Ice within soffit

[tbough]

I have a direct vent gas insert installed in my fireplace. The vents are run up the existing chimney with an external wood framed chase.
My problem is that I get a build up of ice inside the aluminum soffit adjacent to the chase and on both sides. This causes problems in the spring
when the ice melts and water runs towards the window frames. By this time the soffits are completely plugged off for several feet.

I have taken down some soffit and the temperature on the plywood on the side of the chase was 15C (60F) after several hours running. I haven't got my head around the science but obviously condensation then freezing happening in that space. Running with the soffits removed seem to be the best short term fix although I still get some hoar frost forming.

Last spring when I was investigating this I found the preexisting fresh air vent to the original wood burning fireplace. I assumed I was drafting warm air
up to the soffit area so I blocked that off as best I could. Maybe I need to do more there.

I would appreciate suggestions. Google has not been my friend on this.
Would insulating the exposed sides of the chase be enough?

p.s. I just checked, the fireplace hasn't run since last night and with an ambient temperature of -20c the area of the chase under the soffits is -2c.

Tony

 

[stoveliker]

This my be best in the 'Its a gas" forum

Also, some pics will help you get better help. (And I'm concerned about a fresh air intake in a soffit, i.e. above the stove?)

 

[Qvist]

Do you have any pictures of the area? It definitely sounds like warm damp air from the chase, or somewhere else is getting up into the soffit condensing and freezing.

 

[tbough]

Thanks for your interest. I'll try and include the pictures I have of the east and west sides under the soffit. The new plywood in the one picture I put in this spring. Behind that is a rectangular duct/vent which supplied air to the original wood burning fireplace. This I tried to plug off. When I first removed the soffit there was a build up of solid ice on both sides.

 

[Qvist]

It's hard to see what you are talking about in the pictures. I'm thinking that either warm air from the inside is getting into the soffit, or exhaust gases from the fireplace are. Exhaust gases have water vapor in them so if they are getting up there and cooling it could cause your issue. Is the old fresh air vent connected to both east and west sides of the chimney or only one side? Since you are getting ice on each side the same source or sources of warm air are present in each side. It's also somehow connected to the fireplave being on because it doesn't happen when it's off . The chimney chase could also be acting like a chimney for warm air to flow into from the building. Search the area for warm air when the fireplace is running assuming it's safe. Anything that is much warmer than the outside air in the area is suspect. Having someone come with a thermal imaging camera to look for the warm air would help.

 

[tbough]

It's hard to see what you are talking about in the pictures. I'm thinking that either warm air from the inside is getting into the soffit, or exhaust gases from the fireplace are. Exhaust gases have water vapor in them so if they are getting up there and cooling it could cause your issue. Is the old fresh air vent connected to both east and west sides of the chimney or only one side? Since you are getting ice on each side the same source or sources of warm air are present in each side. It's also somehow connected to the fireplave being on because it doesn't happen when it's off . The chimney chase could also be acting like a chimney for warm air to flow into from the building. Search the area for warm air when the fireplace is running assuming it's safe. Anything that is much warmer than the outside air in the area is suspect. Having someone come with a thermal imaging camera to look for the warm air would help.

I'm coming to the conclusion that the problem is warm air from the room going up the chase and/or air duct. The original wood burning fireplace had the air duct, with fan, on just the west side. Last winter, when that duct was open to the enclosed soffit area, the ice build up on that side was much worse than on the east.

I can't explain why but in that enclosed micro environment inside the soffit it seems to be the reverse of condensation on the windows happening. Cold moist air ( 70% RH and -30C) contacting warm wood, moisture dropping out and freezing. This happens much less with the soffit area opened up.

When the contractor installed the new insert it was just placed in the existing fireplace and the vents run up the chimney. The old air duct was left in place along with the electric fan inside the duct that was just disconnected. I wonder now if that fan could be rotating backwards and adding to my problems?

I think, in the spring, I need to expose that west side of the chase outside and see what I have and hopefully remove the old air duct.

I appreciate your thoughts

Tony