# Thermostat keeps resetting



## GASCo (Jan 23, 2017)

I can't post links yet, but the image is here:


https://postimg.cc/image/b2p8nqtd9/


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## yuri (Nov 29, 2008)

Do you have #18 wire to the thermostat all the way from the furnace?

If it is smaller like phone wire or less than #18 it can cause problems.

Limit controls on most modern furnaces don't shut off the 120 volt power to the transformer. Back in the 70's and mid 80's they did. Now they shut of the W circuit or gas valve 24 volts.


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## GASCo (Jan 23, 2017)

Thanks, Yuri!

I can't see the whole length of wire because the basement is finished but as far as I can tell the 5 wire thermostat wire that leaves the furnace gets all the way to the wall where the thermostat is. The distance is only about 20ft.

Can it be that the transformer at the furnace is failing? Because I think if the problem was the wire, the resetting would occur randomly and not only when furnace runs.

I have a 20VA transformer laying around but I believe it wouldn't be possible to power the thermostat with a separate transformer, since then the thermostat would do C+W from different supplies to call for heat and that would be weird to say the least, right?


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## GASCo (Jan 23, 2017)

The furnace is a Rheem from early 2000's


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## yuri (Nov 29, 2008)

Rheem of that era I would be 99% sure will interupt the W wire or gas valve not the transformer.

You also need a 35- 40 VA transformer.

Check the wiring behind the thermostat in the wall for splices and make sure the wires that attach to the tstat are not brown and corroded. Scrape them down to bare copper.

Make sure R and C have not been switched around in colors. It may work backwards but not properly.

You should be able to see if it uses the same LVT cable and compare it to what is at the furnace.

Maybe you got a dud/bad tstat. Have heard of problems with Nest but not Ecobee.


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## supers05 (May 23, 2015)

What's the exact model number of the furnace. 

Cheers!


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## beenthere (Oct 11, 2008)

Does your system have a condensate pump with its safety switch wired to cut power to the thermostat.


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## GASCo (Jan 23, 2017)

Thanks for the questions guys.

Well, last night I checked the wires and they all look good and making nice contact. Measuring the voltage R-C through the connector itself I measured 27.4VAC. Of course at this time to thermostat was working fine. I couldn't see it off last night but now I see online it reseted a couple times around the time I was measuring.

My furnace is a 80%, so no condensate pump.

The actual model is UGDG-12EARJR (Ruud).

Coincidence or not, the filter was loose before (not completely functional) and now that I fixed the location and there is therefore MORE resistance and pressure drop it seems that the resets are less frequent, but once they occur they seem to keep repeating for as long as the temperature is being maintained.


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## GASCo (Jan 23, 2017)

Just got home and the Thermostat was off. Quickly I measure R-C voltage and it was zero. 

So we know now that the 24V line is actually cutting off.

I went downstairs and noticed that the humidifier was also off (probably because ithere is no 24V to tell there is a heat call).

In the furnace, the main blower was on, the gas valve was off as well as the exhaust fan.

I hope this clears the situation a bit!

Thanks!


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## beenthere (Oct 11, 2008)

Carrier has some furnaces that kill power to the thermostat when the high limit trips. While I am not aware of Rheem having any models that do that. They may. So check to see if its tripping the high limit.


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## BayouRunner (Feb 5, 2016)

Not that this will be a lot of help but on that particular furnace the transformer supplies 24 volts to the control board and it goes through the control board and then you pick up your 24 volts at r,c. where rcwg are on the board. So if all connections are good on the low voltage wiring. You could check for low voltage at two places to eliminate if the board was bad. One at the low voltage leads going into board and again where it comes out of the board at r and c. Problem is when you take the cover off its going to kill the power to the board and when you reapply power to test everything may work properly. Perhaps you could temporarily bypass door switch so that you could check the unit if you could catch it in default again. Please don't forget to remove the bypass after you rule it out. I doubt it's the transformer, it's possible it could be your power to the furnace itself so check your plug and receptacle as well. I've seen a lot of pressure switch failures on those units but I honestly can't remember the sequence of events when it fails. Although I doubt it cuts off 24 volts. So basically what I'm saying is check all connections, then 24 volts into and out of control board (during failure) if that checks out, look at power going into the furnace. A temporary problem like that can be tricky to find. Good luck!


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## Bob Sanders (Nov 10, 2013)

GASCo said:


> I went downstairs and noticed that the humidifier was also off (probably because ithere is no 24V to tell there is a heat call).


The stock transformers that come built in to these furnaces are not that big... maybe 40va, so the question I have is how much stuff are you running off that transformer? Is the humidifier running off that transformer as well?


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## GASCo (Jan 23, 2017)

Bob Sanders said:


> The stock transformers that come built in to these furnaces are not that big... maybe 40va, so the question I have is how much stuff are you running off that transformer? Is the humidifier running off that transformer as well?


No, the humidifier has its own 20va transformer. It only gets R-C from the furnace to determine if heat is on.



BayouRunner said:


> Not that this will be a lot of help but on that particular furnace the transformer supplies 24 volts to the control board and it goes through the control board and then you pick up your 24 volts at r,c. where rcwg are on the board. So if all connections are good on the low voltage wiring. You could check for low voltage at two places to eliminate if the board was bad. One at the low voltage leads going into board and again where it comes out of the board at r and c. Problem is when you take the cover off its going to kill the power to the board and when you reapply power to test everything may work properly. Perhaps you could temporarily bypass door switch so that you could check the unit if you could catch it in default again. Please don't forget to remove the bypass after you rule it out. I doubt it's the transformer, it's possible it could be your power to the furnace itself so check your plug and receptacle as well. I've seen a lot of pressure switch failures on those units but I honestly can't remember the sequence of events when it fails. Although I doubt it cuts off 24 volts. So basically what I'm saying is check all connections, then 24 volts into and out of control board (during failure) if that checks out, look at power going into the furnace. A temporary problem like that can be tricky to find. Good luck!


I'll try that next! 



beenthere said:


> Carrier has some furnaces that kill power to the thermostat when the high limit trips. While I am not aware of Rheem having any models that do that. They may. So check to see if its tripping the high limit.


Where would be the high limit switch? I see one switch right above the burner inside the exchanger, is that a NC that becomes an open when too hot? Is that what I should test?


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## beenthere (Oct 11, 2008)

Above that switch, to the left or right.


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## supers05 (May 23, 2015)

R is interrupted by the limit switch if I got the right manual..... Not sure if I did. The model numbers are a bit wonky on the cross branding. 

http://www.jmargolin.com/furnace/RUUD_GPH_Manual.pdf


Cheers!


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## Bob Sanders (Nov 10, 2013)

supers05 said:


> R is interrupted by the limit switch if I got the right manual..... Not sure if I did. The model numbers are a bit wonky on the cross branding.
> 
> http://www.jmargolin.com/furnace/RUUD_GPH_Manual.pdf
> 
> ...


Good catch.

It appears the blower door switch as well (interrupts on the 120 side) so I would check that to make sure it's not intermittent.


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## GASCo (Jan 23, 2017)

Guys,

Thanks a lot for all the help. I finally found the culprit after taping the door sensor switch on so I could open the door when the furnace was misbehaving.

Turns out that for the past week or so, after I taped the switch the thermostat hasn't reseted not even once. 

I believe that switch is going to be taped for the rest of the furnace's life. Is that OK? I am believing that that switch is nothing but a mechanical safety device for when you go replace the filter. If this is true I am smart enough to turn the furnace off, or not to stick my hand into the blower.


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## beenthere (Oct 11, 2008)

Yes its a safety switch for changing air filters, or working on the board.


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