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I have a Rheem furnace and a Ecobee3 smart thermostat. I'm having a problem where the Ecobee3 randomly loses power and reboots. I've traced this back to the control board (IFC) inside the HVAC unit. I don't know exactly what is causing the power to cut out, but my guess is that it is some type of kill switch built into the unit. Regardless of the reason the power is cutting out, I want to see if I can first fix my Ecobee3 issue.

To fix the Ecobee3 issue, can I remove the R and C wires from the IFC and wire them directly into the transformer inside the HVAC unit?

EDIT: Here is the wiring diagram image.

Wiring diagram

Also, the IFC model number is 1012-925A and the HVAC unit is a Rheem Classic 90 Plus. I don't know the if the number on the wiring diagram is the model number, but that number is 90-24007-03.

EDIT 2: I pulled two different switches. One of the limits for the burners (first image) and one for the pressure switches (second image). When the burner switch was open, the thermostat lost power. When the second was open, the thermostat didn't lose power.

It looks like the HVAC system or the IFC is opening one of the limits, which is cutting power to the board. This must be how it prevents the house for exploding. :)

I don't think it is normal that the HVAC/IFC is killing power multiple times per day, but it sounds like it might be time to have someone take apart the whole system and see why that is happening.

Limit switch

Pressure switch

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2 Answers 2

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Based on the ladder diagram, it looks like the R terminal is only energized when all the limits (main limit and however many rollout limits there are) are closed. So if any of the limits open, the thermostat loses power (maybe).

enter image description here

I can't say for sure; since I'm not familiar with that board, but if that's how the furnace disables itself during a problem. Then bypassing that safety system can be quite dangerous. I'm not sure if the board monitors the limit circuit, or simply cuts power to the R terminal in the event of an open limit. If it's the latter, then connecting the thermostat directly to the transformer would be hazardous.

WARNING:

The following procedure requires working on energized equipment. If you're not comfortable with that, please contact a local licensed HVAC technician.

  • Connect the thermostat as normal, with the system powered on, and the thermostat not calling for heat/cool/fan.
  • Open the access panel for the furnace, and locate one of the limit switches.
  • Remove one of the low voltage signal wires from the limit.
  • If removing the panel cuts power to the system, replace the cover.

In this state the furnace will not work. But what you're looking for, is whether or not the thermostat has power. If not, then you're not going to want to bypass the IFC. It also means that whenever a limit opens, the thermostat is going to reset. Which is not a terrible thing, as it makes it obvious that there's a problem with the furnace.

Right now the thermostat is connected to the IFC terminals like this.

enter image description here

If instead you connected the thermostat directly to the transformer, then it would look more like this.

enter image description here

You'll notice that the limit circuit (highlighted in red), is completely bypassed by connecting the thermostat directly to the transformer. Which means even if one of those switches open, the thermostat will still be able to signal for heat/cooling.

Again, I'm not familiar with this IFC, I'm simply basing this on the diagram provided. The IFC may in fact monitor the limit circuit, which is why I recommend testing it.

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  • Can you expand on "connecting the thermostat directly to the transformer could be hazardous"? I'm not sure why that would be the case. It seems the same as plugging your smart phone into a transformer via a USB cable. It's just straight 24v power that is being drawn directly from the power grid. Unless the amps on the transformer are too low or too high, isn't it the same thing?
    – voidmain
    Commented Feb 3, 2016 at 2:36
  • @BrianPontarelli Not hazardous to the thermostat, hazardous to your life and property. If you bypass the safety limits by connecting directly to the transformer, the furnace could over heat, or you could have flames roll out of the furnace and start a fire. The safety limits are there for a reason, and should not be bypassed.
    – Tester101
    Commented Feb 3, 2016 at 2:40
  • Huh? I'm not going to bypass the safety switches that kill power to the board. I'm simply powering my thermostat. This is the same thing as a battery powered thermostat that continues to call for heat even when the thermal switch has flipped and killed power to the IFC. There are a number of videos on YouTube where HVAC contractors power the thermostat using a plug-in transformer and it seems like this is the same thing.
    – voidmain
    Commented Feb 3, 2016 at 15:27
  • @BrianPontarelli Please re-read my answer, and look at the photos. If you connect the thermostat to the red and blue dots in the image, instead of the R and C terminals on the IFC. Then you may indeed be bypassing the limit circuit. Again I'm not sure if the IFC monitors the limit circuit, but based on the diagram it doesn't appear to. A battery operated thermostat only uses the battery to power the display, it still uses the power from the furnace to call for heat/cool.
    – Tester101
    Commented Feb 3, 2016 at 15:40
  • Maybe cutting power to the thermostat IS the method the IFC uses to shut those things off.
    – Harper - Reinstate Monica
    Commented Feb 3, 2016 at 19:00
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Yes. Old thermostats are passive devices - a bimetal strip with contact blades. The heater provides 24 volts AC via a transformer, used to switch the gas solenoid, fan relay, A/C relay etc. via the thermostat.

Smart thermostats need power of their own. That's easy: there's a 24v transformer right there in your furnace, its terminals are called R (hot) and C (common/neutral). R is probably already done for you, just now do C. That is to say, wire the Ecobee C wire straight to the transformer, not the furnace's control board, and don't change the relationship between that control board and the transformer, i.e. Don't alter your furnace, simply grab the C wire.

Passive thermostats never needed C, which is why it's not there now. To spare you the need to run a C wire, smart-thermostat designers created some inventive ways for the thermostat to leech power off current flowing to the solenoids/relays. This doesn't always work.

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    Just grabbing the C wire won't help if the board is cutting the voltage on the R wire. I think I need to grab the R and the C wire from the transformer to fix the problem. Not just the C wire. My plan is to splice the wire from the transformer to the board and tap the wires to the thermostat there.
    – voidmain
    Commented Feb 2, 2016 at 15:19
  • My concern is that tapping R off the transformer directly means you are bypassing several safety interlocks that do something. That is, with direct access to R, the Nest could activate devices without realizing that the interlocks are saying Do Not Activate.
    – Harper - Reinstate Monica
    Commented Feb 3, 2016 at 18:32

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