The furnace went out this weekend. (No problem. Only 15 below zero...
Fahrenheit, mind you... with winds howling over 30 miles per hour.)
Turns out the intake pipe (that leads outside) was clogged. The
technician lifted the pipe out of the socket where it connects to the
furnace and moved it aside an inch or so, allowing the furnace to suck
in air from the room instead. He assured me the furnace can operate
just fine on room air until temperatures warm up outside (i.e. spring,
which is weeks away) and the pipe clears, at which point I can
reconnect it. My question: Is he right? Is room air just fine for
my furnace? If so, then why was it designed to use outside air in the
I'm also wondering why they put the exhaust pipe right next to the
intake pipe, since it was probably steam from the exhaust pipe that
clogged the intake pipe. Geez, the intake pipe must be sucking in
exhaust all the time! That can't be good! Seems like they should be
separated as much as possible...
This shouldn't be a problem other than with the exhaust going outside (as it
should) the inside air used for combustion will have to be replaced, and
that means cold outside air will be drawn into the house where ever it can
be - generally around doors and windows. If the furnace is in an
unfinished basement the air is probably drawn in thru cracks and openings
around the foundation, so the heat loss isn't as bad as it would be if the
furnace is in a utility closet off a living area. This is in fact exactly
how old-fashioned low-efficiency furnaces worked (and still do). Drawing
combustion air from outside is a good energy savings measure with new
The exhaust pipe on the other hand should NOT be disconnected in such a
manner, for obvious reasons.
As for the location of the intake and exhaust pipes, the manufacturer has
specs for how far apart these should be, and how far they should extend
beyond the house, to prevent the sort of problems you are describing. If
installed correctly the amount of exhaust sucked back in the intake should
be minimal. Additionally, of your furnace is exhausting so much moisture
that the exhaust is clogging the intake, it would seem there is either an
issue with the installation, or with the furnace' functioning. While my 90%
efficient furnace will produce some steam on cold days, I've never noticed
any ice forming on either of the pipes. We haven't hit -15, but have been
down near 0 with howling winds and high humidity.
Have you actually inspected the pipe to confirm you suspicion it is ice that
is blocking it and not something else? If it is disconnected at the furnace
it should be fairly easy to run a wire of fish tape up the pipe to see how
far in the blockage actually is.
one more thing..
(this is a different Mark by the way)
I would seal off the unused intake pipe that leads outside.
This is because you said the outside opening was right near the
exhaust pipe. If the intake pipe should clear itself one day and you
are unaware, it will be open and can draw in the exhaust gasses CO etc
into your house. Seal it off on the inside so you know it is sealed.
Nobody else said it so I will- has anyone tried clearing the intake tube
from the furnace room end, with a drain snake? Odds are it just an an
animal nest or something, with maybe a layer of ice. On PVC pipe, it
can't be stuck very hard, unless the tech who put it in was a slob, and
left lots of burrs in the joints. If it is iced up, directing a
hairdryer up there for a few minutes may be enough to clear it. The
exhaust from a shop vac, or even a leaf blower (if you can narrow the
output end down) may be helpful as well. (I use my leaf blower to clear
downspouts when I am up there cleaning gutters.)
No one else has said this so as gas fitter I will if you are operating
the firnace without the air intake hooked up and some one more than
one told you it is okay suggest you find a new service person. I
have put in 40 to 50 high efficent furnaces and never had one ice up
as you describe . This can lead to carbon monoxide being sucked in to
your house from the gas watertank possibly putting it out. If this
service person worked for me he would be fired as he is an idiot.
What a terrible disaster waiting to happen. Room air will kill your furnace,
in a hurry. You need air from behind the garage, under the wood pile, and
next to the bird bath. Your furnace is in such great risk, you are about to
Having started with a joke. Sure, the cellar air is just fine. You should
(this doesn't seem intuitive) leave a window open a crack, if possible. The
air has to come from some where. Formerly, it was coming in the PVC tube.
Now, it may be coming down the water heater chimney.
A properly installed exhaust should be farther away from the house compared
to the inlet. and usually pointed away. So, cycling back is seldom an issue.
I would be a little worried that if something was blocking the intake
that same something might be blocking the exhaust too. Have carbon
monoxide detectors installed? Trust them with your life? Might be a
good idea to go figure out what is causing that blockage.
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