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Old 01-09-2010, 10:26 AM   #16
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Furnace shuts off due to clogged exhaust


Ozman, you need to get a copy of the instalation manual. Some furnaces have a 6" max between vents, some reduce the exhaust to the next size down on the end for increased throw to avoid recirculation. Most that I have installed want the exhaust vent extended 8-10" past the intake. Each manufacturer has their own way of venting and those specific directions must be followed. As a general rule the vents can exit over a walkway of yours but not a common or public sidewalk, too much liability of ice forming on the walkway.

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Old 01-09-2010, 01:33 PM   #17
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Furnace shuts off due to clogged exhaust


I put screening on my furnace and tankless water heater exhaust ( and intake ) for the same reason. I don't have a problem with condesnsation though. My mesh is fiberglass. Is your mesh perhaps steel, and possibly it freezes over more readily because it transfers heat quicker ?

My furnace is handling it fine, but the tankless shot some dust/debis out and shut-down the unit. I took the screeing off. Only a very small obstruction would shut it down.

I think the true answer is like Tommy2 said -- remove the grating. Then in the spring come up with a better critter trap.
I need to do that too.
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Old 01-09-2010, 02:23 PM   #18
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Furnace shuts off due to clogged exhaust


Quote:
Originally Posted by michaeljc70 View Post
I've been hunting around the web and several have said that winter is when rodents/birds will try to get into a warm place. That is my concern.

To clarify, the exhaust is an upside down J PVC pipe. I would guess it is a 4 inch pipe. The grate is wire grid with the squares maybe the size of a pinky finger.
We pull the screen out after inspection. Because customers hate to be without heat because that stupid screen causes the freeze up.
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Old 01-09-2010, 03:20 PM   #19
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Furnace shuts off due to clogged exhaust


Park one of those fake Owls on the pipe and that will keep the critters away. LOL
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Old 02-15-2010, 03:30 PM   #20
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Furnace shuts off due to clogged exhaust


wat going on?
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Old 01-13-2011, 12:48 PM   #21
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Furnace shuts off due to clogged exhaust


I live in Winnipeg, Manitoba, plenty cold here. I've had similar issues with my HE furnace and here's what I found: My pipes are 2" ABS, the exhaust not far from the intake, but both are installed correctly. I found as others have that the moist exhaust was getting sucked into the intake causing a restriction from the formed ice/frost. My intake fan could not draw enough air to begin combustion so the furnace would shut down before the burners would light. I thought I solved this by placing (upright) a piece of 2'x4' plywood between the pipes, and for the most part it's worked. Instead of 3-4 times a winter, now I'm down to being plugged once or so. I believe the culprit now is high humidy when cold. That can form a "Hoar Frost"( very white trees, etc,) and also seems to be able to plug my intake (again). Scrapping out the elbow always works, but now I'm scared to go away in winter with my house unattended. Thinking of possible trying a short run of heat tape to see if it helps. I'm a bit scared of too much heat on the ABS from the tape, anyone else try this yet??
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Old 01-04-2014, 02:07 PM   #22
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Furnace shuts off due to clogged exhaust


When the temperature gets near zero F, the intake pipe of my high efficiency furnace gets plugged with snow-like flakes and stops. Combustion produces some water vapor. The cold air condenses the water which would gradually accumulate in the intake pipe as snow-like flakes. This has been a repeating problem - fortunately it does not get that cold every winter here. It happened a couple of times during recent cold snaps (where is global warming when you need it ). After some prodding from my wife, I got out the furnace installation instructions. My vent and exhaust pipes were not properly installed and much too close together. Today, I installed 3" PVC pipe according to the attached instructions. Tonight is supposed to be cold. Will let you know if the problem is solved.

Note: The PVC is press-fit together and not glued. May have to disassemble to clean sometime in the future.

Update: It was 3F when I got up the next morning, and the furnace was working. Looks like extending the intake vent worked!
I examined the PVC fittings later in the day. This design is more sophisticated then it initially appeared. Some snow flakes accumulated on the inside walls of the section of the T that is vertical - where the weight would cause them to fall to the ground before causing a blockage. But, none in the intake.

Science behind why the tee is critical:
Upon further consideration, it occurred to me that the condensation from the exhaust is not the cause of this problem – at best a minor contribution. In an earlier post, the person who put a large piece of plywood between the exhaust and intake continued to have problems. After digging deep into my engineering education, I think I have the explanation including why the tee works.
The outside air during the cold winter has high humidity. This is counter intuitive because the heated air inside the house is dry. When cold outside air enters the house, it is heated. It expands and can hold more moisture – thus lower humidity.
During the daytime in the winter, the outside air absorbs water from the snow and other sources, and reaches a high humidity – often near 100%. As night falls, the outside temperature drops along with the air’s ability to hold water. The air can reach a state where it is “super saturated” with a humidity above 100%. This excess humidity is unstable, and a disturbance will cause the water to condense on any object (you may have noticed frozen dew on the grass or tree branches early in the morning before the outside air warms).
Air sucked into an intake vent has turbulence that will cause the super saturated air to condensate on the inside of the vent. Grating installed to block critters would be particularly problematic. The ingenuous tee attracts this condensation in a place where it does no harm i.e., the condensate falls to the ground before it builds-up enough to block airflow. After the tee, the air is no longer super saturated and enters the intake without a build-up of snow-like flakes.
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Furnace shuts off due to clogged exhaust-highefficiencyfurnance-vent-intall-instructions-repaired-jan4-2014.jpg  

Last edited by Ralph_handy; 01-11-2014 at 08:41 PM. Reason: Added that the change fixed the problem; and the science explanation
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Old 01-04-2014, 02:44 PM   #23
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Furnace shuts off due to clogged exhaust


turn the intake 180 degrees and put a 3-4' length on it and a rain cap

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