How to test which wire is R and W in a 2 wire only thermostat?

sh500 · February 2

I currently have a battery powered thermostat with 2 wires. It is working fine with the corresponding wires going to the R and W terminals.

I tried replacing it with a Honeywell T8775A1009 digital heat only thermostat https://honeywellhome.com/us/en/products/air/thermostats/non-programmable-thermostats/premier-whiter-digital-the-roundr-thermostat-t8775a1009-u/ that has no batteries, connecting the terminals as they were wired on the old thermostat. The thermostat turns on, but as soon as there is a call for heat, it turns off.

The vent damper turns on to open the vent at the boiler but the gas valve/burner do not turn on.

Is it possible that the wires are reversed at the thermostat? Or, because of the multiple safety controls on the steam boiler, is this type of thermostat likely incompatible?

I don't want to open the electrical connections at the boiler. Is it possible to safely check the wires at the thermostat with a multimeter without shorting anything? If so, how can that be done?

Thanks.

KarlW · February 2

If you have a Boiler, you are likely out of luck. The test cycle of the boilers often cut power while it’s testing. You may need to go back to a battery powered Thermostat.

Teemok · February 2

Well.....let's start with, that's not the best thermostat. I would stick with a battery type. I might be corrected by those who have more electrical theory than I but here's what I understand. We are talking about a/c current alternating positive to negative 60 times a second. Electrons going one way and then the other. That device likely charges a capacitor to hold its "switch" (physically not a switch but technically) closed for the call for heat. They don't even mention it's a power sealing thermostat in the instructions. I can't imagine that the thermostat cares what the color of the wire is but I've been wrong before. My bet is it's rubbish and that's why they discontinued it. My cousin used some and had to replace them all. My condolences. What KarlW said

.
If your vent damper opens, that would indicate a call for heat. There might be another problem in the ignition sequence. Twisting or have someone hold the two wires crossed together eliminates the thermostat altogether placing a permanent call for heat. Remember to untwist after the test.

Teemok · February 2

I don't think this will help you out as you don't want to open anything up but I'll put it here so maybe someone else who wants to find which wire is R can. https://support.johnstonehvac.biz/hc/en-us/articles/360039702214-Transformer-and-Power-Phasing-Reversed-Polarity

Jamie Hall · February 2

Try @Teemok 's test of twisting the wires at the thermostat together to simulate a call for heat. The boiler should run through it's start cycle and fire up normally, then stop when you untwist the wires. That will eliminate the wiring and boiler from the equation.

Now in a 24 VAC simple system such as yours, it shouldn't matter which wire is connected to which terminal on the thermostat. However, it might though it seems unlikely. So swap the wires around and see how it behaves. If that works... splendid. I'm not counting on it, though, and the suggestion for a battery powered thermostat instead makes sense...

Chris_L · February 2

I don't know anything about that thermostat. But you should be able to identify the R wire with a voltage detector like this:https://www.homedepot.com/p/Milwaukee-1000-Volt-Dual-Range-Non-Contact-Voltage-Detector-2203-20/205441488. (The R wire should always show voltage; the W wire should not when it is disconnected from the thermostat, but should when connected and with the thermostat calling for heat.)

Big Ed_4 · February 3

If your stack damper opens , your thermostat circuit is working . If the circulator also comes and no fire , you need to check the burner circuit , which is in series for a break .

sh500 · February 3

Thanks for all the input. I put back the old thermostat and it works just fine. I guess after the damper opens, the thermostat losing power shuts off the call for heat.

I read that it is a power stealing thermostat and there is no visible battery on the circuit board.

This is a steam boiler. Not sure if the low water intermittent test shut off the circuit but that usually happens at some point after the burner fires.

If the transformer circuit loses power at any point during the cycle, then this thermostat won't be compatible it seems.

Was hoping to get rid of the 17 year old battery thermostat and just put this non programmable non battery thermostat since I don't do serbacks and just leave a constant set point for steam.

Big Ed_4 · February 3

Don't over think it ...

Alan (California Radiant) Forbes · February 3

The Honeywell T87 is not a power stealing thermostat.

EBEBRATT-Ed · February 3

I had some issues with that thermostat had one at my house worked fine for a few years and then went wacky. It was just connected to a Honeywell relay to T & T. It would sometimes pull in and drop out. I finally tossed it and put something else in.

It has to power steal somehow as it has a light on it.

Teemok · February 3

@Alan (California Radiant) Forbes Please clarify. "The Honeywell T87 is not a power stealing thermostat." No C, no batteries and no bi-metal or contacts. Does it operate on faith? Is it a "ten year" non replaceable battery like modern smoke detectors?

EdTheHeaterMan · February 4

Teemok said:
@Alan (California Radiant) Forbes Please clarify. "The Honeywell T87 is not a power stealing thermostat." No C, no batteries and no bi-metal or contacts. Does it operate on faith? Is it a "ten year" non replaceable battery like modern smoke detectors?

No C.
Correct

No batteries
Correct

No BiMetal
incorrect. there is a BiMetal that chances the position of a sealed mercury bulb

No Contacts
Sort of. The contacts are enclosed inside the sealed mercury bulb

operate on faith?
Not really. more on physics.

Ten Year battery?
no battery

Here is the IO sheet for the T87F

https://usermanual.wiki/Honeywell/T87F.1096874439/view

dko · February 4

The T87 and the T8775 are different stats.. op is talking about the T8775, not sure how the T87 even came into the discussion.

T8775 has a digital display, has no battery backup and is indeed power stealing.
When we mention T87 now, we should be talking about the current edition and not the discontinued mercury one unless specifically mentioned. The current T87K is also technically power stealing, but battery assisted(CR2450).

Jamie Hall · February 4

Teemok said:
@Alan (California Radiant) Forbes Please clarify. "The Honeywell T87 is not a power stealing thermostat." No C, no batteries and no bi-metal or contacts. Does it operate on faith? Is it a "ten year" non replaceable battery like modern smoke detectors?

No. It operates on a very simple bimetal strip which changes its curve in relation to temperature. As it changes the curve, it turns on or off an equally simple little switch which, in turn, turns the heat on or off. They last forever (I've seen some earlier models almost a century old, still running just fine).

Teemok · February 5

Pile on related to mistaken thermostat ID. It's understandable. I bet Honeywell intended that exact mistake when they chose the model number of their new questionable quality thermostat. I'm aware of how the old mercury tank standard works.

Teemok · February 5

@Jamie Hall I have a gallon paint can in a protected spot in my shop with many mercury tubes in it that I collected over decades. Lots from old gravity air systems in San Fransisco. They were very reliable but no mater how you talk to your pocket computer it will not control or schedule them. This thread reminded me, I need to properly handle my little toxic threat.

How to test which wire is R and W in a 2 wire only thermostat?

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