High efficiency furnace size to use 2inch vent and intake
 

Hi

I am designing the layout of the basement in a 60 year old home, theres not alot of room there (5' 9" from the support beam to the floor), for this reason its taking me days of work to arrange ducting and venting pipes for everything in the 3D CAD software.

I notice in 1000-1200 sq ft houses the high efficiency furnace are venting with 636 2inch pipe (same as in take), this is 1000 sq Ft, however thought i would ask the experts instead of assumnig this is ok, what size high efficiency furnace will need 3 inch 636 vent duct (to make sure 1000 sq ft will use a small enough furnace to require 2inch only)?

Theres only room for 2inch in this place which i think will be fine, but always best to check on these things.

Generally 40 and 60,000 BTU units. And some 80,000 BTU units.
However. it also varies with the length of the vent. You could have to go to 3" if its too long, or has too many ells in it.

Don't forget. You will also need to slope the pipe so the condensate van drain back toward the furnace while it is running.

You probably can get away with a 70,000 BTU Lennox G51 or G61 using 2" venting for that size of house. Need to do a heat loss calc to find the correct size.
Load Calculator

And as Beenthere said need to keep the pipe length and elbows under control. Check this for better idea.

http://tech.lennoxintl.com/PDFs/506508-01.pdf

p.18 and on, not all brands use 2" in that size of furnace so you need to find out B4 you buy.

Thanks for the install guide, this helps alot as it has the locations with sizing for all pipes (so i can route them in the design), do you happen to have access to the G61 as well, would like to have a look.

Exact same furnace, just has another firing rate/stage (70 and 100%).