Should have read the manual - 90% Heil furnace, 100K BTUs calls for 3" PVC pipe. Allows 5 elbows (90s) & a 40' run. I have 2" PVC with only two 90s (sweep) & one 22.5 with a 15' run. Furnace is running well but I should probably change out the vent pipe. Any advice? - Dave

Yes, change the pipe to meet the manufacturers spec. Many high efficient furnaces now use a 4" PVC. You must have installed a reducer and that is where you should start with the replacement.

Yes, change the pipe to meet the manufacturers spec. Many high efficient furnaces now use a 4" PVC. You must have installed a reducer and that is where you should start with the replacement.
Will do, although I didn't use a reducer, it was 2" out of the starting blocks. I'll add a 2" to 3" on the vertical right out of the housing and change out the pipe.

I don't know why some manufacturers do that but my Amana furnace comes out of the chamber as a 3" PVC but the specs then call for a 4" exhaust. I think that possibly if the exhaust is larger than the chamber it reduces pressue in the exhaust and makes the flow of exhaust move more easily.

If you did the install and start up yourself, did you check the air flow? If you set the inducer speed high enough you may get away with restrictive vent.

There are a number of issues with a too small of a vent, where the factory spec. is 3 inches. As the inducer get old, it will not be able to pull in the pressure switch, and at this point, you may be on the very edge of the pressure switch making. By checking the rise through the furnace, you may find that it is on the high side... where it should by about 55 degree F difference with correct air flow through the heat exchanger and circulation blower on the proper tap. Another important issue is that you may not a long life out of the heat exchanger... it is best to change it to 3 inch, use an increaser close to the furnace. ( if this is a 2-pipe furnace... be sure to change both to 3 inch )

I think TSA did a good job of explaining the issue. I believe that your warranty would not be honored if you did not follow Heil's install instructions.