wolf359 Offline
Wolf359  Offline
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Posts: 5,576
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.

Re: Furnace venting [Re: Wolf359] #4627158
01/07/18 04:25 PM
01/07/18 04:25 PM
Joined: Apr 2008
Posts: 37,916
Ontario, Canada
OVERKILL Online content OP
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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.


2018 RAM 1500 Big Horn EcoDiesel
2016 Grand Cherokee SRT
Re: Furnace venting [Re: OVERKILL] #4627167
01/07/18 04:35 PM
01/07/18 04:35 PM
Joined: Apr 2012
Posts: 5,576
MA
Wolf359 Offline
Wolf359  Offline
Joined: Apr 2012
Posts: 5,576
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.

Re: Furnace venting [Re: OVERKILL] #4627170
01/07/18 04:36 PM
01/07/18 04:36 PM
Joined: Apr 2008
Posts: 83
Leesport, PA
MedicRxDoc Offline
MedicRxDoc  Offline
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Posts: 83
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.

Re: Furnace venting [Re: MedicRxDoc] #4627172
01/07/18 04:39 PM
01/07/18 04:39 PM
Joined: Apr 2008
Posts: 37,916
Ontario, Canada
OVERKILL Online content OP
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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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