Not sure what you meant 27v at thermostat baseplate- is it the C wire pin?

On my Honeywell smart thermostat, the C wire provides power to it. Your ports may be similar. Check the C wire at the thermostat location to see if you have any voltage (should be ~24v). If no voltage there, note the color of the C wire when you go to the other end to check wire connection and the voltage of the wire at the transformer output. If you ruled out the transformer and the thermostat, it could be a loose wire connection or damaged wire somewhere in between.

Also want to add that my condensing furnace can also shut down my thermostat if there is a backup of condensation, as a safety feature. Your boiler may have some kind of protection to shut down the thermostat if something is not working right. I would have the boiler checked out.

My question is whether the 27V coming out of the transformer is high enough to shut down the thermostat. I don't think so. What else should I try?

The 27 V should not be a problem. This an AC voltage which is converted to DC to power the components of the thermostat.

For what I read, you used the old base plate with the new thermostat. Maybe the old base plate has a dirty contact or does not seat properly when you push the thermostat on to it.

Does the boiler have water at the proper level in the sight glass? There may be a safety feature which shuts down power to the thermostat if the water level is too low. Maybe the safety device is malfunctioning. I would think you would read no power if the safety feature was cutting off the power. But it is worth investigating.

A simple test you can do is to connect the R terminal to the Y terminal. This is what the thermostat does when calling for heat. If the boiler does not fire up, then it is a possible wiring problem or a furnace control. Is the wiring very old? Do you know if the wiring was replaced when the RTH9585WF was first installed?

Thanks for everyone's suggestions. They reflect my thinking.

Regarding Resideo and Honeywell, Honeywell stuff was rock solid but I have had problems with Resideo stuff and don't think the quality is up to par. Also their tech support and documentation are terrible.

I am seeing 27VAC at the C-wire/R-wire pair at the thermostat baseplate. R is nominally unswitched 24V. There is also 27V on the C-wire/Rc-wire pair. Rc is nominally switched 24V, which is the output from the hydrolevel. I agree that 27V is within the acceptable range so I doubt that is the problem, nor is a symptom of the problem.

I replaced the baseplate with a new one. No change in symptoms.

The thermostat wire was replaced when the thermostat was installed, but the boiler wiring and the transformer are original with the boiler.

I'm still stuck. I think the next step for me is to buy a cheap thermostat and try it. I need to get one that is at least semi-smart so that it requires voltage on both the R and Rc wires, so that it tests what doesn't work on the old one(s).

I really don't want to call a professional. It would cost a fortune, plus they'd probably start replacing expensive highly-marked-up things instead of finding the problem.

If anyone has any additional suggestions I would welcome them. Thanks again!

Another test is to measure the voltage at the base connection while the thermostat is connected. This would show if there a voltage drop while the thermostat is drawing current. I know you can't get your voltage probe under the thermostat. You can try clipping leads to the base and thermostat connectors and then measure the voltage while it is connected.

"I have had problems with Resideo stuff and don't think the quality is up to par. Also their tech support and documentation are terrible."

That can be frustrating. I have both pre spinoff Honeywell/ and post spinoff Resideo equipment and both have been trouble free. I've had occasion to call Tech Support a couple of times (because of things I wanted to do differently, not because of problems). I think for each of several calls, there was something like a 10-15 wait on hold. The people I spoke to were, I believe, based at a Resideo location in Mexico. All spoke English very well, knew their stuff, and were quite helpful. Sounds like you haven't had such good luck.

Your expectation that a pro coming in would "probably start replacing expensive highly-marked-up things instead of finding the problem." describes someone who doesn't know what they're doing. A competent pro finds the problem (or tries to do so) before doing anything further. There are a lot of hacks around who give the trade a bad name. At the same time, we all know that HVAC systems don't last forever and in the best of cases, parts can and do need to be replaced. In the worst of cases, new equipment is needed.

Good luck.

Well I got it working (a good thing since it's in the 20s at night now). The problem was a combination of the low-water cutoff being triggered and the thermostat being wired wrong.

