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do two-wire nest thermostats age out

dathilacha
Community Member

hi all. my furnace has developed a problem of cutting out--it runs for a day or so, then stops and has to be manually restarted. the oil company i have a service contract with is saying the problem is the thermostat, a nest, two or three years old. my nest runs on two wires; service guy says in most thermostats there's a third wire, that nests can run on two (as mine has just fine for the past two or three years), but that then whatever system allows them to run on two ages out and dies, and the furnace starts cutting out. so he thinks i need a new thermostat--doesn't think there's anything wrong with the furnace. has anyone ever heard of this, and does it make sense? thank you, david frankel. 

1 ACCEPTED SOLUTION

laurentbourg
Silver Product Expert
Silver Product Expert

The best way to diagnose whether you have a furnace issue is to revert to a basic thermostat such as the one you had before your Nest. If the thermostat calls for heat and the fan runs with no heat does seem to point to a furnace issue, especially in a 2 wire setup where there is no direct way for the thermostat to activate the fan independently. 

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8 REPLIES 8

laurentbourg
Silver Product Expert
Silver Product Expert

The service professional is referring to is a C wire as being the third wire. I have not heard of cases where not having a C wire for a certain period of time works fine and then suddenly the system would require one. There are cases where installing a C wire is trivial, did you ask your service professional how much would they charge you? There are also C wire adapter you can purchase cheaply to validate that it is a C wire issue, or even keep those installed to resolve the issue if you don’t mind them esthetically. You should probably first replace your Nest with your old thermostat if you kept it to ensure that it is not a problem with your furnace. 

thank you so much, i appreciate your answering. no, he didn't say what he would charge, but wasn't really offering to do it--thought i should either get a different kind of thermostat (an ecobee) or get an electrician to put in a third wire. maybe he just doesn't like nest--he said he'd heard of lots of problems with it, whereas my experience of it so far has only been good. in any case, since he left and since i posted this question, something occurred to me: the thermostat is still asking the furnace to start up when the temperature sinks below the set temperature. the furnace comes on, and stays on, but blows cold air because there's no flame. that would seem to say the problem is with the furnace, not the thermostat, i would think?  thank you again.

laurentbourg
Silver Product Expert
Silver Product Expert

The best way to diagnose whether you have a furnace issue is to revert to a basic thermostat such as the one you had before your Nest. If the thermostat calls for heat and the fan runs with no heat does seem to point to a furnace issue, especially in a 2 wire setup where there is no direct way for the thermostat to activate the fan independently. 

Thanks so much for your help

Jake
Community Specialist
Community Specialist

Hey dathilacha,

 

I want to ensure you are good to go, and everything is working properly. Please let me know if you are still having trouble, as I will be locking the thread in 24 hours due to inactivity. 

 

Thanks so much laurentbourg, for the helpful information. 

Best regards,
Jake

dathilacha
Community Member

 thank you jake, i think i'm fine. meaning, it doesn't seem like the problem is in the thermostat! have asked the furnace-service company to come back for another visit. thank you, df

Jake
Community Specialist
Community Specialist

Hey dathilacha,

 

Glad to hear you may have isolated the issue. Please let me know if you are still running into any issues after the service company has come out, as I want to ensure you are good to go and everything is working properly. 

 

Best regards,

Jake

dathilacha
Community Member

thank you so much. the service guy can't find a problem in the furnace so insists it must be in the thermostat, but i'm not sure i believe him: the thermostat is turning the furnace on when the temp. goes below the setting, and the furnace is turning on--in that the fan blows--but the flame doesn't ignite so the fan just blows cold air. (erratically; more often than not the furnace comes on fine, but every now and then it doesn't.) i don't see how the thermostat can be at fault, or if it is, i don't understand how. if anyone thinks the thermostat could produce this behavior, it would be great to hear from you; in the meantime i'll keep arguing with the furnace company. thank you, david frankel.