I suspect my furnace of reaching a temperature limit and cutting power to my thermostat as a result. How would I determine if that is the case?

First off, my situation is very similar to Furnace turns off power to the thermostat, but I have a slightly more basic question. It may be helpful to read that question first though.

I recently replaced my 4-wire thermostat with a 5-wire wifi thermostat. It now will regularly lose power. I've checked and replaced the wiring from the thermostat down to the furnace. I had been thinking a loose connection was the cause, but now I'm not so sure.

Like the other post, I have an older natural gas furnace that (based on comments from HVAC professionals) is likely getting less than ideal air-flow. How would I confirm that the furnace is actually hitting a limit and thus cutting power?

If it helps, the loss of power seems to be more common when it is colder outside and the furnace has to work harder. It's now bad enough that the furnace doesn't run enough to keep my house quite up to full temp on very cold days. The thermostat can take many minutes to reset and then doesn't keep the blower running for a time after the heat is off.

  • What make and model is your furnace, and can you post a wiring diagram for the unit please? Commented Dec 3, 2020 at 2:47

2 Answers 2


I will assume your thermostat is a digital one. Digital thermostats can get power 3 ways:

  1. From the C wire on the furnace transformer
  2. From leakage current across the relay coil
  3. From batteries in the thermostat

MOST of the digital ones on the market today have a C terminal. However there are some that "steal power" these are intended for when you are using antique thermostat wiring that does not have enough wires to have a C wire. Some of these had batteries some did not.

The batteries are for BACKUP in case of power failure. NOT for a permanent source of power.

I suspect you do not have a properly hooked up C wire. Your old stat had backup batteries in it and stole power off the relay coil. It also lost power too but during those times it could not get power off the relay coil it used it's batteries. Your new stat lacks backup batteries or it has dead batteries and your C wire isn't correct so the stat has to get power from the relay coil that is why it cuts out periodically.

Without more info we cannot possibly tell you how to troubleshoot your thermostat wiring. I would normally suggest an HVAC pro but my experience reading the professional HVAC forums is a LOT of HVAC techs out there don't understand thermostat wiring anyway. That is sort of understandable because furnace and thermostat designers seem hell bent on complexifying something that really should be very simple. Take pics of the stat and supply it's model number, take a pic of the terminal block in the furnace that the stat wiring is connected to and make sure the pics show the wiring colors used on the furnace terminals.


There are so many directions an answer could go and with more information we can help diagnose the root cause further. But the original text asks twice "how can I tell whether the furnace is hitting a temperature limit" or, said differently, "how can I tell whether the furnace stops heating before the thermostat said to?"

Disconnect the thermostat and connect the R and W wires directly. Use a paperclip jumper wire across the terminal block on the mounting base, or pull the two wires from the terminals and bend them so that their springiness holds them in contact.

This creates a never-ending call for heat and the furnace should run until you're sweating and the house feels like a sauna. If the furnace stops running before you open the R-W connection then yes, the furnace is hitting some kind of limit.

If the furnace does shut down and you see the LED on the furnace controller has gone dark, or disconnect R-W and measure about 0 AC volts between them, then the furnace has cut power. Depending on how in-depth you wanted to go you could then find and test switches inside the furnace to determine which one caused the shutdown.

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