I have a Carrier weathermaker gas furnace which is about 12 years old and works well. The exhaust piping goes up (4" pipe) then connects to a 6" pipe and then connects with elbow to an horizontal pipe (about 5' long) than connects upward to chimney. The horizontal pipe is angled slighlty downwards (about 11/2inches over the 5' lenght). Is there a safety issue with the horizontal piping sloping slightly downwards ?

What would happen if you cut the 4" pipe and drop it? Would the slope improve? ASHRAE code calls for 1/4" per foot positive slope. CO is not something you want in the house. You can buy piping at HD or Lowe's to make changes and I would also use approved metal tape to seal the piping.

Sloping downward toward the furnace, or the chimney? If it is sloping down to the furnace, you are fine. If it is sloping down to the chimney, that's not ideal.

What would happen if you cut the 4" pipe and drop it? Would the slope improve? ASHRAE code calls for 1/4" per foot positive slope. CO is not something you want in the house. You can buy piping at HD or Lowe's to make changes and I would also use approved metal tape to seal the piping.

I can drop the 4" pipe to improve the sloping but that will cut the headroom substantially i.e.. The horizontal pipe would be about 5 1/2' above floor (lowest point) as opposed to about 7' now. The reason it would have to drop so much is that the exhaust pipe must run above or below a water pipe. Currently it runs above the water pipe, hence the downward slope to the chimney. FYI, this was brought to my attention only recently - I haven't had any problems for 10 years. Should I go with the lower headroom for peace of mind or is it unlikely to be a problem given that there hasn't been a problem for a long time (my CO detector has never gone off) ?

If nothing else ties into the six inch pipe, it could be reduced to 4" since it is only a five foot run. Only going to gain an inch here, unless you can raise it at the chimney, which would give you a true two inches of adjustment.