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Frosting and ice build-up


Darren
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I'm scratching my head on this one...

9 year old 2-story house vacant since Dec 1, 2007. During today's inspections (25 degree weather), there was ice on the shingle nails in the attic. When I conducted the exterior inspection, I found evidence of ice around the B vent cap. When I went into the attic, you can see where a piece of ice fell down off the B vent.

Also note there are 2 furnaces, 1 in the attic and 1 in the basement. The basement B vent connectes to the attic vent and there is only 1 vent that goes thru the roof. Humidifier on 1st fl unit only, inoperable.

Upon entry of the house, the 1st floor stat was on 64 while the 2nd fl was on 54 (temp on 2nd floor was 61).

More than enough soffit, ridge & roof vents installed. No bath fans, kitchen fan is exterior vented.

Does anyone think the cap was frosted completely over forcing the exhaust to leak back thru the connections?

If not, what else would cause this?

Darren

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Hi,

You said that there was a B-vent cap. Was there a storm collar above the bib flashing where the vent passes through the roof and was the storm collar caulked to the vent pipe with high-temp silicone to prevent condensation from draining through the bib and dripping off the vent?

Was the vent from the collar in an unheated area that's allowing exhaust gases to cool so rapidly that condensation actually ices up at the terminus?

The ice on the nail heads could be ordinary house humidity that's condensing on the cold tips of the nails. After the sun gets high in the sky and heats the roof a little bit, the moisture will evaporate.

OT - OF!!!

M.

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Darren, my guess would be that the cap iced up. Seems that only the furnace in the basement was firing with any regularity due to way the thermostats were set. With the stat set low and the recent cold snap, the flue never got hot enough due to only occasional firing of only one furnace, and I'm sure the water heater was set to low. The single flue is now oversized and not drafting well enough to remove all of the moisture.

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Originally posted by hausdok

Hi,

You said that there was a B-vent cap. Was there a storm collar above the bib flashing where the vent passes through the roof and was the storm collar caulked to the vent pipe with high-temp silicone to prevent condensation from draining through the bib and dripping off the vent?

No, I could see that water could possibly drip down into the attic, but where would the ice (that was found in the attic) come from?
Was the vent from the collar in an unheated area that's allowing exhaust gases to cool so rapidly that condensation actually ices up at the terminus?
The 1 furnace is in the attic; it's vent connects to the basement vent that runs up thru the attic (after the vent gets larger).
The ice on the nail heads could be ordinary house humidity that's condensing on the cold tips of the nails. After the sun gets high in the sky and heats the roof a little bit, the moisture will evaporate.

OT - OF!!!

M.

What humidity? The house is vacant?

Neal, that was my thought except if you look at the first picture, there's lots of black stains around the nails indicating it's been going on for a long time.

Darren

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"Also note there are 2 furnaces, 1 in the attic and 1 in the basement. The basement B vent connects to the attic vent and there is only 1 vent that goes thru the roof."

Maybe I am reading this wrong, but are the attic and basement furnaces sharing a flue in the attic? These are both natural gas?

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