Furnace venting

[OVERKILL]

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:

 

[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...

 

[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.

 

[irv]

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.

 

[supton]

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.

 

[OVERKILL]

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

 

[doitmyself]

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.

 

[OVERKILL]

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.

 

[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.

 

[OVERKILL]

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.

 

[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.

 

[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.

 

[Wolf359]

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.

 

[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.

 

[OVERKILL]

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.

 

[bmwjohn]

This is why I really appreciate BITOG - and donate sometimes. Almost any question asked will have at least ONE answer that is at least 50% or more correct. And in the US, and probably Canada, skilled technicians are rare, and you get unskilled workers (and sometimes instructors as well). Old story....I read in the Revolutionary era apprentices could stay in England an extra year to become a real craftsman, or quit and move to America and be considered a craftsman from the get go.

 

[irv]

Originally Posted By: doitmyself

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.

Unionized has nothing to do with it as the same type of work, whether unionized or not also exists out there. Many certified trades people exist that are not unionized. The trick is to pick a good, reliable, dependable company that has great reviews or references from friends and family that will back/correct their work if a problem arises.

 

[irv]

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.

Like what happened to, Overkill, if that were the case, the furnace will trip, or should. It is also the homeowners responsibility to clear the snow around those if that becomes an issue. It is unreasonable to blame this on anyone but the homeowner, unless, of course, the vents/intake were only a few inches above the ground.

 

[JHZR2]

Originally Posted By: OVERKILL

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.

Kris's idea is IMO a good one, just dont cement them in place, observe, and adjust. I think that cutting a custom thig will not look as "finished" personally, and really is just for some feel-good anyway. Like I mentioned, ours is coaxial, so has some minimal heat exchange on the pipe itself, but it vents horizontally out without issue. Its beneath the eave so I dont have any real concern of captured rainwater. Maybe some other location/geometry would differ...

 

[dnewton3]

Do not worry about rain getting into either the exhaust or intake. Both systems are designed to deal with moisture. In fact, there's likely a p-trap in the system so that there's no gases that pass back into the system. If you remove the door and really study the system, you'll see what I mean. Both intake and exhaust have a tie into a drain (or at least should) that will exit the unit. I'd just stub them out like the clean install you modified and leave it at that.