flashheatingand

Granted it's the middle of summer, but, when taking off the 4" s.w. pipe while removing the blower, frequently, the inside of the pipe is rusted. I know it's the condensation and the white crud is the byproduct.

This is what I think is the case, but not positive. If the the system has a lot of rust, it is likely that the furnace short cycles. Perhaps it's due to oversizing, or inadequet ductwork, but nonetheless, the furnace is on for three minnutes or so, and off for say eight min.

Sure some condensation is inevitable, but a lot of rust within the s.w. pipe is a sign that something is amiss with sizing and/or the ductwork.

I think I know a bit about hvac, but there is certainly room to learn more. I just wanted to hear what other hvac guys have to say about the rust and byproduct.

NickTech

sometimes a lack of draft air or secondary air causes rust. oversized units that short cycle can cause it as well, if the chimney doesnt heat up right or not for a long enough period of time moisture will linger and rust the vent pipe.

flashheatingand

I don't like to just vent to the chimney, I will run b-vent. I guess one can get away w/ single wall, but to avoid problems, I just go w/b-vent. Most of the time, the furnace is in a closet or garage, so the chimney thing isn't an issue.

I don't like to see appliances short cycle, thats one reason I reccomend programble stats and letting the temp drop/rise significantly while the premesis is unocupied.

NickTech

2 things i can think of, perhaps the blower is too fast and the flue gases are too cool and allow for condensation, or the temp surrounding the flue is too cold doing the same.

flashheatingand

Well, usually the situations in which I described the furnaces are located in garages, so that would explain the area around the flue being too cool. I have never checked the actual rpm's on an inducer. The other day, while pulling the blower, I checked out the he, and the exterior had some superficial rust.

Just hoping to hear what some of the probable causes are.

NickTech

not so much the venter rpm, but the blower pushing air over the HE. if there is too much air then it will cool the HE to quickly leaving little heat in the flue gases causing them to rise slower on a natural draft, but on a forced draft the cooler discharge temps would allow for the H2O vapor to condense in the pipe. yer looking at approx, 400 deg stack temp for gas. check what the specs are.

flashheatingand

Never took exhaust gas temps. That's an interesting way of checking how the unit is operating.