Two Nest tstats for gas-fired steam boiler with tankless loop - failed install
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.
Comments
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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…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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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?
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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.
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If the Taco is still functional and was working, yes. I'm concerned that it may have fried.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
I think the taco as a separate fuse at the xfmr which is probably now blown.
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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.
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I will talk to my contractor when he comes on Friday to repair the system.
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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.
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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.
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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.
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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.
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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.
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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.
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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.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
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.
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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?
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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.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Granted, the Nest requires perfect wiring to function properly. But, I fail to see how it could disable the entire system if R and W are reversed. The system does not care if R and W are reversed. The Nest might not function but this result suggests another issue.
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Think about the wiring. G on the boiler control is connected to the hot side of the boiler transformer — labelled R. Now if the wire from Y on the boiler control goes to W on the thermostat and the wire from G goes the the thermostat R connection, no problem. But what happens if R on the thermostat goes to C instead, and W on the thermostat goes to G? Dead short when the thermostat closes. There are other ways to accomplish this… that's just one.
In fact there is one, and only one, way to get it right with this control: G on boiler to Rh on thermostat, C on boiler to C on thermostat, and Y on boiler to W on thermostat. Note that this will also cut power to the Nest if the low water cutout opens, but that's minor.
And no, you cannot connect R on the boiler to Rh on the thermostat, as that will bypass the low water cutout wheb the Nest is calling for heat, and that is a very dangerous condition.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
A further comment: if you use a relay, you can get around the low water cutoff problem. One wire from R on the boiler to Rh on the Nest. C on the boiler to C on the Nest. A wire from W on the Nest to one relay coil terminal. A wire from the other relay coil terminal to C on the boiler. Wires from the normally open terminals on the relay to G and Y on the boiler.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
If you didn't use the c wire for the hot water loop then where is the c wire going that you connected to the 501?
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The mistake that could have been made is that Y goes to R and G goes to W. That's easy to do at the 'stat. My contention is that this will not cause a short anywhere. The Nest may not operate correctly but the result that he is suffering could not be caused by this reversal.
Of course, anything connected to C but the C wire from the Nest will absolutely cause a problem.
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@pedmec , the Nest Power Connector has one set of wires that go to the W, R, and COM terminals on the 501. It has another set of wires that go to W and R on the thermostat. It has some internal circuitry that makes this all work out.
Setting aside the NPC, I did notice that the wiring diagram for the SR501 indicates that one can insert a resistor across W and COM to support a smart thermostat. Maybe the NPC does something like that.
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@LRCCBJ , regarding "anything connected to C but the C wire from the Nest will absolutely cause a problem", I can see that the C terminal on the transformer is connected to "24V GND" on the UTEC Ignition Control. If I'm reading the wiring diagram correctly, the LWCO and the damper are also connected to the C terminal.
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I have no idea how the NPC works. And i don't want to know. However, if R on the boiler is connected to R on the Nest, W on the Nest to Y on the boiler, and C on the boiler to Com on the Nest… it should appear to work properly. It would appear from the description that this is what, in some arcane manner, the NPC does.
However, on this boiler, that connection bypasses the low water cutout.
This is an extremely dangerous, though not obvious, safety hazard.
I don't really care how you do it, but whatever you do do it so that either the Nest is powered from G on the boiler, not R, or the Nest is isolated from the boiler by a relay, as I have suggested.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
Correct. They are all the ground side of the system.
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I beg to differ.
The LWCO has a power feed from R; It also has a power ground to C. That is correct, as the LWCO requires 24 VAC power to operate. What is NOT perhaps obvious is that the output of the LWCO is connected to G, is not connected to ground. That terminal is hot when the LWCO is closed — water is above the sensing level (note the dotted NO contact in the LWCO symbol). That NO contact set is closed when there is water present; when the thermostat is also closed this completes the safety/demand circuit around to terminals 3 (fused) and 5 on the damper control, thus connecting those terminals to R, 24 VAC hot. This completed circuit causes the damper to open which, in turn, powers damper terminal 2, which in turn powers the gas valve.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
It also has a power ground to C.
Therefore, the proper answer to his query remains "Correct". There is no disagreement.
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Um… well… yes. Sort of like saying that one's three prong wall socket is connected to ground. Or at least one terminal of it is…
Personally I think that there is a good deal to be said for protecting equipment — such as transformers as we have here — with fuses on the secondary side. Anything electronic should be fast blow. Motors can be slow blow. But that's me.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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i know how the NPC works but if you have a third wire up at the thermostat why would you need the NPC. you just land the third wire on the C terminal of the Nest. The NPC lets the nest run with a 2 conductor if that's all you have at the thermostat. unless I misread your comment you said you had a 3 conductor at the Nest.
You don't need the resistor if you have a C wire. Only if your running on 2 conductor.
