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I have a Honeywell T5 WiFi Thermostat. I am running a gas furnace and an AC system that uses the furnace air handler. There are H, R, G and Y wires coming from the furnace. There was an unused 5th wire (blue) that I hooked up to the C terminal at the furnace and on the thermostat.

- The problem is that when the T5 calls for heat, the heat comes on normally but after about 6-8 minutes, the heat goes off, and then after a few minutes it comes back on and then shuts off again after 2-3 minutes. This 2-3 minute cycle repeats until the house temp reaches the set point.

  • The thermostat reboots when the heat goes off, so I assumed there was a voltage drop causing it to lose power. I put a meter across the furnace's R and C terminals, and there are +/- 28VAC. Same at the thermostat. When I monitor the R to C voltage at the furnace, I see a drop to about 4VAC just before the furnace shuts off. It then comes back to 28VAC. I have tested the limit switches immediately after the heat shuts off and they have continuity.

- When I put my old 4-wire, battery-powered Tstat back on, the system runs continuously until the set- point is reached without any problems.

- I would like to add a second transformer to provide continuous, clean voltage to the thermostat, but I am not sure if this is OK to do that with this model thermostat. I could add it either at the furnace and use the existing R and C wires (disconnecting them from the furnace terminal block) or I could add the new transformer at the thermostat as the video shows (to the R and C terminals) and disconnect the C and R wires running from the furnace to the thermostat. I could also connect the transformer wires to the Rc and C terminals and remove the jumper between the R and Rc terminals, but I am concerned that having 2 sources of 24VAC to the thermostat would cause problems. The Furnace is a Heil 7000 Model # NUGK100DH08 867.769437 MFG in 1990.enter image description here

Does any of this make sense with this thermostat? Also, any idea what causes the voltage to drop at the furnace? Any feedback would be appreciated. Thanks for your help.

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  • Brand/model of the furnace might be helpful.
    – FreeMan
    Jan 20 at 12:29
  • It is a Heil 7000-about 30 years old. The transformer is not so easy to change, and since the furnace/AC functions perfectly with my old thermostat, I am looking for a way to provide clean, continuous power to the Tstat without getting into the furnace components.
    – RGM
    Jan 20 at 19:05
  • Can you post photos of the wiring diagram of the furnace please?
    – ThreePhaseEel
    Jan 21 at 15:38
  • ThreePhaseEel Thanks for your help. I added a photo of the wiring diagram. The Furnace is a Heil 7000 Mode # NUGK100DH08 867.769437. Any help would be most appreciated. Thanks.
    – RGM
    Jan 22 at 20:37
  • Have you verified that you aren't getting a limit or rollout switch opening? That'd be much more likely than the transformer itself being at fault
    – ThreePhaseEel
    Jan 22 at 22:24

2 Answers 2

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your short cycling is being caused by an overheat of the furnace. you just replaced the thermostat and the old one was battery operated. The battery operated thermostat is not able to be turned off like the hardwired one is. So you never experienced an issue until you replaced the thermostat. The thermostat is not at fault. Your furnace is trying to protect itself. Pull your furnace filter out and see if that solves the issue, if it doesn't then make sure all of your registers are fully open and check around the furnace for possible dampers that have been installed into the ducts. If you find dampers make sure they are fully open.

If you are still having an issue I will need the model number and manufacturer of your furnace. I will also need to know the size and number of registers in the house and also the size and number of returns in the house.

Your exact problem happened to me and it ended up being that the furnace was too large for the size of the ducts in the house. Need to do the number crunching in order to figure that out. Also when I said that I need to know the size of the registers I am referring to the size of the pipe going to the register and not the physical register itself. You will have to take the cover off to measure the size of the pipe.

Another way you can induce more airflow is to take the cover off the furnace where the blow is, If there is a switch that shuts the furnace off when you take the cover off you will have to jumper across the 2 wires connected to it to keep the furnace on. If you still have short cycling that is occurring you can test the wiring coming out of the transformer. You will find that it still has 24VAC. If it has that then it is a confirmation that the furnace is overheating. The things that are powered by that transformer do not pull a lot of amperage and the transformers when they go bad they short out or stop working all together. If you pull too much current from it it is going to melt the windings in the transformer or it will blow a fuse in the furnace. I have never seen one cut in and out from too much current.

You can see the circuit I am talking about in the wiring diagram you posted. I zoomed in to the area in the attached photo.

enter image description here

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You already know what the problem is.

The transformer is not powerful enough to provide power to the gas furnace the AC and the new Thermostat.

Find a new transformer that has more power and use it.

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  • That's what I suspect, but changing the transformer on the furnace is not so simple. Do you think that since it works fine for 6-8 minutes, does it then overheat and the voltage drops for few seconds? I hooked up the T5 right at the furnace (2 feet of wire) and the system ran fine. Do you think the long run to the actual tstat location is causing the problem? I would like to power the tstat with a second transformer, if possible, so I won't have to get into the furnace components.
    – RGM
    Jan 20 at 19:12
  • @RGM yes the long run of tstat wires can do that, probably has bad insulation somewhere, Check the wire continuity with Ohm meter
    – Ruskes
    Jan 20 at 20:05

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