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Old 12-28-2010, 02:37 PM   #1
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My christmas odyssey of WTF is wrong with my furnace

It all started 3 weeks ago.

Noticed that my furnace would short cycle, cutting out before hitting the requested temperature.

Called out a tech, he couldn't replicate the problem. About 4 days later it happened again, cutting out after about 10 minutes.

Called out another tech who couldn't replicate the problem but suggested it was hitting the high limit of 200f. He checked everything to make sure there were no obstructions. Check the A coil, no issues except for a couple of damaged fins in a small 1x3 inch area towards the front of the A coil. So he suggested to move my fan speed from medium low to medium high. Worked fine.

However, when looking closer at my inducer housing, I saw that it was cracked in 3 places and covered with high temp silicon that had started to seperate. I replaced that inducer with a new cast metal housing.

Everything was fine up until the day after Christmas. My wife said something smells like it's burning. It was coming from either the floor registers or return vent. I went into the furnace room and All i heard was the inducer fan on and a lout humming noise. Instantly I knew it was the blower. I swapped it out with another capacitor my HVAC tech neighbor had. Blower still hummed.

Replaced blower, everything went smooth. Put the fan speed back to medium low thinking the overheating motor probably caused the high limit switch to trip. Well after 5 minutes of operating it short cycled again. I switched the fan speed to medium high and the temperature stayed in between 145-150 in the heat exchanger area. I let it run for 10 minutes and shut it down. When attempting to restart it about 40 mins later, it wouldnt. The HSI wouldnt light. And then when it did, the pressure switch shut the furnace down and the circuit board would show the blink the "Ok" led twice. It did this 2 times.

I blew in and sucked into the pressure switch a few times and then it started up fine. Ran until the thermostat was satisfied. not being satisfied I was finally able to call out a tech from if not the best, one of the best HVAC companies in my area. Im awaiting on him now. Tired of playing the replace this part game. Getting a new Furnace is not in the cards right now due to chronic unemployment in my field (union electrician). Crossing my fingers that it's nothing more major. I can live with replacing the pressure switch.


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Old 12-28-2010, 03:38 PM   #2
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Replace the pressure switch. It has a history of nuisance tripping. I change a half dozen of them every year.


"Cut it twice and it is still too short".
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Old 12-28-2010, 06:58 PM   #3
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turns out that my return ducts are not providing wnough return air. im at -.35, i should be at -.05. So I'm getting cavitation, low fan speed cant pull enough air. Among other things he told me to replace the pressure switch. With my fan setting on medium high, im keeping a perfect balance with my heat rise. My gass pressure was a bit high too (.365).
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