stack corrosion

[mdman]

Re:

what's you take on this.

This a 1991 Carrier Furnace. He's got corrosion or combustion products where the stack vent attaches. Is this a spill from this positive pressure vent or is he condensing in the stack? This vent joins a common vent shared with a gas DHW heater.

Is that single wall or B-vent?

Thanks, mdman

[kangaroogod]

the single wall vent should be replaced ith b-vent to your b-vent verticle chimney. the temperature rise should be set on the furnace along with verifying proper vent pressure. it is condensing for sure.

kgod

[mdman]

how can you tell it's not just spilling and condensing in the cool space (yes, I realize thats not good and needs to be sealed)?

Are you saying that the flue gas is condensing in the stack due to...
...under fire
...over blower
...low TR
...single wall vent
...too long a vent

and doing this

If flue gas is condensing down the inside of the stack that's one thing. If flue gas is spilling at the collar and condensing on exit that's another.

Which do you mean?

[Black Adder]

It is for sure condensing in the stack, look at the staining on the joints on the c vent. Possible causes are poor draft on chimney, chimney passing through uninsulated space makes the b vent too cool. Change vent to b vent from furnace to main b vent. Do you have any back venting on the water heater? This would be a sure indication of venting problems.