| High efficiency gas furnance: Intake freezes shut|
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|Jay NE Ohio|
My sister just moved into a house in town. It is an older house, but has a high efficiency gas furnace. The air intake is 2" schedule 40. The furnace is in the basement and the intake pipe goes out the block wall, up about 2 feet, elbow and then down 6". When we had that cold spell a few weeks ago, the intake froze shut at the end of the intake pipe. Of course the furnace shut down due to lack of air.
We took the elbow off and put a tee, two elbows and two 6" pipes. It didn't freeze up again untill this morning when our temperatures went to -12F. My sister went outside at 3 am and took off the tee and brought it into the house to thaw out. The furnace worked fine the rest of the morning without the tee.
What would be a good solution? She does not want to get up every cold morning and thaw the pipe! The house needs more insulation,,, that would cut down on the furnace run time, but might not solve the problem. Should we go to a 3" pipe? Or can we draw intake air from inside the basement?
|ours draws intake air from the basement, and has a 4" powered exhaust, we haven't had any problems with it freezing shut, I would just let it get air from inside the house.|
|You need to get the intake away from the exhaust but within the limits allowed by the manufacturer. The intake drawing in moist air from the exhaust is whats causing your problem. Most furnaces can be used with just the exhaust vented outside but you would need to check the installation instructions.|
|wrap the intake with electric heat tracing you can get from a hardware store, that will keep the ice off. Insulate around the outside of it. You might put some sort of timer or maybe there's sensor activated models that would turn on and off on demand.|
|I just bought a house and it does the same thing. I need to raise the pipes up off of the ground more. They need to be moved up because the previous owner put a deck on and that trapped the exhaust gases where the wind will not move them away from the house fast enough. They told me to raise the inlet above the deck so it is not in a hole|
Many yrs ago when I first had hi-efficiency installed, I had the same problem a couple of times. I'd also wake up to a cold house with the scavenger draft fan only running. Apparently this problem is more common than one thinks?
My solution for MY furnace (I don't have a CCA therefore I make NO recommendations) was to cut my intake pvc in half, only a few feet from the furnace. I then took one of those rubber sleeves that they use to join pvc together & I drilled a 1" hole in the side of it. I slipped the sleeve over the pipes & made sure pvc pipes stayed spread far enough apart to allow the hole to breathe. Then I tightened the hose clamps. Now, when/if the intake freezes on the outside of the house, it will draw air from within the basement. However, if it's not frozen over, there seems to be less resistance to the outside air source & I feel very little air entering the hole via a venturi effect.
That was MY solution & I do NOT recommend anyone else modifying their furnace without proper supervision from a qualified installer.
The photo below is my unauthorized modification. You can see the hole in the slipjoint. The additional slipjoint to the right does nothing now. It has NO hole in it. I slipped it over the pvc when I had them split just in case my modification did not work. I wanted to be able to quickly join the pvc again & go to plan B. That was probably 15 yrs ago. There's been no further freeze problems since the mod.
Jay, you might see the same idea where I got it from. The crankcase breather tube on most recip airplane engines has a "whistle tube" in it to prevent total line freeze over when flying in sub-zero temps.
|I would NOT draw air from the basement. Burning outside air is the whole concept behind a high efficiency furnace. Think about it, if you burn inside air and exhaust it outside, more cold air has to infiltrate into the house to replace it.|
|So...tnt, what's your suggestion for solution to frozen intakes on the outside that have no normal melting mechanism for the icing problem? What would you do when it becomes cold in your house & your furnace won't run?|
|I was not knocking your idea nor was I replying to you. The problem has to be with the exhaust being to close to the intake or the configuration of the intake piping. How do you know when the pipe freezes up. That one inch hole won't provide enough air as the regular pipe and will still probably shut down the furnace after a while.|
Lac qui Parle County Minnesota
|Dolittle is right on the reason. You have to add some pvc to your intake. Around here alot of them run a 45 degree elbow straight up, add another foot of pipe to that and top it off with another elbow. Its like he said, the warmer moist air is being sucked back into the intake and freezing on the much colder pipe.|
Edited by Gromit 2/5/2009 23:07
|Put the intake below the exhaust with just a 90 elbow pointed down. Have the exhaust blow out straight away from the house.|
Hagen Brothers farms,Goodrich ND
|I recently watched a "this old house" type home fix it show where they installed a tankless gas water heater. |
The air intake / exhaust pipe for this heater was a pipe within a pipe type deal. The intake pipe was about 4 inch with the hot exhaust pipe running back out the center of the larger intake pipe. This let heat from the exhaust pipe be transfered to the intake air, which pre heats it enough to prevent freezing and increases the efficiency of the heater a bit.
The small exhaust pipe extended about 1 ft beyond the end of the larger intake pipe to reduce mixing exhaust air with intake air.
Edited by Jon Hagen 2/6/2009 01:30
|The coaxial exhaust and intake vents are fairly common for the High Efficiency furnaces. However, I had a furnace with pvc vents that went up the old chimney and never had a problem in weather as low a -22F, on this lash up the exhaust went straight up and the lower intake pipe had double elbows on it so it pointed down to keep rain from getting into it. Those furnaces produce a lot of water so the exhaust is very damp. The condensate pump on mine quite once and I had a real mess of water that smelled like vinegar in the basement. I would guess that the best fix would be to use the pipe in a pipe arrangement. Next get the intake about a foot below the exhaust.|
Edited by JohnW 2/6/2009 02:40
tnt...this reply is to you.
You're right. That solution did not fix my furnace that would not light even after scavenge fan running & furnace going thru 3 light attempts, then never attempting to relight until power was removed & then retried again (per mfg). That happened on multiple instances prior to my fix. However not once in 10+ yrs since, nor has furnace shut down.
So, you're right, it won't work.
Again, I do NOT recommend anyone else trying that for a fix.
|Jay NE Ohio|
|Apparently the installation was done incorrectly. The intake is about 6" ABOVE the exhaust! So we will re-plumb it correctly and hopefully that will take care of the problem. Thanks for the tips!|
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