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Rheem Furnace sometimes takes a couple of ignition cycles before

Symptoms:
1) Thermostat sends signal to furnace when temp drops.
2) Exhaust fan comes on for a minute or so.
3) Ignitor comes on and glows.
4) Gas turns on and ignites.
5) After about 10 seconds gas turns off.
6) After a few seconds ignitor glows again.
7) Gas turns on and ignites.
8) Sometimes gas will stay on, heats the house and turns off as designed. Sometimes it repeats steps 5,6 & 7. Other times I have to turn power off to reset the logic.
Is this a faulty thermostat in the plenum? Is this common with Rheem Furnaces?

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  • 306 Answers

Try cleaning the flame sensor. It is located right besides the last burner. Looks like a thick piece of wire. Take steel wool and gently remove the carbon that has built up. Should only take 4 or 5 strokes. Let me know how it goes after that. ken

Posted on Dec 23, 2008

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Posted on Jan 02, 2017

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Gas furnace short cycles 2 minutes or less. Multiple troubleshoot steps and replaced components - no help. Any advice?


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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Rheem 90+ NG Furnace. When furnace starts, draft inducer starts, ignitor glows and gas ignites. Burners stay lit for 10 seconds or so, then the flame reduces for a few seconds then goes out. Main blower...


Check the draft inducer air switch ( vacuum diaphragm switch) If the switch is opening when the burner starts, you might have a blockage on air piping coming into the furnace. Also prime the condensate trap on the condensate drain from the combustion fan. Also check the gas pressure, if your gas pressure is dropping, the gas pressure switch will shut it down. If it's too high the moment it comes on could raise the pressure in the furnace dropping out the combustion fan proving switch.

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Furnace cuts off before reaching set temp.


You can make sure it is not the thermostat by crossing or jumping the R and W terminals (or associated wires) together. This will keep the furnace calling for heat. As it is approaching set temperature, get in front of the furnace and inspect the air filter to make sure it is clean, watch the control module for any error codes that may be turning off the system prematurely.

On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).

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Furnace shuts down 60 seconds after startup cycle then repeats


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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I have this same POS model. It is not the thermostat. this model is just garbage and cant wait to get something that works right. i have been fighting this thing for 4+ years now.

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Flip the lower panel over and look for a wiring diagram. On that diagram there will be a diagnostic chart. Let me know what 5 blinks represent. Then I want you to read on.

On a call for heat, the 24 volt thermostat sends a signal to the control module. The control module will indicate a call for heat with a light on the control either blinking or remain solid depending upon model. The inducer (exhaust) blower will purge all gasses from the furnace and pressurize a pressure switch. Once the pressure switch tells the module to continue, the electronic ignition will energize and send 120 volts to the igniter. The igniter will glow and you will be able to see it if viewed thru the small inspection port. Once the igniter gets hot enough, it sends a signal to the module opening up the gas valve (24 volts). Either a pilot will come on or the burner tube will ignite then spread the flame to all burners. Lastly a safety sensor will be looking for a certain temperature within a few seconds and the furnace will continue to operate and the room air blower will turn on in a minute or two.

What could go wrong? The unit will not run if there is no signal from the thermostat (bad thermostat or broken wire), the control module does not sense a signal from the thermostat (bad control), the inducer does not energize (bad motor), the pressure switch does not close (blocked vent piping, bad switch, plugged condensate hose), the igniter does not energize (bad control, bad igniter), the gas valve does not open or there is no gas (bad gas valve, broken wire, no gas), the pilot does not light (dirty pilot), the burner does not light (bad burner, plugged orifice, not enough combustion air), the flame does not spread to each burner (bad flame spreader, dirty flame spreader, more bad burners), the flame safety sensor does not detect flame (dirty or bad flame spreader, bad flame sensor, broken wire, bad control), or the room air blower does not energize (bad fan motor, bad control).

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