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Posted on Jan 06, 2010
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Furnace shuts down 60 seconds after startup cycle then repeats

I have an older Rheem gas furnace, converted to propane. Horizontal with forced induction air. Ser# CM 1 D302 F5092 1384. I installed it in 2000 but it has never worked right. It goes thru cycle perfectly but shuts down about 60 seconds after house air fan comes on. Then it repeats. Polarity is right and ground is good all the way to connector on controller board. Tried disconnecting AC signal. Tried moving ignitor a bit incase its overheating. Tried defeating the overheat sensor. Disconnected exhaust vent to eliminate possibility of obstruction. Troubleshooting chart says it's ground or ignitor overheat of the controller board. Time to buy a board?

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Could be. But you should try changing the lower limit of the bi-metal thermostat switch located in the plenum for the induced (house) air. It could be that the plenum gets cooled off by the fan (too much air). The propane has lower BTU values than the gas the furnace was originally designed for. Because of this, the induced air plenum will warm,, the fan will turn on, but then the plenum goes below the low temp because the gas cannot maintain temp because of lower BTU values (or for some other reason), and the furnace "thinks" that it has done its job heating the house for a cycle. After shutting off, it is then rudely notified by a complaining wall thermostat that some more work must be done pronto. That is why it cycles. Good luck.

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If it shuts down shortly after the room air fan starts, have you thought about a cracked heat exchanger? If the flame pattern changes as soon as the fan comes on there is a very good chance you could have a crack or contamination. Or it could be a bad board but I don't think so.

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Heil h9mpv075f12d1 90000 btu Turns on fine, runs 5-6 minutes at slow fan speed. Then flame shuts off. Then for about 45 seconds fan goes to higher speed. Repeats this cycle over and over.

https://diy.stackexchange.com/questions/130130/furnace-runs-for-about-5-minutes-only-and-shuts-off?rq=1
Why does my furnace run then stop? A common reason a furnace short cycles is because the heat exchanger, the part that heats your air, is getting too hot due to a lack of airflow. Once the heat exchanger overheats, it trips the high limit switch, which shuts the furnace down to protect it.Dec 9, 2016
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I did not see that your indoor fan motor started. Near your indoor fan motor there is a temperature switch that is called the fan delay relay. On a temperature rise the relay shuts a contact that turns on the indoor fan motor. If the contact does not close the high temperature limit will shut the gas controls off like your getting. A fan delay relay also allows the fan to run a minute or so to carry away the remaining heat in the furnace and help cool the furnace down. The fan delay relay is normally mounted in a recessed area in the fire wall of the furnace plate. I have also seen them on the top of the furnace heat exchanger. Anyway, the fan delay relay will probably not be available on a sunday, so temporarily until parts are available, try turning the thermostat fan from auto to on and try the furnace again. By turning on the fan to on the high temp limit will not shut you down again.

I have had high temp limit change there trip points, but you said you bypassed it and did the same thing.

Another cause could be using the wrong type of air filter. The new pleated filters sold block to much air flow. If air mass flow rate drops the difference in temperatur go up and when the difference in temperature go up, it gets nearer the high limit and causes it to trip. All pleated filters drop normal air flow through ducts by at least 35%. To make your air duct system work properly use the what is called throw away filters which cost about 1/3 the cost of the pleated filter. Remember to either clean or change the throw away filter monthly to reduce dirt being drawn into your duct system.
Hope this helps.
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Is this a new furnace? or possibly the furnace has been converted from propane to natural gas? Was it working correctly at an earlier time and nothing has changed since? Or was there a change such as fuel change or another appliance was added? Any specifics you can add will help. The problem says furnace is not getting enough gas, why do you think that is the case? In other words what symptoms are you seeing that lead you to that conclusion?
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