Ice/Condensation Around Aluminum Chimney HVAC Exhaust

[jp1818]

Here is the situation, moved into this house about 1 year ago in the winter. About 6 months later we replaced the roof, the roofer also replaced the aluminum chimney that wraps the exhaust vent for a high-efficiency gas furnace. The duct/vent is 8" and it runs thru an unheater attic area before exiting the roof via the false chimney wrap. The duct work is insulated in the attic, except for the last few feet beyond the roof line.

Prior to having the roof replaced I never saw any ice form around the chimney like we have now. When the furnace is running you can watch the water drip down from the top of the chimney, some of it drips back into attic.

Thoughts on what is causing this? Please see the attached pic.

Thanks

 

[MintJulep]

Your flue gas is hitting that big chimney cap and the moisture in it is condensing. The condensate is then dripping onto the roof.

 

[jp1818]

Is there anything I can do to stop it? What would have changed for there to be so much more condensation now? Could there be something wrong with my furnace? Or the way the new chimney was installed?

Thanks!

 

[IRstuff]

What was there before? Was it a full cap or a grill?

TTFN

FAQ731-376

 

[jp1818]

It was basically an identical chimney, it had a full cap. It was just old/weathered so it was replaced.

 

[IRstuff]

So, if the cap is cool enough to condense the moisture within the smoke, the only obvious solution is to keep the caop warmer. This might be doable with a "cap" on the "cap". Some sort of insulation will reduce the convection and radiation losses from the cap, allowing it to reach a much warmer temperature.

Covering it with some spare shingles might be enough. You might also want to overhang the edges, to delay mixing with cooler ambient air until the smoke leaves the chimney.

TTFN

FAQ731-376

 

[MintJulep]

Is your furnace a condensing furnace, or standard?

I would suspect standard if the vent is 8-inch metal.

The cap that you have has too much surface area. You might be able to raise it - allowing flue gas to more fully mix with ambient air before it hits the cold cap surface.

Otherwise, you should get a more appropriate cap designed for an 8" gas vent.

If your furnace is a standard one you need to address this quickly. The condensed products of gas combustion are acidic.

 

[jp1818]

Any thoughts on how to determine if its a condensing or standard unit? Its from 2003, it does have pvc drains running out of the furnace.

The fact that its an 8" vent might just be the byproduct of an earlier heating system, since the house was built in the 50's...

 

[MintJulep]

If the vent connection at the furnace is PVC then it's a condensing furnace.

Go find the manufacturer's web site and look at their recommended vent materials, diameter and termination.

Do you have a gas-fired water heater feeding the same stack?

 

[jp1818]

Its a carrier weather-maker 8000, I can't tell from the manuals if its a condensing unit or not. The connections to the exhaust vent are not pvc, they look like normal duct material(metal). The unit has pvc water drains leaving the unit.

From reading the manual, it seems like the exhaust vent should be 4" not 8", but its pretty technical about all of the different ventilation configurations...

 

[jp1818]

oh and yes the water heater exhaust feeds into the same exhaust stack.

 

[MintJulep]

Ok, it's this?

http://www.albeyers.com/pdf/58wav.pdf

That's not a condensing furnace. It's not even a high-efficiency furnace. In fact, it's a pretty crappy efficiency furnace.

If you have central cooling the PVC drains are probably condensate.

Just get a standard cap for a 8" diameter flue and you should be fine.

 

[jp1818]

well that sucks! Guess I gotta roll with this one until it dies...yes we have central cooling too.