Install C wire for Nest E on Weil McLein PEG steam boiler

gis

Hi,
Need experts' help with connecting C wire to supply power to Nest E thermostat. I have Weil McLein PEG35 steam boiler with a separate forced hot water zone. For now I want to replace just one old t-stat with Nest, in the steam zone. Wiring diagram is here, page 12:
https://www.weil-mclain.com/sites/default/files/field-file/eg-peg-egh-control-supplement-manual_1.pdf
Transformer part number 510-312-166:
https://www.supplyhouse.com/Weil-Mclain-510-312-166-Transformer-120-24V
Transformer has 5 contacts R, C, Y, W and G. Thermostat is connected to Y and G. Where do I connect the C wire?
Thanks in advance!

Jamie Hall

Nests don't play well with steam. Do you really want to do this?

gis

I really want to install a smart t-stat, not married to Nest, although I have one already. Are there other smart t-stats that play better with steam? What do you mean - does not play well with steam?
Thanks!

Jamie Hall

"Smart" meaning... wifi connected? If so, Honeywell makes a number of models with varying capabilities. Ecobee is fairly well regarded as well.

The problem with smart thermostats and steam -- or, for that matter, hot water or radiant heat -- is that they are intended for forced air, which responds rapidly to setbacks. The whole idea is that they "learn" when you are there and raise the temperature, and when you aren't and let the temperature drop. Works for forced air. But steam and hot water may take an hour or two to come back from even a 3 degree setback -- and radiant may take a day or two -- so there is little advantage to setbacks, and thus little use for the "smart" feature (so it is advisable to disable it).

The other problem, of course, is that they are connected to the internet -- which means that unless you have a very robust login and password ("admin" and "12345678" just won't do) everyone else on the internet potentially has access to your thermostat -- and your house, including when you are there and when you aren't. Very handy for some folks...

gis

Thanks for the detailed explanation! I currently have a simple programmable t-stat where i use setbacks quite successfully, so i don't see why the Nest be any different. The zone heats up in the morning from 62 to 68 in 30-60 min. I hope that Nest (or any other) will give me additional flexibility to change my preset schedule. I also want to see how long the boiler running day to day.
I looked at ecobee and it will require C wire as well. I assume Honeywell is the same. So my question still stands.

Jamie Hall

Some Honeywells do not require a C wire -- battery powered. And the Ecobees come with a power supply kit if it happens you don't have the C wire.

The Nest -- and its friends -- will indeed give you flexibility to change your preset schedule. In fact, in smart mode, they will happily do it for you. Whether you will save any energy or not is another question; I can only note that over the years many folks have found that setbacks less than a few hours or more than a few degrees don't, except for forced air. Your mileage may vary.

gis

Just looked at Honeywells and they all require C wire. As far as ecobee, it also require it for two-wire systems:
https://support.ecobee.com/hc/en-us/articles/227874667-Heat-only-2-wire-boiler-furnace-installations-for-ecobee-thermostats
Their extender power kit only works for systems with at least 4 wires:
https://support.ecobee.com/hc/en-us/articles/360009155051-Installing-your-ecobee-thermostat-with-the-Power-Extender-Kit-no-C-wire-

JUGHNE

Looking at your boiler wiring diagram, it looks like the best fix would be another transformer and an isolation relay. IMO

From what has been written about the Nest for heating, you may think you program it. But apparently it thinks it knows better and decides to override your wants. It may work for quick heat systems like forced air.
I don't know for sure, don't have nor want one .

SuperTech

You could add a separate transformer and wire it to the RC and common terminals if this thermostat isn't used for cooling as well.

I have used a device called a common wire adder if a thermostat only has two conductors going to it.

I would encourage you to pick any other thermostat for steam or hydronic heating. A Honeywell thermostat will give you energy reports that show hours of operation, if you like that sort of thing. And it won't change what you set it to without your input.

And yes, set backs are best used for forced air.

gis

Thanks for all the suggestions! I'll definitely look into Honeywell as far as functionality and not being "too smart". But will need a C wire in any case. A separate transformer is an option, but I do have two additional free wires run (4 total) to the t-stat, so it a pity to spend money and effort on the new transformer and isolation relay, if I all I have to do is to find the right spot to connect it. Unless you see any other reasons why it's not a good idea.
To give you complete picture - I installed Nest initially (about a week ago) on two wires in my steam zone, because their web compatibility tool said it will work. It did work fine for some time. But then I noticed that when my second t-stat (forced hot water zone) calls for heat, Nest loses power. I think this is what happens. The two t-stats are connected in parallel (there is a Taco SR501 relay in play, so technically steam t-stat and Taco are in parallel). When the other one calls, it causes the voltage on the Nest to drop and it turns off. Even worse than that, when it gets power back (after the other zone is satisfied), it shows room temp lower than it actually is. This causes my steam zone to overheat.
So bottom line - Nest works absolutely fine on two wires, if it is the only t-stat in the system. As soon as you add the second zone/t-stat, two wires in not enough to power it, you'll need a C wire.
Thanks again!

ChrisJ

If the PEG series is the same as the EG series, and I think they are. All you need to do is connect your C wire to the ground on the boiler.

I've had a C wire on my by EG series since 2012. There's no need for a separate transformer.

gis

Thank you Chris! This is what I was looking for! Easy enough to try and if it doesn't work - I will consider a transformer, or maybe just forget about this whole idea.