When I first installed it I tried mightily to get some clarity as to what the various wires did, with no luck, as is now apparent. It's no help that there isn't consistency in the naming and function of the various wires, and there's no concept of a neutral wire in the wire function names. My understanding is that there are four wires: 1) a neutral, 2) always-on power, 3) power that is only on if it's safe to turn on the gas valve, and 4) safe power switched by the thermostat and going to the gas valve. I think I have #3 and #4 switched, but I'm not sure.

I'm warm now so I'm not motivated to tinker with it. Also I ultimately want to put the rat's nest of wiring at the boiler on some clearly-marked terminal strips, and I can't really do that until I don't need the boiler running.

Anyhow, I do appreciate everyone's comments and help. Hopefully this will be useful to others.

Is the person who installed the thermostat originally, the same one to discover and remedy the problem?

Yes, it's all just me. And I haven't fixed it yet -- the only symptom is that the thermostat goes dark if the boiler runs out of water.

I am glad to hear you were able to resolve the problem before this recent cold wave. When you first reported you were measuring 27 V at the thermostat base, I assume it was between the neutral wire and the always on power. I assume you only had the switched power (low water cutoff) as the only power source. Looking at the installation manual for a steam steam, I believe to avoid the thermostat for going blank in the future, the wiring should be as follows:

neutral -> C terminal

always-on power -> Rc terminal

safe to trun on boiler -> R terminal

power to gas valve -> W terminal

Remove the jumper loop wire between R and Rc - this isolates the thermostat power

This wiring should power the thermostat even when the low water cutoff disables the boiler. The drawback is you will have no indication of the problem when you get no heat.

I had asked in a previous post if the sight glass indicated there was sufficient water in the boiler? If there is, then maybe the water cutoff is not functioning properly and needs to be replaced.

Thanks for the help with what wires go where; it will be a big help in getting this right. It sounds like I have the R and Rc wires backwards. I don't know if that was due to mislabeling the two wires, or a misunderstanding of their function. I need to check my notes.

Regarding the low-water cutoff: I stupidly didn't check the water level when you suggested it because I had made some assumptions about what was wrong. I always get in trouble when I make assumptions when troubleshooting. I have a friend who is very good at troubleshooting motorcycle electrics. He always checks everything and assumes nothing. I should learn from him.

The low-water cutoff seems to be working fine, but I need to replace it and have bought a new one, because it is about 30 years old (it seems like just yesterday when I installed that boiler...). It's important because I have to add water to the boiler manually when it gets low. It uses very little water and I usually only have to add water once per year, but still...

I was going to suggest installing an automatic water feed. But if you are only adding water once a year then it may not be worth it.

I find it interesting the low water cutoff is wired to the thermostat. Was it wired this way when the boiler was installed 30 years ago? I imagine the original boiler used a simple two wire thermostat and the additional wiring was added at a later date.

" Yes, it's all just me. And I haven't fixed it yet "

When a person gets to the point where the problems they're experiencing are ones they caused themselves by their own errors, I think that's a signal that they're dealing with something they don't know enough about to handle properly and it's time to call in someone who does know.

HVAC equipment is expensive to replace if other mistaken steps are taken that cause damage. Boilers are a specialized area too, not all HVAC contractors will work on them. Know when it's time to try a different approach.

Good luck.

The low water cutoff has to be in series with the gas valve and the thermostat. You're right that I had a simple two-wire thermostat connection until I installed the Resideo smart thermostat, which required four wires with the addition of always-on power and neutral. The low-water cutoff, and other safety stuff, has been in series with the thermostat all along.

Regarding the comment from the person suggesting that I'm not capable of doing my own work, I kind of take offense to that. Nobody knows everything, and everyone makes mistakes, even professionals. And you don't know me. I'm cautious enough to avoid damaging things, I learn everything I can about what I work on, and I'm not afraid to ask questions, which is what this thread is about. Yes there are many people who shouldn't work on complex stuff, but I'm not one of them.

Pros use trial and error for diagnosis, not for fixes. Most people, having found mistakes they made when trying to do something beyond what's familiar, would recognize the need to call in a pro. If you're doing what you're doing because of financial limitations, that's a different matter.

Good luck. Don't give yourself a task beyond your own knowledge and capabilities by trying to fool yourself by denigrating the capabilities of experienced skilled tradesmen.