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simonb 06-11-2009 05:54 PM

New furnace combustible air vent question
Im in the process of getting my furnace changed and the guy installing the furnace is at the vent part. He advised me that most people in my city (Timmins, Ontario) take the combustible air from the basement because of the cold weather and such.

I did a little research and found that i should have 50 cubic feet of fresh air per 1,000 BTU. I have a payne 100,000 BTU input (model PG9MAB) and my hot water tank is 36,000 BTU which gives me a total of 136,000 BTU. If my math is correct;

136,000 BTU / 1,000 BTU * 50 = 6800 cubic feet needed.

Problem here is i measured the basement and i have a total 5800 cubic feet.

Is this a big problem? I have an older home and its the basement isnt finished, yet.

Also, i was told i could use normal ABS pipe for the combustible air intake, is this correct?

yuri 06-11-2009 06:45 PM

Your guy is all wrong. All high efficiency furnaces in Canada have to be vented with 636 grade PVC pipe (CGA code). Your furnace is designed for a 2 pipe system and is more efficient with a combustion pipe attached to the furnace and a exhaust pipe (636 for both). He may be lazy or trying to save install $$. The idea is to take the combustion air for free from outside not the warmer air from your house which is worth $$. We 2 pipe all our systems except for 1% which are physically impossible to drill holes in the wall. We then vent it up the old chimney. You need 1 cu.ft of air /100,000 BTU's if I remember my code. Call your gas co and talk to an inspector about all this in case your local rules are different. That furnace probably uses a 3" pipe for intake and exhaust. Make sure he has taken out a gas permit and gets the furnace inspected by the gas co.

see this for the pipe:

simonb 06-11-2009 07:10 PM


The vent pipe will be 636 for sure, there is no doubt there. I was talking about the combustible air intake using ABS. Im reading the manual and it states


In Canada, construct all combustion-air and vent pipes for this unit of CSA or ULC listed schedule-40 PVC, PVC-DWV or ABS-DWV pipe and
pipe cement. SDR pipe is NOT approved in Canada.

yuri 06-11-2009 07:21 PM

Not sure we are talking about the same thing. That furnace should have 2) 636 pipes. One for combustion air for the furnace burner and the other for its exhaust. NO other combustion air is required. Some installers cheat and only drill one hole in the wall and take the combustion air from the house instead of outside which is inefficient. Older homes leak enough air into them so that no extra combustion air pipe is required for the water heater. Newer airtight homes by the code require one and that is subject to local building codes. We drill a 5 or 6" hole in the wall and run insulated plastic flex pipe to the furnace area, gooseneck it at the floor and that meets code. I hate seeing the cheaters single pipe units if not necessary, some newer furnaces are approved for single pipe venting, some older ones not approved. Your code info for furnace venting (manual) is out of date, ABS used to be approved but not anymore. Click that link for more info. Combustion air for a furnace is one issue, combustion air for a furnace room with water heater is a different topic.

simonb 06-11-2009 07:55 PM

Well we are somewhat on the same page, i think im just not explaining myself too well. The hot water tank only has the exhaust vent. Its taking air from the basement. The furnace is supose to have 2. One for combustible fresh air and the other for the exhaust. There is no third to provide air to the basement.

The exhaust will have 636 3" venting outside. The combustible air intake he tells me that they have allot of problems in deep winter up here and they end up cuttin it and leaving it take the combustible air from the basement. I dont know why but he sais they end up doing it to almost all of them. He also told me that since its air intake, it has nothing to do with exhaust, so there is no need to go 636. Also the installation manual does have an entire section on taking the air from the basement.

Dont get me wrong yuri, im not not beleiving you, just giving you the info im reading in the manual.

simonb 06-11-2009 08:09 PM

Is it also possible to install the flue vent bellow a window if it has enough clearance? The way i read the manual it tells me that as long as im 12" from the ground and 12" from the window im ok, and the picture seems to show vertical and horizontal.

Take a look here at page 36. Letter A and B

yuri 06-11-2009 08:18 PM

We have colder weather than you. The only problem is if the snow gets into the intake and to solve that we extend it up from the ground about 2 feet. Lennox has a kit for that. You need to keep the snow away from the intake to prevent exhaust recirculating into the intake and frosting it up (maybe happening in your climate if humid etc). I installed lots of those Carriers/Paynes and they don't have a recirc problem if you follow their recommended setup. If you get lots of deep snow then the intake needs extending up and can be ugly with 3inch pipe. I would prefer the 2 pipe system as it is much more efficient. Why burn warm house air. Your house is not so airtight that an extra combustion air pipe is required for the water heater UNLESS you enclose it in a sealed furnace room. That pipe can be plastic flex pipe or insulated galvanized. Insulated flex is easiest to work with. A open combustion air pipe will leak lots of cold air into the basement and I only recommend them if necessary. You may want to talk to a gas inspector to get the exact ruling. Ontario is quite strict about following codes.

yuri 06-11-2009 08:22 PM

You got ahead of my last post. You should be 1 foot above the expected snow level. Under 100,000 BTU 1 foot from a window is okay. I like to stay away from windows as much as possible or the exhaust can fog them up. P20-22 help with your combustion air calculations but I would talk to an inspector as the local guys have to approve it and can get fussy.

simonb 06-11-2009 08:29 PM

I wish i could go with 2" instead of 3", would make my life easier. But after looking in the book, measuring the lenght and the amount of elbows, im stuck using 3". About 15 feet and 5 elbows, not including the terminal elbow.

Myself I dont want to take air from the basement, I would much rather have the cumbustion system intake and vent 100% from the outside.

yuri 06-11-2009 08:33 PM

You absolutely need 3" with that unit!! Read my updated post above yours. DON"T use a concentric kit, nothing but freezeup problems in our climate. Buy yourself a nice Lennox G71MPP and you can go with 2 1/2" and have a self calibrating ECM ventor etc to eliminate venting problems. I'm back tomorrow.


simonb 06-11-2009 08:41 PM

Once again thanks for the answers yuri.

My furnace is the 100,000 BTU input model, does that mean i fall in in the 12" range or the higher range?

I just took two pictures to give you an idea. If i could place the intake and the flue vent bellow the window, i will be saving myself allot of work and i will be able to go 2" with 2 elbows. The pipes come out at 21" from the ground and the window is 5 feet from the 21" mark.

The white exhaust you see in the picture is for the hot water tank. The black was for my old furnace.


Originally Posted by yuri (Post 286211)
You got ahead of my last post. You should be 1 foot above the expected snow level. Under 100,000 BTU 1 foot from a window is okay. I like to stay away from windows as much as possible or the exhaust can fog them up. P20-22 help with your combustion air calculations but I would talk to an inspector as the local guys have to approve it and can get fussy.

yuri 06-12-2009 06:32 PM

Good thing you posted pics. You have several issues/problems.

1) Need to be 3 feet from the water heater vent by code, or it will recirc into the furnace combustion air pipe.

2) Need to be 3 feet from electrical meter and whatever vent is on the left wall

3) I would NEVER put pipes near a corner or wall as the wind will swirl/cause exhaust recirc into the intake and can also cause nuisance pressure switch tripping problems

4) 100,000 is okay but 1 BTU greater puts you into a higher category

Post some pics of the other location and I may be able to help

beenthere 06-12-2009 07:04 PM

Here, its 48" from the gas meter.

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