Condensations in Vent pipe

[Steve Lowery]

'Morning all, this past Saturday I inspected a house for my nephew. Built in '62, new(er) heating unit, auto ignite, older Amanna condenser outside
with dog whiz corrosion in spots.

Couldn't get on a Rheem site to confirm age (I'm guessing 1 -2 Years from how clean it is) but my concern is that it vents into single wall galvanized pipe ( 3" ) which dumps into a 7"-8" transite pipe above the ceiling line.

There is a white fuzzy material growing along lines that have obviously leaked fron the pipe joints and the heater has a powered vent.

Would the condensing stop if it were vented in B-vent carried all the way to the roof jack. How are the condensates kept rom draining back down to the unit and rotting things away?

[Nick Ostrowski]

Steve, the chimney flue may need to be relined to take into account the higher efficiency of the newer heating unit. A house built in 1962 had a 1962 heating system which had much lower efficency than today's systems. Systems with lower efficiencies had higher flue gas temperatures which helped ensure proper drafting in the flue. Fast foward to an 80% efficiency heating system of today's standards and the flue gas temps will be lower (flue temp drops as efficiency levels rise). The inducer fan on the furnace can only do so much assist in the drafting when you have a flue that is oversized.

[Matt Fellman]

There's a white fuzzy material that is often found around gas burning furance exhaust parts. It's a reaction between sulfur from combustion of the natural gas and the galvanized (zinc) on the vent pipe. Sometimes, there's quite a bit of it. Without a picture it's hard to say but what you describe might be this stuff. I'd think of condensation stains as being two-demensional... the stuff I'm talking about it more 3D.

[Gunnar Alquist]

Steve,

As you have surmised, water vapor will condense on the interior of the cold transite pipe and drain down to the metal. Since the condensed moisture is somewhat acidic, the metal flue pipe will eventually rust out. I have seen some pretty good ones over the years. The transite should be removed and a b-vent used to replace it.

On a similar note, I have been told by HVAC guys that single walled should not be used at all. That the manufacturers' installation instructions require the use of a b-vent for the full length. I have not been able to find that, though.