Furance cycles on and off many times before a heating cycle...


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Old 01-23-11, 11:31 AM
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Furance cycles on and off many times before a heating cycle...

I have a Comfortmaker RPJ II (believe it is original, from 1995 when my 1340 sqft. condo was built-upper level) paired with a Honeywell RTH230B (a few years old). The thermostat is programmable, currently I have the 4 time slots a day set to the same temperature of 70 degrees Fahrenheit. A few days ago when the problem started, (or when I noticed it) I had the thermostat un-programmed and set to 69 degrees Fahrenheit. We live in Ann Arbor, MI. The last few weeks we have had highs of 14 or so, and the windchill puts it below zero, especially at night.

The furnace currently cycles on and off about every 30 secs. While watching it, the blower turns on, the heating elements glows and gas flows and ignites and stays on for a few secs, then everything shuts off. This continually happens for hours while the temperature drops in the condo from the set point of 70 to 65 or so most times and has gone as low as 61. Eventually the furnace kicks on and stays on and heats the condo to around 70-71 and shuts off and resumes its schedule of cycling on and off...

Here are a few pictures of the set-up:



And here is a video of what is happening, which I believe to be out of the ordinary but am unsure:
http://www.youtube.com/watch?v=2cP_y_eIIWE This is my first time working with any kind of furnace and I am unsure of what I should be checking, I change the filter about 2 months, and when its changed it never really looks too dirty, but I do it anyway.

Any help would be greatly appreciated, I am about to call a HVAC company, but I want to make sure it isn't something stupid before I spend the money.

Thanks!
 
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Old 01-23-11, 12:53 PM
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From your description the odds are the flame sensor is coated with invisible oxides and is not detecting that the burners are lit. That causes the furnace to shut off and repeat the ignition sequence.


To clean the flame sensor you need to get access to the burners. The hot surface ignitor that lights the gas is usually on the right side o9f the furnace and the flame sensor on the left side.

The flame sensor is a rod about 3" long sticking up into where the burner flames would be. It has a single wire coming from the bottom and going back to the ignition control.

Remove the flame sensor and clean it with a wire brush.


Cleaning the flame sensor should be done once/year as routine maintenance.
 
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Old 01-23-11, 03:13 PM
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Ok, I am pretty sure I know what you are referring too. Looks like a probe hanging in the pathway of the flame on the right burner correct? If so, are there any precautions to take when trying to clean? Is it brittle or fragile at all?

It definitely hasn't been cleaned in a while I would assume, I moved-in in June, and it was rented before, so I will give it a shot!

Thanks!
 
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Old 01-23-11, 03:23 PM
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Ok, I snapped a shot of what my burners look like, but now I am unsure of what you are referring too as I re-read your post and you said it was likely on the left side... I see high temp jacketed wire running to the left side from the bottom, a bolt in probe in the path of the burner on the right, and the heat element which glows before the burners actually fire is also on the left.

Thanks again.
 
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Old 01-23-11, 03:48 PM
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Thanks SeattlePioneer! I went ahead and just search for flame sensor and it is indeed on the right side. Cleaned it off with some fine steel wool, turned the furnace back on and it seems to be working flawlessly now!

Thanks again!

Are there any other things I should be doing annually to ensure worry free operation? Thanks
 
 

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