Hydrostat 3250-Plus Only 2 Wires, No C Wire

chaceM83

@EdTheHeaterMan thank you very much in advance good sir!

I have an old Tstat I want to replace with the Honeywell T9. We do not have A/C so the Tstat just controls the furnace. I have searched everywhere inside the Hydrostat 3250-Plus and cannot find either a C-wire terminal OR the transformer that converts the 120V to 24V at the 2 T terminals on the top left corner of the Hydrostat 3250-Plus.

So, as I understand it I need to make a C-wire, and to do so I was going to buy a separate 24V transformer (wall plug version because I have a outlet in the basement right below the tstat) and install it to run up to the Tstat, essentially creating two separate circuits. One that is on the Hydrostat 3250-Plus and one that provides the constant 24V (C-wire) required by the T9.

I will add some pictures below and try and describe what you're seeing. My question is after reviewing this can you think of another way to make a C-wire that I am missing? What other information do you need?

OK, so this shows the R and W wires (top left corner) they go out the left, below the box and up that pipe on the right. Yes, even as a hobbyist electrician, I judge this wiring ;)

Thus us a 10V transformer I assume runs the valves in the radiators. My original thought was to run to this white wire terminal until I realized it was not only 10V but a separate circuit.

This is a picture straight up to show the box and the 10v transformer.

This is where the two wires go up, one controls the Tstat and the other the valves (sorry if that's not the right terminology)

Old Tstat showing only two wires

front of the furnace in case you need that too.

Reference for another Thread on a more complicated situation:

https://forum.heatinghelp.com/discussion/185655/hydrostat-3250-plus-cwire-terminal

EdTheHeaterMan

Interesting problem. You are correct when referring to the Hydrostat. not having a C location in that control. I actually purchased one to make some tests to see if there is a hidden C terminal anywhere on that control. There is not.

So you MUST use a different transformer to power the Thermostat Rc and C terminals on most thermostats. Then you can use the Rh and W to call for heat on the Hydrostat. If for some reason that does not work then you will need to add an isolation relay.

This 24 V Transformer for Smart Thermostat is what I might try first to see if it can be done without an isolation relay

The transformer you show in the picture above looks like a doorbell transformer and may have nothing to do with the heating system. You mentioned that your heating system may have valves in the radiators. I find that unlikely. Any valves that operate on low voltage, are usually in the boiler room. If you have valves in the radiators I would like to see what your radiators look like and perhaps a picture of those low voltage valves.

That said, you will need a dedicated transformer for the thermostat. This 24 V Transformer for Smart Thermostat is connected to the new smart thermostat like this:

If that does not work then an Isolation Relay and transformer is needed. That might look like this:

If you want to save on the parts you can get this combination Transformer/relay and connect it like this:

chaceM83

Thanks @EdTheHeaterMan I'll give it a shot and let you know. Might be a few days as I will need to wait for the part, then I need to fish it up from the basement, which is always fun. 🙃

EdTheHeaterMan

https://forum.heatinghelp.com/discussion/comment/1838854#Comment_1838854

Actually you can use that part from amazon.com by plugging it into the wall near the thermostat and run the wire in the wall to the thermostat.

chaceM83

I missed Mr. Ed's post below. Much cleaner and easier to follow - thank you again sir and sorry I missed this!

OK, final wire method for me was:

• Furnace W to W
• Furnace Rh to R
• Transformer load to Rc
• Transformer return to C

THANK YOU SIR!!!

(No sir, I didn't leave the wires like that ;) they were flush when I finished it just wanted to make sure it worked ;-)

Looks unlevel, but I assure you it's the paint!! Thanks again and hope this thread helps another!!!

EdTheHeaterMan

STOP

EdTheHeaterMan

TO BE CLEAR:

• Rc means R for cooling
• Rh means R for heating
• Rc to C is where you want to connect the transformer.
• W to Rh will turn on the heating system.

So with your new thermostat that has labels R and Rc You want to slide the blue tab DOWN, Then connect the transformer to Rc and C to place power to the thermostat, Finally the heating wires go the Rh and W to operate the heater