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Old 01-26-2014, 05:33 PM   #1
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Blocked Air Intake Vent Shuts-down and Stops High Efficiency Furnace in Winter


When the temperature gets near zero F or below (particularly during the evening and night), the air intake pipe of my high efficiency furnace gets plugged with snow-like flakes and the furnace shuts-down. This has been a repeating problem - fortunately it does not get that cold every winter here. This problem occurred a couple of times during some recent cold snaps. After some prodding from my wife, I got out the furnace installation instructions. My intake pipe was not installed according to the manufacturer’s recommendations. I installed 3" PVC pipe according to the attached instructions (attached).
Note: The PVC is press-fit together and not glued. You may have to disassemble them to clean (critters) sometime in the future.
Update: It was 3F the next morning, and the furnace was working. Also kept working during a couple of more cold nights near 0F since then. Looks like extending the intake vent worked!
I examined the PVC fittings later in the day after a particularly cold night. This design is more sophisticated then it initially appeared. Some snowflakes 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 another post, a person put a large piece of plywood between the exhaust and intake, but 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 (note: 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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Blocked Air Intake Vent Shuts-down and Stops High Efficiency Furnace in Winter-highefficiencyfurnance-vent-intall-instructions-repaired-jan4-2014.jpg  

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Old 01-27-2014, 04:21 AM   #2
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Blocked Air Intake Vent Shuts-down and Stops High Efficiency Furnace in Winter


Actually, the air never holds more then 100% RH. If air/exhaust from the exhaust pipe is reaching the intake, it isn't installed right.

Many manufacturers don't use the tee configuration yours does, and they don't have a problem.

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Old 01-27-2014, 11:50 AM   #3
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Blocked Air Intake Vent Shuts-down and Stops High Efficiency Furnace in Winter


a nice concentric works well




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Old 01-27-2014, 04:46 PM   #4
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Blocked Air Intake Vent Shuts-down and Stops High Efficiency Furnace in Winter


Quote:
Originally Posted by hvac instructor View Post
a nice concentric works well




as long as you pull the donut hood off and clean the spiders webs out once in a while.
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Old 01-27-2014, 04:48 PM   #5
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Blocked Air Intake Vent Shuts-down and Stops High Efficiency Furnace in Winter


That tee config is what they use after they test the setup in calm conditions. Has to be far enough away to not recirc. Has to do with the velocity of the discharge and how much suction the intake has also. Lennox uses an accelerator in their exhaust pipe and shoots it far out and is a rather clever and cool method.
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Old 02-02-2014, 04:06 PM   #6
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Blocked Air Intake Vent Shuts-down and Stops High Efficiency Furnace in Winter


[QUOTE=beenthere;1297382] "Actually, the air never holds more then 100% RH."
Rain, snow or sleet comes from air exceeding 100% humidity, water condensing on particles (dust) in the air (forming clouds), and the fine droplets combining to form rain drops. As moist warm air rises, it gets cold in the upper atmosphere and becomes supersaturated (exceeding 100% humidity).

Similar conditions can occur as the air near the ground cools during the evening. The result is dew or frost on the ground.

Supersaturated air is unstable. Turbulence - like in a thunderstorm - causes more rapid condensation and heavier rain fall.
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Old 02-02-2014, 07:31 PM   #7
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Blocked Air Intake Vent Shuts-down and Stops High Efficiency Furnace in Winter


[QUOTE=Ralph_handy;1300741]
Quote:
Originally Posted by beenthere View Post
"Actually, the air never holds more then 100% RH."
Rain, snow or sleet comes from air exceeding 100% humidity, water condensing on particles (dust) in the air (forming clouds), and the fine droplets combining to form rain drops.

Its the fine duct particles that prevent the air from having more then 100% RH.

Moisture condenses on any thing that is at dew point temp.

As moist warm air rises, it gets cold in the upper atmosphere and becomes supersaturated (exceeding 100% humidity).

Similar conditions can occur as the air near the ground cools during the evening. The result is dew or frost on the ground.

I can put a cold glass outside and have moisture condense on it. Its because the glass is at or below dew point. The same as cold ground or grass. Not because the RH exceeded 100%.

Supersaturated air is unstable. Turbulence - like in a thunderstorm - causes more rapid condensation and heavier rain fall.
If you want really supersaturated air. Then you need to purify it so it has no particles large then about a .1 micron. That doesn't happen in nature.

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