# Gas furnace cycles on and off...



## kleinbiker1 (Oct 29, 2012)

I'm sure this is common and easy for someone who is HVAC knowledgable but it's stumping me.

I have what appears to be an older Rheem. There is no computer nor any electronic controls. Once the stat calls for heat the exhaust fan comes on and a few moments later the gas fires and stays lit just fine. Once everything heats up the blower comes on. The gas will then stay lit for another 10 to 40 seconds where it will suddenly then turn off as well as the ventilation fan. The blower will keep running as the mechanical temp controlled switch mounted in the initial duct keeps it on. 

I'm not sure why it's shutting down. I've bypassed the pressure switch that's activated by vacuum created from the exhaust fan so that doesn't seem to be it. The mechanical device that turns on the blower and shuts it off when everything is cool enough appears to work fine. I can see it's dial turning appriopriately with temperature changes.

I know in newer furnaces there is some type of electric thermo couple to make sure the furnce has lit properly and will shut down the gas if it doesn't detect heat in a pre determined amount of time. The only thermocouple I thought I had was the one that keeps the pilot light lit. I thought I would pull the burner out and take a look. I did find what looks like the end of an elecrical thermo couple with no wire attached to it. I can't however find any stray wires that might have gone to it. 

I took several picures of the inside of the furnace and what not to hopefully help someone help me....

TIA,

Chris


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## kleinbiker1 (Oct 29, 2012)

More pics...


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## Wuzzat? (Oct 30, 2012)

If I could read the schematic I could see all that can cause your symptoms.  The ladder diagram on the right third of the diagram is more useful so a closeup of that might work.
 Or post a parts list from the Rheem manual.


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## kleinbiker1 (Oct 30, 2012)

Does this help?


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## Wuzzat? (Oct 30, 2012)

kleinbiker1 said:


> Does this help?



Jawohl, Herr kleinbiker1!

There is a wire running vertically from the right side of the transformer 24 vac secondary down the lower right side of the diagram and it could be called the "common" lead or terminal.  Put one DVM lead on this point. 

You can see that the "hot" 24 vac lead goes to terminal R on your thermostat.

You see the assembly marked "36C84"?  I assume this is the gas valve.  

There are four switches, the contacts of which need to be closed for this assembly to be powered: 
the heat switch contacts in the thermostat, 
the switch marked CAL, 
the switch marked SNP, and 
the switch on the crease in the paper.

You already know the gas valve is shutting off so that's not the first point I would monitor.  Using your other voltmeter lead, monitor the point between CAL and SNP.  

If the ~24 vac stays here then the switch that is opening is downstream.  
If not, it's upstream.

Continue this procedure until you find which one of these several switches is opening, or not closing, and why.

Dig your test prods into the connections: sometimes they have tarnish or invisible insulating grime on them.  Watch for sharp sheet metal edges.

Your furnace does have a "computer" and this arrangement of relays is it.  It uses "relay logic".  
None of the relays are of the latching type so this computer does not have memory.
My '82 furnace gizzards are similar to those of your '81 furnace.


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## kleinbiker1 (Oct 30, 2012)

Ok thanks for the very thorough reply.  I'm in the process of getting ready to head to work for the night so I will have to try it out tomorrow. The one problem I foresee is translating the ladder diagram into what I'm physically looking at. I certainly know where the transformer is but may not necc know which wires are which. Also something to note, I'm having a sneaking suspicion that things have been changed inside this furnace at some point in its life.  In wondering what that "probe" is that's missing its wire located between the pilot gas supply and the thermo couple?  The corner of the last schematic photo I posted says it has a white Rodgers automatic pilot relight control.  Would that be the unwired "probe"?  Also there def no stray wires that would have been connected to it.


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## Wuzzat? (Oct 30, 2012)

kleinbiker1 said:


> The one problem I foresee is translating the ladder diagram into what I'm physically looking at.


