Two Nest tstats for gas-fired steam boiler with tankless loop - failed install

DanielC

I want to use Google Nest Thermostats to monitor and control my heating system while I’m away for extended periods. I got two from the utility company for $1 each. I don’t intend to use setbacks or learning functions – it’s a set it and leave it approach. I installed one tstat successfully and had a massive fail on the 2nd. I want to know how to set up the 2nd tstat.

Sorry about the long post but it’s complicated and I don’t know how to write succinctly. I posted a shorter one on Google’s Nest Community forum but haven’t gotten an answer.

Overview. I have a gas-fired steam boiler with a tankless hot water loop.  There are steam radiators in the original house, controlled by a Lux 1500 tstat connected directly to the transformer. The hot water loop heats an addition using cast iron baseboard radiators, controlled by a Honeywell analog mercury thermostat.

Hot water loop. The Honeywell was connected to the boiler via a two-wire cable that ran to a Taco SR501 1 Zone Switching Relay and then to the 24V transformer. I wired a Google Nest Power Connector (NPC) to the T/R, T/W and C terminals on the Taco, then ran the R & W wires from the NPC to the Nest tstat. It worked fine.

Steam heat. The Lux thermostat was using only W and RH.  The Google Nest Thermostat compatibility tool told me I could replace the Lux with the Nest Thermostat if I ran a C wire from the transformer to the Nest.  The thermostat wire was 18/3 with one conductor set aside and not used. I made all the connections as instructed (W, R, and C). When I turned on the power at the boiler, the Taco went through the self-test then went click click click and went dark. Now there’s no 24V power at the transformer.  There is 120V power in the boiler room. (I’m assuming that I’ll be able to get power back with help from my heating guy.  But he’s not very familiar with the Google Nest Thermostat so I’m asking about that here.)

I pulled out the wiring diagram for the boiler.  There are two anomalies.  First, the thermostat wire had been connected to W and RH on the Lux.  However, the wiring diagram indicates that the thermostat sits between the Y terminal on the transformer and G on the transformer.  The G on the transformer then goes through the McDonnell & Miller PSE-802-24 Low Water Cutoff and back to R on the transformer.  There are other controls and switches in the circuit, too, according to the wiring diagram.  Secondly, ChatGPT suggested that I can’t use the C terminal on the transformer with the C terminal on the thermostat.

Here's the wiring diagram.  I would really appreciate help with this, it is different from what the compatibility tool told me.  Is there any way for the Nest Thermostat to work with this system (once the power is restored)? Can I use the C wire from the SR501 to the steam tstat (in other words, two Google Nests connected to a single C wire)? Would two Google Nests draw too much power from the transformer, so I’d need a 2nd transformer? I’m also wondering what shorted out the system and whether I need to get the original transformer replaced, or simply check/replace the fuse in the Taco, or …?

Thanks very much.

Jamie Hall

Oh dear. I wonder what fried. Probably the transformer in the boiler control… or Taco… or both.

Now. There is one, and only one, reliable way to control that arrangement with a Nest. Use a completely separate transformer and a relay. The Nest is wired with Rh and C to the transformer. The relay coil is connected from W on the Nest and the other coil terminal back to C on the transformer. The normally open contacts of the relay are connected to G and Y on the boiler.

For reference, probably what happened is that you got the transformer for the Taco connected in series but out of phase with the transformer for the boiler. They don't like that…

DanielC

Thanks for the insight. Just for clarification, does that mean I would leave the Nest and Nest Power Control connected to the Taco for the hot water loop, and use the new transformer and relay for steam?

HVACNUT

Can you get a 4 conductor thermostat wire?

Rc on the Nest to R on the transformer relay.

C on the Nest to C on the transformer relay.

Rh on the Nest to G on the transformer relay.

W on the Nest to Y on the transformer relay.

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1853297#Comment_1853297

If the Taco is still functional and was working, yes. I'm concerned that it may have fried.

mattmia2

I think the taco as a separate fuse at the xfmr which is probably now blown.

DanielC

https://forum.heatinghelp.com/discussion/comment/1853298#Comment_1853298

It'd be great if we could run a new wire from the basement to the living room. But we have a 90-year-old house and the path isn't easily accessible.

DanielC

I will talk to my contractor when he comes on Friday to repair the system.

LRCCBJ

I'm a bit confused with the explanation.

You are using the Taco for the hot water loop………….agreed?

For the steam control, why would you connect anything to the Taco or try to use the transformer in the Taco? The steam system has its own transformer and control circuit. You would simply run the C wire to the negative side of the transformer (C on the ladder diagram) on the boiler and the Taco is completely out of this loop.

DanielC

Yes, @LRCCBJ, I'm using the Taco for the hot water loop. I connected the thermostat side of the Taco to the Nest Power Connector and then the NPC to the Nest thermostat that controls the hot water loop. That worked for the hot water loop. (The NPC eliminates the need for a C wire to the Nest.)

For the steam system, I connected the Nest just as you said: ran a wire from the C terminal on the boiler's ladder diagram to the C terminal on the Nest, used the W & R wires on the Nest just as they'd been used on the old Lux tstat, and left the Taco out of the loop. That crashed the system.

LRCCBJ

How is the Taco currently wired to the boiler to enable the Taco to start the boiler under a hot water demand? You must be sure that 24V from the Taco transformer is not going to the boiler..

If it was working properly before the Nest installation, then this is probably OK.

If the two transformers do not interact in any way, then we're missing something that is wired differently than stated.

An easy mistake to make at the boiler's transformer is to make an error in the determination of which terminal is negative. Sometimes they don't have any letter identifying them and you need to be very careful which is which.

DanielC

Here's the TACO wiring diagram. The wire back to the boiler is at the bottom. The red wire is connected to a black wire marked Y that runs to the boiler transformer; the white wire is connected to a Honeywell component that controls the temperature on the boiler water. I uploaded the boiler wiring diagram, too.

LRCCBJ

That is proper. The 24V from the boiler is on the load side of the Taco relay. There is no crossover from the 24V transformer on the Taco to the 24V transformer on the boiler.

It does not appear that you can screw up the C terminal at the boiler transformer. Those three green wires go directly to it.

DanielC

Where I'm confused (and I'm just a homeowner and no expert here by a long shot), is that wiring diagram appears to my eye to indicate that the C terminal on the boiler transformer is connected to equipment ground. I thought the C terminal was supposed to supply power.

Jamie Hall

C terminal is ground. R is hot. The Nest must have power between Rh or Rc and C to operate, and it switches W to Rh when heat is called for.

All the boiler control is looking for is continuity between Rh and W — G and Y on the boiler control. Where you can blow the boiler control is hooking C on the boiler to G, directly or indirectly.

LRCCBJ

The Nest needs the C wire to go "C" on the boiler transformer…………….the terminal with the three green wires. If it went elsewhere, you found your problem.

DanielC

I used that exact C wire, the one with the three green wires. I thought I moved the W from the old tstat to the W on the Nest, and the R from the old tstat to the R on the Nest. Maybe I reversed the R & W wires at the thermostat? Would that have caused a problem?

Jamie Hall

https://forum.heatinghelp.com/discussion/comment/1853366#Comment_1853366

The old thermostat likely didn't care which was R and which was w. Nests do, and you had a 50/50 chance… heads I win, tails you lose. But yes, that would have caused the problem — and the question now is what blew and what needs to happen to fix it.