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
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.
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.
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.
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
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.
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
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
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
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