I have an older home that has only a millvolt heating system and no central AC.
I want to install a special thermostat that requires 24 volt power and supports power stealing. Since a millivolt system has no power connection, I wonder if I can use this thermostat by addint a 24V transformer plugged into the wall. I have tried this and connected wires a few ways, but with little success.

The instructions for my thermostat do address the question of a millivolt heater in a system with a 24 V AC unit and show that the connections should be:

Millivot heat is connected to the thermostat at RH and W.
24V AC hot connected to RC and shows the other side connected to Y and G with a jumper to C (Common) optional).

Since I don't have 24V AC, I tried connecting a transformer that converts 110V to 24VAC and connected the hot to RC and the common jumpered to Y, G, and the C (Common) connection on the thermostat.

But this gives me more power.

One night I got desperate and connected the 24V transformer hot side to Y and the common side to C on the thermostat. The thermostat did get power for a while, but then burned out my power supply, so I'm back to square one.

Also, I know one solution is to find a different thermostat. But I have special need to use this one if at all possible.

Any help is much appreciated.

Good luck

Just wire in a relay to close the millivolt power contacts when the thermostat tells it to. Use the on-off contacts in the relay to tell the millivolt signal to turn on and off thus the heater will turn on and off.

You will need a 24 volt supply for the 24 volt thermostat and if you are smart you will also use a 24 volt coil relay so you can power both off the same transformer. Install a inline fuse to prevent transformer burnouts if you mess up the wiring. Good luck.

Millivolt contacts are usually silver coated due to very low voltage. I don't know if anybody makes a spst relay for millivolt.

The Idec RM or RY series should work:

http://www.alliedelec.com/Images/Products/Datasheets/BM/IDEC_CORPORATION/814-3072.PDF RM is preferable.

What you don't want to do is use a relay reted for 10 A. The RM series has a contact resistance of 30 milli-ohms.