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I'm considering using this answer to connect a common wire to my thermostat. I have the exact same setup, and have verified with my multimiter that the red T terminal outside F terminal beeps, while the red T terminal and inner F terminal does not beep.

My question is, is this dangerous for some reason? For instance, this answer.

However, this thread says the alarm expressed above is unwarranted.

What I'm wondering is, am I doing something dangerous by tapping into this lower F terminal for my common connection? Why or why not?

Thank you for any help!

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Yes, it would be dangerous.

The power supply embedded inside your Honeywell controller is not designed or documented to provide power to arbitrary external devices, and the exposed terminals are meant only for the purpose documented.

The schematics in the manual are intended to provide a better understanding for the purpose of installing it in the intended way. They are just enough for that purpose. They are not intended to provide the basis for reverse-engineering, such as for tapping power off the embedded transformer.

Technically it might work: for an engineer tinkering with this controller for fun in a workshop, it will probably work.

Practically, in the sense of leaving this unattended in a home for months and years hoping nothing ever goes wrong it is untested, unproven, undocumented and therefore: dangerous.

Danger is not binary. This is not a case where you need to choose the least dangerous path forward. You have innocuous alternative approaches:

  • Buy a battery-powered thermostat. True, that rules out ones with color displays and Wifi.
  • Install a new transformer near the furnace to supply thermostat power
  • Install a power supply (wall wart) or a large battery pack near the thermostat to power it.

Many smart thermostats will take power from the separate A/C side and if you don't have A/C will allow you to connect a dumb power supply for the sole purpose of supplying power, via Rc/C.

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  • See Figure 8 on Page 7 of the R8184 manual, which depicts an L4064T fan limit connected to none other than the F2 terminal of an R8184 (N version in the diagram, but I see no reason why it wouldn't work with an R8184G for that matter) -- considering that the L4064T is a limit switch, and that a fan relay would be on the 'other side' of that connection, I suspect that the folks saying it's dangerous simply haven't seen that diagram Jan 16, 2022 at 17:16
  • I draw the opposite conclusion! That diagram raises questions. The transformer seems to be center tapped so that T1-T2 is 24V (documented) and T2-F2 is 24V (implied by the inclusion of the L4064T but not documented). The L4064T is documented to draw 0.085A and perhaps by its inclusion in the manual, to have been tested and deemed safe. I don’t have one of these so can’t draw any more conclusions. Is T1-F2 24V or 48V? Does the documented 0.2A maximum apply only to T1-T2 so can current from T2-F2 or for that matter T1-F2 be added to that? What is the current draw from OP's stat?
    – jay613
    Jan 16, 2022 at 17:38
  • ... and I'm only addressing one issue, overdrawing the transformer. Are there other issues to be considered? I'm not trying to outperform you @ThreePhaseEel at interpreting figure 8 on page 7. That would just be hilarious. I'm reenforcing my point .... by doing this you are entering experimental territory, WAY WAY outside the realm of being safe in a domestic heating/wiring context.
    – jay613
    Jan 16, 2022 at 17:40
  • Actually, taking another look at the diagram, you are right -- that center tap raises a lot of questions -- I'm going to delete my answer off the other question as a result Jan 16, 2022 at 19:56

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