Is this a safe application for thaw wire?

[Howard Burger]

Hey, folks. I think I have a solution to a condensation problem but I want input from you folks who have been around the block.

Problem: in the fall (like, now) and spring, when it?s in the low 50?s outside and we get all-night rains, my stove hood leaks onto the stove. It's a condensation problem ? I?ve got about 60% inside, and it?s 98% outside, so it?s not humidity going up the pipe that?s condensing. I've moved the pipe from straight up into the roof to up, 90 and then out through the attic (gradual downslope) to vent out the back of the house, and it still drips. And the straight up pipe is insulated.

I?m thinking my solution is to warm up the pipe (I?ve been leaving the fan on low and one burner on at night to keep the pipe warm, and it seems to be working). But is a better solution putting some thaw wire around the pipe, to be turned on when we get one of these fall/spring rains? I?m not sure thaw wire is designed for this, and back in '97 I discovered a fire when I worked up at Prudhoe Bay which was a thaw wire burning up visqueen and insulation near it. So, I don't entirely trust the stuff and of course, there is no smoke detector in the attic (yet). I would sure appreciate any feedback about my idea.

 

[masterinbama]

I would try an inline gravity style damper first. Put it as close to the fan as possible, the same as you do on a bath fan.

 

[Rick Christopherson]

Your problem is from the warm humid air being cooled down as it enters the house. When the air is warm the humidity may be above 90%, but when this same air cools down, that same air's humidity will go above 100%. I would look at ways of stopping the air from entering the vent. It sounds like you don't have any type of flap on the outside vent.

Edit: I didn't see the previous posting. I don't believe you want the damper/flap closes to the fan. It needs to be located at the outside where the vent pipe is at outside temperature (or at least at the point where the vent pipe is no longer cold).

 

[Howard Burger]

flappers

flappers

About the damper, the hood is a GE Monogram and has a spring loaded flapper just above the squirrel cage motor above the stove. It closes, but of course opens with the fan on low. As to the outside flapper, it is not spring loaded, just sort of sits there, to keep the bees out I guess; I bet I can figure out a way to put a light spring on it.

 

[ceb58]

Hey, folks. I think I have a solution to a condensation problem but I want input from you folks who have been around the block.

Problem: in the fall (like, now) and spring, when it?s in the low 50?s outside and we get all-night rains, my stove hood leaks onto the stove. It's a condensation problem ? I?ve got about 60% inside, and it?s 98% outside, so it?s not humidity going up the pipe that?s condensing. I've moved the pipe from straight up into the roof to up, 90 and then out through the attic (gradual downslope) to vent out the back of the house, and it still drips. And the straight up pipe is insulated.

I?m thinking my solution is to warm up the pipe (I?ve been leaving the fan on low and one burner on at night to keep the pipe warm, and it seems to be working). But is a better solution putting some thaw wire around the pipe, to be turned on when we get one of these fall/spring rains? I?m not sure thaw wire is designed for this, and back in '97 I discovered a fire when I worked up at Prudhoe Bay which was a thaw wire burning up visqueen and insulation near it. So, I don't entirely trust the stuff and of course, there is no smoke detector in the attic (yet). I would sure appreciate any feedback about my idea.

I agree with Rick, you need to stop the cool humid air from entering the pipe. More than likely if you just left the fan on you would not have the problem because you are putting positive pressure on the pipe which will not allow the outside air to enter the pipe. A damper on the discharge hood should eliminate the problem.
But it is hard to imagine your "fall" when its 98deg here:grin:

 

[Howard Burger]

98?

98?

Well, it's that time of year. I've been up here so long I don't know if I could handle 98 anymore. However there are compensations, the berries are ripe and duck hunting opens in two weeks (which is bad news if you're a duck, I guess). We told our kids we'd wait until they settled down and come visit the ones living where it's warm in the winter.

 

[Cold Fusion]

--- It's a condensation problem ? I?ve got about 60% inside, and it?s 98% outside, so it?s not humidity going up the pipe that?s condensing. ---

I would guess it is warm air going up the pipe that is condensing or you have a leak and it's rain coming in.

Cold moist outside air coming in doesn't condense out the moisture. As the air warms up it will hold more water vapor, so the vapor in the outside air does not condense.

And there is no temperature/pressure differential to drive the cold air down the tube. The cold air outside is denser than the warm air inside, so the pressure is trying to push the warm air up the tube

Here is what I would guess is happening:
Your exhaust tube is arrange up-hill all the way from inside to outside. That forms a natural thermal driven siphon - just like a stove chimney. With out the fan/burner runing, there is enough temperature differential to push the inside warm air past the hood damper and outside flapper, and rise up the tube. The warm air cools off and dumps it's moisture. When you turn on the fan/burner, the inside air is heated and moving faster - more volume. So the inside air does not cool off enough to dump it's moisture.

Considering your installation, I'm not sure what the best way is to fix it. Spring loaded, better sealing outside flapper - maybe?

Or - the rain leak is on me and I'm all wet.

cf

Saw a little color in the trees this week. Moose season opens in two weeks and one day - not that I'm counting:grin:

 

[Cold Fusion]

I've got a 3" waste vent that sometimes freezes up in the winter. It's a little different issue than yours - doesn't happen til it gets colter than -20F.

My plan for that one is to use a dead front gfi in the closet ceiling so I can see it if it trips, to a receptacle in the attic to power a plug-in freeze protection tape and wrap the line/tape in foil backed fiberglass - secure with aluminum tape. probably put in a switch next to the dead front gfi to turn it off in the summer.

cf