2 stage thermostat with 2 24v transformers

gchristman

I have both radiant and secondary baseboard heat. I have a taco zone valve controller for the baseboard heat and taco zone controller for the radiant pump. Each controller has its own 24v transformer and I need to get both of them to work with a single two stage thermostat. The thermostats I've seen only allow one 24v transformer, does anybody know of a thermostat that will work or a way to get the transformers to work? I'm trying to avoid 2 thermostats. Thanks.

Ironman

If you parallel the transformers on both controls, you can run "W1" from the stat to the radiant control nod "W2" to the baseboard control.

The problem that you're gonna have though, is that the algorithms in standard 2 stage stats are not design for radiant. Look at Tekmar's. I think it's model 549 or something near that that's 2 staged.

You may just simply have to add a relay for the second stage.

Jamie Hall

Please don't parallel the two transformers. That sounds attractive, but unless they are perfectly matched which is unlikely you might get some real problems.

Use a relay.

Zman

Ironman said:

If you parallel the transformers on both controls, you can run "W1" from the stat to the radiant control nod "W2" to the baseboard control.

The problem that you're gonna have though, is that the algorithms in standard 2 stage stats are not design for radiant. Look at Tekmar's. I think it's model 549 or something near that that's 2 staged.

You may just simply have to add a relay for the second stage.

Jamie Hall said:

Please don't parallel the two transformers. That sounds attractive, but unless they are perfectly matched which is unlikely you might get some real problems.

Use a relay.

These are both good suggestions.

Technically, what Iron Man is suggesting will work, provided both transformers are exactly the same and wired to the same 120 volt power source. If they are wired to different phases, you will end up with 48 volts and something will smoke.

I have also experienced what Jamie is cautioning you about where the 2 transformers were slightly different and the smoke came out of both of them.

A relay would be the safe move.

Solid_Fuel_Man

I too would use a relay for the second stage. You can use ONE of the controls to power the thermostat and the first stage also goes to that same control. For the second stage the dry contacts of the relay go to the thermostat input on the OTHER control. This isolates the two controls from each other. You do not want two different transformers connected in any way in this type of situation.

Taylor

SWEI

Add the relay or use a bigger transformer to power both.

gchristman

Could someone suggest a relay to use?

Solid_Fuel_Man

Functional devices, RIBU1C is my go to. Will work on either 24v or 120v.

Taylor

ratio

RIB relays are fantastically easy to use. I've taken to opening them up & nipping off the unused power leg, & usually the NC wire as well, to save space in the box and to make it harder to screw up the installation.

gchristman

Would someone mind providing me with a simple wiring diagram for this application along with the recommend thermostat? I would like to be sure I fully understand it so I don't burn up anything. Thanks

ChrisJ

I'm glad this thread was created.

I've run into an issue where I want to control an air handler and my boiler using one thermostat that doesn't have an RC connection (VP TH8321U1097). It came as a huge surprise to me as all of the lower models have the RC connection.

I figured using a relay for the boiler was the way to go but wasn't sure how acceptable in the industry or "normal" it would be. If someone else ever has to troubleshoot the system I'd rather things be appropriate.

ratio

A relay should be fine, but IIRC there's an R for the stat/cooling & an RH for the heat. Not necessarily an RC, but still the heat & cool can be separated. Is this the green backlight or the new white backlight?

ChrisJ

ratio said:

A relay should be fine, but IIRC there's an R for the stat/cooling & an RH for the heat. Not necessarily an RC, but still the heat & cool can be separated. Is this the green backlight or the new white backlight?

Older green backlit one.

All lower models have an RC and an R.
This one, apparently, does not.

ratio

Huh. The universal relays use up all the terminals I guess. A RIB relay at the boiler, or an IDEC tucked into the wall maybe? That'd keep him guessing!

ChrisJ

ratio said:

Huh. The universal relays use up all the terminals I guess. A RIB relay at the boiler, or an IDEC tucked into the wall maybe? That'd keep him guessing!

I'm glad you're as surprised as I was.