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david.hill4@comcast.net

new member

2 post(s)
posted on 11/9/2007
Hi all,
I have a Rheem 100k BTU gas forced air furnace.  
With the thermostat calling for heat the unit fires off fine, then 30 seconds later the blower turns on.  
The unit will operate for only another 30 seconds or so and then both the gas burner and blower shut off.  
The unit will repeat this cycle in another 3 minutes or so. 
I've checked the draft in the flue (it's strong) and just replaced the limit switch, but it still operates the same.  
Any suggestions for the next step?  
 
Thanks in advance!
Dave

 
 
Replies [ post reply ]
brian@pacificgas.ca

senior member

31 post(s)
replied on 11/9/2007

if it was the limit switch the blower would keep running. if the flame drops out and the blower keeps running (even a few seconds) check the sensor (measure microamps) if the gas valve and blower drop at the same time you will have to check what is dropping the voltage at the board or the voltage to the furnace - which means check the grounding too. let us know Brian

brian@pacificgas.ca

senior member

31 post(s)
replied on 11/10/2007

There should be a sensor that has one white wire going to it at the opposite end of the burners from the ignitor. but you have not told us how old or model. -  (note) if this is a older Rheem / Ruud hot surface ignition with no flame sensor - you will need a tec to get a retro kit (not expensive) but I really doult this is the problem unless you have bought a older model that someone just was in stock. the board manufacture replaced a resistor that changed the values and the flame would drop out. It was not Rheems fault but there was a lot of problems in the late 90's - ( there should be a led light to tell a fault code - so if you do not have this it could be the problem mentioned above. but if this is not the case you will need to measure the voltage to w terminal if you do not loose voltage there when the  gas shuts off try the limit switch if voltage is ok there try the pressure switch if voltage is ok there try the gas valve - I use a 12 volt test probe light to check these as it is quicker than a meter and easier to notice voltage drop offs. - it seems that the call for heat is being interupted you just have to determine where. let us know - but I recommend getting a good tec but this should give you info as to what he should be doing. good luck Brian

david.hill4@comcast.net

new member

2 post(s)
replied on 11/10/2007

Brian:
Thanks for the prompt response! We've ruled out the limit switch, but here's another clue. When the flame goes out the blower keeps running, but only for another 3 to 5 seconds. That probably rules out a grounding issue, right?
You mention "check the sensor (measure microamps)" but I'm not sure which sensor you mean.  Can you advise? 
Thanks again for your patience and advice!

Dave
 


garrett.garris@pse.com

senior member

27 post(s)
replied on 11/11/2007

If the blower and flames stop within a couple of seconds of each other, it sounds like a bad circuit board. You could pull the board out and look at the back to see if there is a discolored or hot spot look to it. Thats what I'd bet on.

david.hill4@comcast.net

new member

2 post(s)
replied on 11/12/2007

Thanks for the updated responses! 
The unit is an older Rheem.  I decoded the serial number and I think it was manufactured in 1992 or '93. 
It's a basic 'builder's model' and as I recall the unit isn't very efficient (about 70 to 72%) so it may be worthwhile to upgrade to a newer unit and be done with it!   
I'll check the voltage to the leads as mentioned and see what's up.  I will probably then get a local tech to check it out as I'm about at the end of my DIY abilities.  
Thanks again!
Dave
 



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