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Originally Posted by cvsmoove

Is there a formula for determining if I need 2" or 3" venting? I have a 2" pipe on my current furnace, about 22' length w/ 1-90 and 2-45 degree turns to outside wall. This does not include the outside piping. I would like to use this if I can, but don't want to cheap out at the risk of performance.

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The advantage of 2 pipe over 1 pipe is that you can enclose the whole closet or area around the furnace but with 1 pipe system you have to introduce air in the closet or area around the furnace. 100,00Btuh input is required to have 25 sq inches of open space for natural convection so A 25 Sq In hole is 5"X5" If you run a long way up the out side you will have to make the pipe pitch down hill so the water on the vent outlet will drain out DO not set it level or make a trap in it. If your chart showes a listing for the next size furnace (I m guessing a 125,000btu) and it is shown with 3" and yours is just shown with a 2" 2"will be fine.
If yours has both take the number of feet total add on 5 more feet for each 90 (and / or for each pair of 45 ells) And most of the time you don't have to count the 1st ell. Your chart and instructions will show the maximum feet you can run the vent. Use that as rule.:)

Here is the reason I've been told to use fresh air from outside for combustion(it makes sense, but I really don't think its as big of a deal as they say.) They say that to get the full eff out of the furnace you will need to take fresh air form outside of the house for combustion,, because,, by drawing the combustion air from inside the house, you are using warm(heated air) for combustion, which means this combustion air is being exhausted out of the house through the exhuast pipe.(wasting warm air). If you bring in cold air from outside, your eff will be better because you don't have to heat this air, and you are not losing warm air from in the house. Also, cold air(believe it or not) combust better, giving you the highest eff that you can get out of your furnace.

To put it simply... if you are using indoor air for combustion, you are losing whatever cfm is required for combustion... its going right out the exhuast. Now, this is one way it was explained to me, I really don't think that its going to make much difference in efficiency either way, maybe a buck or two a month, just thought I would give a little insight as to what I have been told. Lets hear your thoughts.

PS. Just to clarify,, I use one pipe settup more than two pipe. I only use two pipe when job REQUIRES it. Never had a problem with one pipe.

Here in wv we have been using just 1 pipe as much as possible. The gas company has so much moisture in there gas that with a two pipe system and breathing colder air has been causing too much condensate

The two pipes need to have the same atmospheric pressure, utilizing combustion air from inside the house will not be the same pressure and termination is important in regards to vent pipe and combustion air inlet pipe.A single pipe system still must pull combustion air from outside or a vented attic or crawl space. Not all furnaces can be used as a single pipe system, you will need to follow the manufacturer's instruction.

Wow, I don't know where to start correcting you, so ill just give up.lol.

Oh, yes you are right about the two pipes having the same atm press, but when a one pipe system(exhuast pipe only) is used, this becomes nill and void, as long as you have enough usable combustion air in the area furnace is installed.

Its obious that the last poster has not read the manufacturers installation instructions, not all 90 percent furnaces can be installed utilizing only one pipe, the single pipe installation should never be installed where it derives its combustion air from within the conditioned space as was suggested, this is contrary to to the installation instructions which state that the combustion air on a one pipe system be taken from an outside source, (vented attic, crawl space) Before offering corrections do your home work.

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Originally Posted by acetc

Its obious that the last poster has not read the manufacturers installation instructions, not all 90 percent furnaces can be installed utilizing only one pipe, the single pipe installation should never be installed where it derives its combustion air from within the conditioned space as was suggested, this is contrary to to the installation instructions which state that the combustion air on a one pipe system be taken from an outside source, (vented attic, crawl space) Before offering corrections do your home work.

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Thousands of units have been installed with 1 pipe systems, that have not had air pulled from a attic or crawl space. Lots of homes don't even have attics or crawl spaces to get the air from. And all the units I have installed with no intake what so ever have work fine now for over a decade.

When a contractor installs a single pipe 90% furnace and uses conditioned air (air already heated by furnace ) he is compromising the efficiency of the furnace. This air that is used for combustion when drawn from within the building structure has to be replaced by outside air drawn in through cracks around windows and doors, this outside air then needs to be conditioned. If you had a large room that was air tight and pulled combustion air from this space you would develop a negative pressure within this space. You say the units installed in this manner work fine, well if this is the case then why even vent the furnace? This is a sardonic question, installing a furnace improperly does not make it right.

No house is sealed so tight that it will develop a negative pressure from a 1 pipe system. Inless you put it in a 2 foot room. Why even vent the furnace? What kind of silly question is that you trying to kill some one? And what brand unit are you refuring to that says it must come from a vented attic or crawl space?

I believe we should agree to disagree. The asker has already made provisions to have the furnace professionally installed, so its up to the installer to make his decisions now. Take care all, no hard feelings and lets move on to the next one.