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Old 01-18-2011, 07:24 PM   #1
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Bryant Furnace short cycling...help!


Furnace is short cycling... runs for maybe two minutes, then the CAMSTAT switch inside the heat exchanger box trips (at about 210 degrees), and shuts off the gas and inducer. The blower motor continues to run. The system cools down, and the process repeats.

When I jump-out the CAMSTAT switch, the furnace runs fine, but is obviously getting too hot, because I can smell the residual dirt and dust "burning".

Once or twice, while I had the CAMSTAT jumped out to get some heat in the house (I was sitting next to the furnace the whole time), the flame roll-out switch tripped.

What I've tried to fix it...
1. I've replaced the CAMSTAT switch, and dialed it up to about 230 degrees. Neither helped.

2. I've ensured all of the registers are open, no blockage.

3. I have removed the blower motor/fan unit, cleaned everything very well. And, changed the fan speed from heat-medium to cool-fast, in order to get the most motor RPM's and the most air thru the heat exchanger.

What I haven't done yet...
4. I haven't cleaned the bottom side of the air conditioner A-coil yet. I fear it may be covered in cat hair and dust. But.... I wouldn't think it would be causing THAT much blockage, with the increased blower speed. This cleaning process takes about three hours to do right... and I don't have that kind of time until the weekend.

Any other ideas I haven't thought of?

I've read other's posts about short cycling, and haven't seen much more that I can do.

help!

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Old 01-18-2011, 07:36 PM   #2
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Bryant Furnace short cycling...help!


did the flames roll out? have heat exchanger checked by a pro!

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Old 01-18-2011, 07:39 PM   #3
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Bryant Furnace short cycling...help!


I never saw any flames actually roll out...
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Old 01-18-2011, 07:48 PM   #4
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Bryant Furnace short cycling...help!


you'd be surprised how much decreased airflow can be with a dirty blower and evap coil. How old is the furnace if it using a camstat?
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Old 01-18-2011, 08:03 PM   #5
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Bryant Furnace short cycling...help!


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you'd be surprised how much decreased airflow can be with a dirty blower and evap coil. How old is the furnace if it using a camstat?
Furnace was new in 1998. I typically take the entire thing apart every two years and clean out the cat hair/dust from the A-coil. I was planning on cleaning the a-coil in the fall, but didn't get around to it. It's not any easy, ten minute task... if it was, I would clean it every few weeks.

Furnace has been running fine all winter, thus far. Saturday afternoon, I pulled out the dirty air filter and installed a new filter. Woke up Sunday morning to a cold house, and the LED on the controller board flashing three shorts and three longs at me.... which basically means the camstat switch has tripped about five times in a row.
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Old 01-18-2011, 08:19 PM   #6
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Bryant Furnace short cycling...help!


sounds like the camstat is doing its job keeping the system from overheating. It almost has to be lack of airflow or Heat Exchanger issue.
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Old 01-18-2011, 08:29 PM   #7
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sounds like the camstat is doing its job keeping the system from overheating. It almost has to be lack of airflow or Heat Exchanger issue.
Thanks. I'm gonna pull the heat exchanger "box" tomorrow after work, clean it up, and clean the air conditioner coil too.

I'd love to rip out this furnace, and re-design it and re-install it with about 14 extra inches of space between the heat exchanger and the coil. Then, I could add an access panel that could easily open and allow me to clean that coil.

I'm very good about changing air filters. Many people change thiers once or twice a year. I change mine every 30 days or so... and the freekin cat hair still finds it way in to clog the system. I hate that freekin cat.

If this doesn't work.... are you in the Indianapolis area? Do you do freelance work?
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Old 01-18-2011, 08:31 PM   #8
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Bryant Furnace short cycling...help!


im in Indiana, but I do not do side work. this is a far as I go with helping on the side.
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Old 01-20-2011, 07:48 PM   #9
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Bryant Furnace short cycling...help!


Are you sure if the duct work is undersized? If so it may cause it to get too hot and shut down. Just a thought.
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Old 01-20-2011, 07:52 PM   #10
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Bryant Furnace short cycling...help!


i dont think its ductwork size on a 13 year old furnace
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Old 01-20-2011, 08:20 PM   #11
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i dont think its ductwork size on a 13 year old furnace
In the back of my head, I knew what the problem was all along, I was just trying to avoid it, because it's a pain in the rear to fix it correctly.

Last night, I completely disassembled the entire furnace. When I was done with disassembly, it was basically a sheet metal box with a condenser coil at the top. I pulled several hand fulls of cat hair and dirt off the air conditioner condensor coil, then used soft tooth brush and shop vac to carefully clean the dirt between the fins. Close to two full hours of cleaning time, with my back and arms wrenched in awkward positions, dust in my eyes, sneezing, etc. I'm sure a bent over little portions of a few of the fins on the coil.

After a through cleaning, I put the furnace back together, fired it up, and the problem is gone. My furnace simply wasn't breathing.

This is the fourth time I've had to clean the thing. Basically every two years.

Before cleaning, I could only feel the air coming out of the register vents if my hand was about 12 to 14 inches from the vent. After cleaning, I can feel the air blowing on my hand, 60 inches away.

From start to finish, I spent about thirty minutes taking the furnace apart, two hours cleaning, and about 45 minutes for reassembly and cleanup.

Thanks to all who helped me learn a little more about my furnace though.

The problem is that even though my furance is designed for a 16x25 filter, there is still a half inch gap alone one entire edge that allows air in without being filtered. I think I'm going to make a thin aluminum plate that will fit into the furnace, and take up this gap, ensuring that ALL of the incoming air is filtered before hitting the blower fan.
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Old 01-21-2011, 04:01 AM   #12
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Bryant Furnace short cycling...help!


Besides making that plate and installing it. Use a general pleated air filter. If already using a pleated air filter, then ignore that suggestion.

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