Is this a true statement?

This is taken from the “How to inspect HVAC systems” of the online course. I was wondering if the last paragraph (concerning draining the condensate back to the unit) was a true statement. It seems false to me. Just wondering.

"PVC Pipe Venting

High-efficiency furnaces use PVC piping to vent combustion gases and byproducts outside. You usually see the PVC pipe extending from the furnace through the wall to the outside. A PVC vent pipe will likely indicate a high-efficiency gas heating system. The pipe is typically 2 inches in diameter. PVC is also used to bring fresh outdoor air into the system for combustion. The length could be very long – sometimes as long as 60 feet.

The temperatures of the combustion byproducts are low – 100° F to 150° F. That’s very cool. The PVC does not melt. It will feel warm to the touch. That’s one way to determine which pipe is the exhaust.

High-efficiency furnaces should not be vented into a chimney. The exhaust gases from a high-efficiency heating system are too cool to create enough chimney draft. The cool gases will condense inside the chimney and cause damage.

PVC pipes need a proper slope. The pipe should be sloped down and toward the furnace, or slope up and away from the furnace. Typically, ¼-inch per linear foot is recommended. The pipe should be sloped and adequately supported so that condensate does not form and puddle inside a sagging part of the vent pipe. **The condensate should be allowed to drain back toward the furnace. **

Yes, true.

Here are the installation instructions for a Carrier High-efficiency furnace:

Look on page 5 from link:
“All condensate formed in the vent must run back toward furnace for proper drainage. Install vent pipe with no less than a 1/4 inch per foot slope from furnace to vent terminal.”

Thank you sir. The course was stating proper slope but I failed to understand the direction of the slope. Your link is very informative.


If allowing this to drain to the exterior in the winter would cause an ice dam outside of the home.

And you see a few of those around here in the winter with water draining back into the house. A nice 22 1/2 will fix that but not the cause.

The part that is wrong is the part where it says 1/4 grade away from the unit. That part is false.