That is definitely a problem.  

As you figure this out, make notes about wire colors in the ladder diagram.  

Ideally you want a one-to-one correspondence between what you see when you look into your furnace, the wiring diagram and the ladder diagram.

Also, ask Rheem for whatever service info, sequence of events and Theory of Operation that they are willing to send you.

My requested factory manual, free from Bryant, is covered with my notes, ohm readings, etc.  It's about 10 pages of photocopied material.  
Even with all this, the last problem I had took me 3 hours to find.  It was a corroded connection on the edge of the control board which I indirectly measured at 60 megohms [vs. "zero" ohms].

Dunno' about that unused pin terminal.


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## kleinbiker1 (Oct 30, 2012)

I found the white Rodgers module and its not hooked up to anything. Also of you see in the 2nd picture I originally posted there are actually two low voltage transformers.  They appear to be connected to each other actually. This will certainly take some sorting to figure out.


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## Wuzzat? (Oct 30, 2012)

kleinbiker1 said:


> its not hooked up to anything.
> 
> there are actually two low voltage transformers.  They appear to be connected to each other actually. This will certainly take some sorting to figure out.


That does not make sense, so proceed with caution.

Possibly it's two 12vac units with the secondaries wired in series in the correct phase to get 24vac.  
That is sort of shown on the ladder diagram, now that I look for it.

Make a diagram of what you have now, so that you can return to that configuration.


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## kleinbiker1 (Oct 30, 2012)

I snaped a few more pictures before I left for work...

The disconnected white rodgers module....






Some other module/device mounted to the inside of the furnace cabinet....






Two transformers....the lower one has all it's connections on the back side....








Close up of the blower/fan controller...


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## om55 (Oct 31, 2012)

has anyone checked the heat anticipator on the thermostat, if it isn't set right it will make the furnace short cycle.


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## Wuzzat? (Oct 31, 2012)

kleinbiker1 said:


> I snaped a few more pictures before I left for work...
> 
> The disconnected white rodgers module....



Evidently your gas is ignited by other means than this high voltage module.  
While you're at it, check the resistance between each of the three module terminals, T1 to term stub, T2 to term stub, T1 to T2.

Use the number of wires into each module (four wires and two wires) and the colors of the wires into each module and the larger wiring diagram that shows colors 
to figure out what these modules are.  
The color code abbreviations are in the lower right corner of the wiring diagram.
If the terminals of the modules are marked with tiny letters or numbers, mark these on a copy of the wiring diagram.
Temporarily unbundle wires if you need to, to trace them.

You can check the anticipator resistor with an ohmmeter but the voltage from the 'stat will tell you if the 'stat is commanding this short cycling.


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## kleinbiker1 (Oct 31, 2012)

Yes the furnace has a standard old school pilot light. 

I can't imagine that the stat is causing the shut down.  It's the old school round dial type.


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## Wuzzat? (Oct 31, 2012)

kleinbiker1 said:


> Yes the furnace has a standard old school pilot light.
> 
> I can't imagine that the stat is causing the shut down.  It's the old school round dial type.


I'd try to find out what gas valve you now have.  

The ladder diagram in that area has changed so I strongly recommend you draw a new wiring and ladder diagram based on what is actually in your furnace and then measure some voltages.

Your thermocouple voltage output to the gas valve can be 12 millivolts to 400 mV when connected to the valve's solenoid and twice that when the 'couple output is open circuited.  
The 'couple might be bad.
You can simulate the 'couple output with a 1.5v battery and two resistors, values to be determined to see if the valve remains on.

You can probably test your old HV module by putting 24 vac into it and seeing if you can make a spark jump over a short distance.  I'd hold the wires with plastic or wooden clothespins.  
If the module is bad I'm not sure why the tech didn't just replace it.

Evidently the stubby terminal in the second photo of your second post was supposed to have a heavily insulated wire, like a spark plug wire, running from it to the HV module.


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