Nest Thermostats with Steam Boiler and Hot Water Loop

AMW

A company just replaced my boiler and thermostats. At my request, they installed new Nest Thermostats, which initially they said would be no problem. Now they are saying it may not be possible to get the system working properly with the Nests.

Most of the house has steam radiators, but one room has hot water radiators. There is a loop with a heat exchanger going off the steam boiler for the hot water radiators. There are two thermostats, one for the steam radiators and one for the hot water loop.

After the initial install, what happened is whenever the hot water radiator would call for heat, all the steam radiators would get hot as well, so the two zones didn't really operate independently.

They came back and replaced and relocated the aquastat, but now the way it works is that, once the water in the hot water loop is hot enough, it cuts power to the Nest thermostat that controls the hot water system. So then the Nest thermostat first displays an error message and then loses power.

The company says they don't know of any other way to set it up; they don't know of a way to make the Nest that controls the hot water radiators to stop telling the boiler for more heat without cutting power to the Nest.

Any ideas to fix it, beyond replacing the Nests with another brand?

Jamie Hall

How refreshing. A Nest problem which isn't a problem with a Nest (although you will find that the Nest and steam don't play well together, but that's another story).

What should be happening -- and there may be a missing control on the boiler -- is that when the hot water zone is calling for heat, and not the steam zone, the boiler should fire up to make hot water -- but when the water is hot enough for the hot water zone (probably 160 to 180?) there should be an aquastat on the boiler (or maybe on the pipe from the boiler TO the heat exchanger) which turns the boiler off for a bit, until more heat is needed. The Nest for that zone should be controlling the ciruclating pump for that zone -- and pretty much nothing else. When the Nest for the steam zone is calling, that should fire up the boiler directly (with the usual safeties, of course(. So the aquastat for the hot water zone and the steam Nest get wired in parallel to the boiler.

I would strongly recommend that both Nests be powered independently of what they are controlling. This is easy enough to do, but again, it's a wiring problem, not -- in this case -- a Nest problem.

If your company can't do it, shame on them -- it's trivial -- but we can probably post a wiring diagram for you and you can find someone to wire it up ( @EdTheHeaterMan we need one of your classy diagrams!)

ethicalpaul

Apparently Nest is the new scapegoat for bad contractors! Not that it doesn't deserve it!

Lance

I cut my teeth on controls when I found many could not even tell a wire from a pipe. This problem cannot be resolved in this format. So my advice is;
The installer needs to hire someone who can make it work. Or the consumer needs to find an expert and be prepared to make some changes to make it all work. It takes a control expert, not the service tech, installer, electrician, pipe fitter, carpenter or untrained consumer. Today, the field of controls has advanced to the point even I struggle to keep up. We have to be experts on programming, selection of product, test for compatibility and proper application of controls to make it work. Polarity in wiring is often ignored. How many read the 150-200 manuals anymore? At the end of the job is the worst time to make changes and figure how to make it work. Did you know you can wire up a low voltage zone valve that is properly field wired that will burn out every year? Attention to product knowledge settings and qualified control experts are required to prevent that. Also did you know you can destroy a series circuit thermostat when you jumper out a gas valve to test it? 20 + years ago I found a thermostat that could be used on almost anything. It was the Flexstat. It was a pain to setup, and use, but it did everything from setback to lock out to multiuser authorization, commercial, residential, multistage. After 2 installs I refused to offer it.

ethicalpaul

It's not really that complicated.

The "water loop nest" just has to be in series with an aquastat set to 180f or whatever, and that circuit has to be in parallel with the "steam nest" thermostat.

And like Jamie said, they are powered separately so they aren't losing power all the time.

vhauk

I cut my teeth on differential pneumatics. Then to Honeywell building computer control system. Our software engineer was not up to the job. 

EdTheHeaterMan

@Jamie Hall Here is the "Classy Diagram" you requested. I did not have the actual boiler wiring diagram so I stole this one from another post. I added the Nest thermostat connected to the Circulator relay with the Operating control on the diagram . The orange circuit is what you were talking about in your comment. I hope this helps @AMW with his issue.
The SR501 Switching (or Equal) will provide power with a C terminal to the Nest thermostat for the water radiators

If a common (C) wire is needed on the Nest for the steam radiators, it is important to connect the G on the boiler to the R on the Nest. Then use the C on the boiler transformer. If that does not operate properly then an isolation relay is required.

EBEBRATT-Ed

Not as classy as @EdTheHeaterMan's drawing mine is more generic but here it is for wiw it includes separate power and a relay.

AMW

Thank you all for your answers! I showed the company this post and they sent someone who is good with wiring who fixed it I'm really delighted.

ethicalpaul

Nothing like a little shame to get things going