100 year old hot water boiler and new Sensi thermostat

SteveKC

I have a old coal boiler which was converted to gas with a Honeywell system many years ago. It still works great, but I want to add a new Emerson Sensi thermostat.
The heating system takes over 4 hours of continuous running to get the house warm after it has gotten cold. When we are away for a few days I want to lower the temperature but I want to have the house warm when we decide to come back.
The valve has three terminals that go to the thermostat Red - White - Blue(but the wire is green).
The instructions with the Sensi seem to show that I just add a 24V transformer to the C and the RC terminal - connect W wire to W terminal - connect B(now green) wire to B/O terminal - connect R wire to the remaining RH terminal - and clip the red jump wire on the Sensi.
My concern is that is this all it will take to match up my very old valve control to this nifty new thermostat and will doing this not mess up the system that has worked perfectly for all these years.
I can send photos if it will help.
Thanks SteveKC

SteveKC

I just noticed that the W terminal on my Sensi Model UP500W is labeled W/E, the B terminal is labeled O/B and the R terminals are RC and RH. I hope that using these Sensi connections are the right ones. Please let me know.
SteveKC

Steamhead

That's probably a Honeywell Series 10 thermostat circuit- but there's an outside chance it might be a Series 20. What is the model number of the present thermostat?

The Series 10 circuit is described here:

https://forum.heatinghelp.com/discussion/comment/1600709#Comment_1600709

HVACNUT

Yes pics will help. The thermostat sub base with the wires connected and designated. And where those wires connect at the boiler. You say they go directly to the gas valve?
It's not millivolt?
This is hot water, not steam, correct?
Is there a circulator or is it gravity?
Is there a transformer anywhere?
Definitely pics.

SteveKC

Here are some images. I tried to just send photos but it would not work. It looks like a Series 10 valve.

SteveKC

It is a hot water system. It is a gravity system. There is a transformer that operates the system. It is 16V.

Steamhead

Definitely Series 10. On most of these hookups, Blue and White are the wires you want to use on a 2-wire thermostat hookup.

JUGHNE

So you are trying to match a 24 volt t-stat with your 16 volt system. Seems like needing an isolation relay somewhere??

mattmia2

i bet most t-stats regulate it down to like 3.3v anyhow and would be perfectly happy at 16vac

nicholas bonham-carter

What temperatures are you selecting during setback period, and normal running?—NBC

SteveKC

Hi all,

So will it work if I add the 24V AC to the Sensi C and RC terminals - B wire (now green) to the O/B terminal - W wire to the W/E terminal and cut the RC/RH jumper wire on the base of the Sensi?
Getting and installing the 24V transformer should be easy.

We do not use a setback setting at night since it takes so long for the system to bring the temperature back up again for day use. I just want to be able to lower the temperature when we are gone for a few days and have it warm again when we decide to return.

Thanks for your help.
Steve F.

Steamhead

@SteveKC , I just looked at the Sensi wiring diagram, here:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwjrqPGx_-brAhVCYjUKHYlSDKAQFjAAegQIAhAB&url=https://sensi.emerson.com/documents/sensi-wi-fi-thermostat-installation-guide-10-17-2-en-us-4278840.pdf&usg=AOvVaw15lHDK3JljCkX4nSfU4R1p

You will definitely need a 24-volt transformer and an isolation relay to use the Sensi with your ancient boiler. The reason is that, according to the wiring diagram in the manual, the first-stage heating contact is not "dry" (dry means there is no voltage on it, so the voltage comes from something else). Rather, it sends 24-volts out to operate something. This is not compatible with your setup as-is.

So an isolation relay is needed. Any decent relay with a 24-volt coil will work. I like the Honeywell R8222 series, but there are many other possibilities.

Wire it up so the 24V transformer R terminal sends power to the Sensi R terminal. I like to use a Red wire for this to match the terminal designation on the transformer and the Sensi. A White wire runs from the Sensi W/E terminal to one side of the relay coil. A Green or Blue wire will run from the C terminal of the Sensi to the C terminal on the transformer. Another Green or Blue wire would run from the other relay coil terminal to the C terminal on the transformer. In this way, the transformer powers the Sensi and operates the relay when asked to do so. If the relay is located near the transformer, you can run a standard 3-wire thermostat cable between the relay/transformer location and the Sensi. Or, use the existing 3-wire cable from the Series 10 hookup if it's in good condition.

Connect the Blue and White wires from the gas valve to a normally-open contact on the relay. Then watch it work.

I find that using a "fan center relay" such as a Honeywell R8225 or R8239 series unit makes for a neater installation, since the transformer and the relay are on a 4-inch square plate that mounts to a corresponding size electrical box (called a "1900 box").