I am adding 4 cadet heaters to an addition I have added to my home. I would like to put in relays to that they will all turn on with the central thermostat that controls a propane furnace. It is a 24v thermostat. Has anyone done this, what kind of relays would I need? The heaters will be 240v.

Thanks

Found at:

Low Voltage Controls Thermostat: The Round®

Here is a cut sheet for just the product you need, a relay to be controlled by a low voltage stat, that will control the 240 volt electric heat, made by Cadet:

http://www.cadetco.com/support/specsheet/1018_b.pdf

You could use 4 Double Pole Contactors that will handle the load, and then some.
They need to have a 24 volt coil, 1 side of each coil will go to 1 terminal of all contactor coils, the other comes from thermostat. Be sure the VA for transformer will handle the Contactor coils.
You could also have toggle switches to interrupt 1 or more of the coils.
You could also use a 100? Amp Contactor to feed an 8 circuit panel, when thermostat puts voltage to the one contactor coil, it energizes the whole panel.

You want a White-Rogers Electric Heat Relay.
This is the gold standard for controlling electric heat with a low-voltage thermostat.

http://www.white-rodgers.com/wrdhom/..._06_pg0095.pdf
http://www.white-rodgers.com/wrdhom/.../0037-4165.pdf

Just a suggestion. Don't have the room addition heat come on with the propane heat in the other rooms. Wire it with the 24 volt control, but keep it separate from the existing heat unit o it's own thermostat. This provides you with zone control.