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I give up. However, if you do run three wires from the basement, remember to land Rh at the boiler on B — NOT ON R. W lands on Y, and C on C.
Otherwise, as I stated above, you will have cut your low water cutoff out of the safety circuit and that is NOT GOOD.
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England0 -
The way those boilers are wired through the LWCO, power to the Nest will drop out using the factory thermostat connections. Run a new 4 con wire, go with a Honeywell RedLink and an EIM, or put the old thermostats back (recommended), and get your dollar back. If its only for "monitoring", get a high/low temperature alarm.
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@pedmec , yes, you may have misunderstood the comment. The SR501 <-> NPC <-> 2-conductor wire <-> Nest thermostat connection controls the baseboard hot water heat that runs through the tankless hot water coil in the boiler. There is another setup for the steam heat. It uses a 2-conductor wire from the boiler that's subsequently connected to a 3-conductor wire that goes up to the thermometer. Naturally, that 3rd conductor was unused, so I connected it to the C terminal on the transformer.
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So if I am reading this correctly the baseboard loop is operating properly
The Steam radiator thermostat is the problem
You have three wires from the thermostat to the boiler controls
This is a common problem with the way that most steam boilers are connected to the transformer… To get the smart thermostat to work properly you must have the R from the thermostat connected directly to the R on the transformer. and the C from the thermostat connected directly to the C on the transformer. This is not possible on the Weil McLain boiler as it is currently wired. See Illustration
If you look closely at the diagram you can clearly see that the thermostat location on the Weil McLain diagram does not have a direct connection to the R or the C of the transformer. On the R side of the transformer is located the LWCO and on the C side of the transformer is the vent damper. That is because both of those devices also need to be connected directly to the transformer at one point. Since there are only two sides to the transformer, R and C, there is no place left for the Smart Thermostat.
This is where an isolation relay will make your thermostat operate properly. See this post from last year. It will explain how to use an isolation relay to get you where you need to be.
If you feel qualified to rewire the boiler and can test all the operations of each safety device to be sure that each one is operational, this is another way to do the smart thermostat without the relay. YOU MUST ME QUALIFIED TO DO THIS REWIRE AND MAKE SURE ALL THE SAFETY LIMITS ARE WORKING PROPERLY. If you feel that rewiring the boiler is beyond your capabilities, then you should not attempt this procedure. Stick with the isolation relay above.
Whatever way you decide to use, just know that R from the thermostat MUST be connected directly to R on the transformer. That way, other devices will not be interfering with your smart thermostat.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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So the post I made above tells you Why you have a problem, and you need to go to several locations to find the way to fix it. Here is YOUR system diagram:
To make it work you need to purchase this transformer and relay to isolate the NEST from the boiler transformer. You must also have all the necessary parts working to include:
- Boiler transformer
- Boiler safeties and limits including: LWCO, Vent Damper safety switch, Spill Switch, Roll-out Switch, Pressure-trol
- Taco SR501
- Operator control For the baseboard control.
If any of these parts failed during the part where you lost power top everything, then those items must be put back in working order
Between your professional and yourself, you should be able to follow this diagram to get it operational.
And so you don't let the factory smoke out of any more transformers you should get a couple of these.
and use then on every R terminal of every transformer you have!
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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UPDATE
My contractor replaced the Taco and transformer to bring the system up to operations. We decided that the best way to allow the Nest to control the steam heat would be to install a separate transformer/relay as some have explained.
Doing so is beyond my skill set. The cost of paying a professional to do it is too high given the reasons I had for wanting it. Rewiring it as some have proposed would cost less but would make the connections non-standard, creating challenges for future professionals. So I'm using the old thermostat.
Thank you all for your comments and discussion. I do appreciate your time and expertise.
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@DanielC … Don't sell yourself short. It is only 5 wires that need to be connected to this combination transformer and relay It is easier than you think.
And if you use the fuses, you will not damage any controls.
This is a picture of the wiring diagram you need. The green area is already done, so you don't need to bother with that. Just the 5 wires in the white area along with a part that is less than $35.00 will get you where you need to be.
Edward Young Retired
After you make that expensive repair and you still have the same problem, What will you check next?
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There's no 120V supply to that transformer/relay?
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Don't give up…………
I believe you intended to say "land Rh at the boiler on G"
I agree with everything else you wrote. I do have a question.
G is hot on the transformer at all times (with power on) with the SOLE EXCEPTION being if the boiler is low on water. The old thermostat is connected to G and it is also connected to Y which then allows power to proceed to all of the remaining safeties.
If the Nest functions like a standard switch, it should also be able to be connected to G and Y and operate in the same fashion as any other thermostat. Of course it also needs C.
If it loses power due to low water, that should have no bearing on the operation (which has now ceased). It has a battery backup.
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fat fingers…
Br. Jamie, osb
Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
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