Furnace cycles on and off every 5 minutes

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Old 10-18-13, 06:51 PM
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Furnace cycles on and off every 5 minutes

This is a Janitrol GMT090-3 natural gas furnace; 11 years old.

I have replaced the control board, which fixed the problem of it not coming on. I currently have the thermostat shorted out so that heat is called constantly (while I'm working on it). I have cleaned the flame sensor with a ScotchBrite pad. The limit switch is closed when the furnace is not in use.

When turned on, the indicator light glows steadily, the inducer fan comes on, then the HSI glows and ignites the gas. In 90 seconds, the blower comes on. Everything seems to be fine to that point.

But while the chimney pipe gets good and hot (not hot enough to burn the price labels off my new pipe), the heated air coming from the furnace is only comfortably warm. I'd want it warmer than that coming out of my registers.

Then, in about 2 minutes, the indicator light gives 4 blinks (limit switch open) and it shuts off. The inducer and blower continue until everything is cool. Then, just as the inducer stops, the HSI relights it. The entire sequence takes almost exactly 5 minutes, over and over.

I actually just figured out that I should check the indicator light during the cycle as I was typing up this inquiry. With the front cover on, I hadn't thought of that.

So I guess I need a limit switch and will order one, and should be able to install it. Question is:

1. Should even a bad limit switch cut out at so low a temperature?
2. Should the air get hotter than that in the two or three minutes it's running? Or is this furnace just too small for my needs? (It has four burners.)
3. Should the chimney get that hot?

Any help would be appreciated. It's getting cold and I'm going to need heat here in Buffalo. I'll get an HVAC person out to check over the system when I have it all running to make sure it's safe, but I'm using a free used furnace for a reason.
 
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Old 10-18-13, 07:29 PM
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Let's start with the air temp. In days gone by they had mechanical air temp switches that could be adjusted. Your furnace is on a timer with no regard to air temp. I consider it a down grade but that's progress. Your blower has speed adjustments on it for AC and lower speed for heat. Depending what speed you're on the air might be moving to fast to get properly heated before delivery. That would be a place to start.
 
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Old 10-18-13, 11:16 PM
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Welcome to the forums.

It doesn't sound like the limit switch is bad. It sounds more like an airflow problem. The first place to start is with the air filter. Has it been changed recently ?

Did you notice any difference in the failure time with the blower door open versus closed ?
 
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Old 10-19-13, 08:45 AM
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Thank you for replying. Right now, the blower door is simply open - not even the filter on it.
 
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Old 10-19-13, 09:26 AM
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I've noticed that more 'user friendly' usually means 'downgrade in options'.

I don't believe this furnace has ever been connect to AC. In Buffalo, that would be a bit unusual.

Blower fan - the wires are connected black, blue, red (high, medium, low) from top to bottom. I have no idea how to adjust that.
 
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