Heat tape on Plastic pipe?


Hello,
I just had the furnace guy refire up my energy efficient furnace as it went out last night. We have been having a cool spell (-45 to -50 Celciuse) in Canada here... I have a 2600 sq ft four level split home and they installed the new furnace last year and I have these three inch plastic pipes as an intake and exhaust. Problem is the exhaust had to be run for quite a ways (through the garage and up through the roof) for the installation and the very top of the exaust is freezing over (The exhaust is sloped properly and condensation does drain back) The part of the exhaust is the verticle section coming up through the roof...The verticle section rises about 5 feet over the roof so that it won't be buried in a snow bank........This morning we cut two feet of length as the end of it iced over and closed the exhaust. I don't know weather to use heat tape on these three inch lines or just wrap with insulation.... Thoughts...typing with gloves on here...just got things up and running again...close call...laying in bed this morning and our cat came to bed and noticed he was cool to the touch...LOL...lucky I didn't freeze any pipes Thanks..... Jim
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You want to do the portion above the roof only? I see at least one problem. Plastic does not transmit heat very well and the tape may do no good at all. Wrapped around the outside, the moisture on the inside top inch or so could still freeze and build an ice block.
I'd consider transitioning to a copper tube (expensive as that may be) or 3" electrical conduit if you want to heat it. You may only have to heat the top 12" or so to prevent freezing.
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Jim wrote:

Self regulating heat tape shouldn't get hot enough to damage PVC pipe. I often use it to keep PVC drain lines clear of ice in walk in freezers. I've never seen it damage a PVC pipe.
TDD
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Find a new run for it. In many cases the outlet does not have to go up and out the roof. You can run it out the side as well. Run it out the side wall high up on the garage and you'll still be protected from snow accumulation without having to extend the pipe into the open air.
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Most of the runs of PVC I've done have been horizontal, out a side wall. I can't say as I've ever exhausted flue gasses out a roof. High on the garage side wall sounds like a good choice.
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Where is the Pvc intake, Its common here to run both intake and exhaust pipe out the same general area with about a 2ft inbetween the intake and exhaust, mine is that way and my Lennox pvc flange has 2 holes mine is about 3ft above grade. The longer the run the more problems you can encounter, your instal manual specifies how many feet and how many elbows you can have and maybe at a certain outside temp, did you follow it. Cant you run out the exhaust near where the intake is. -50 is cool, so what is cold up there.
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I don't

Heat trace can melt plastic pipe unless you use the lowest wattage tape. Problem is the pipe is exposed to very cold moving air which is a much worse case than buried pipe which sees only a few degrees below freezing. If you use enough heat trace in a high enough wattage and with tight wraps to gain a positive heat balance when it is minus 40 degrees that would melt the pipe when it was only minus 10 degrees and the heat trace came on.
The wattage of heat removed by minus 40 air in a strong wind can not be handled safely with heat trace alone. You could use a low wattage tape, 4 to 7 watts per foot, plus wrap the pipe with insulation that is approved for use with heat trace. Install the heat trace only in a straight line or you can cause over-heating in warmer weather.
Best solution may be to insulate the whole run of pipe that is exposed to cold air - even the pipe inside the house. Purpose is to keep as much heat in the condensate as possible until it reaches the cold end section. Then there may be enough heat left in the condensate to prevent freezing and allow drainage between furnace cycles.
If you need a quick solution - heat trace only the section of pipe above the roof and cover with a water-proof insulation that is approved for use with heat trace. Use 2 inches thick insulation. Safest method is to use only a sraight line installation for the heat trace but you may get away with a wide wrap - coils at least a hand-width apart. If you can't find a short enough heat trace, then start with the plug end at the roof and work your way up. Carefully feed the excess heat trace down the inside of the pipe avoiding any curls. Cover the insulation with something to prevent UV from destroying it.
Heat trace usually has a builtin thermostat set at 40 degrees F. Make sure the type you buy does.
Making a safe water-proof electrical connection on the roof under snow is beyond me. I'd find a way to put the plug through the roof so it could be plugged in under dry condtions. Staple the plug wire to the uphill sideof the hole so any water leaking does not flow along the wire to the plug.
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Hi all again and thanks for your replies... I got off the phone with the installer whom also came to the house and got me up and running again (Free service). He suggests since the run is so long and the layout is so tough for doing anything different..(Blieve me it was a tough install) He suggests just insulating the 3 inch exhaust pipe that goes along the ceiling in the garage and that should suffice to keep things warm enough to prevent freezing in the verticle run outside.....He said I don't need heat tape?......So I am off to Rona (Canadian Home Depot) to load up on pipe insulation. BTW I can't buy that premade sleeve type insulation that you slide over the end of the pipe because the pipe goes out one wall and into a ceiling so will need an insulation that can be wrapped around the pipe as the ends are not accessable if you follow what I mean. Hopefully I'll find something that will look half assed decent and finished. Anyways, some suggested running pipes out the side of the house...we did that with two intakes (Also have high efficient tankless hot water system) and had no more room by the design of the house to do anything different unless we tore half the house apart and I wan't going to get into doing that. I just wanted to thank you all for your replies and let you know what I hope to do...we're getting a break in the weather...has warmed up to about -25 C this afternoon...Jim
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Above the roof, use an appropriately sized larger pipe to sleeve the vent pipe, protect the insulation and a better appearance. Probably need to come up with a way to seal the gap between the end of the two pipes so moisture doesn't compromise your insulation. One thought is a cap on the larger pipe with a hole to tightly fit the smaller pipe and then silicone caulk. Another idea is a bushing that matches the fit diameter differences between the 2 pipes.

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