Instead of rigging something up yourself, you should get with a 'smart thermostat' installer. This isn't a job for someone who is 'not good with "electricity stuff"'. Try posting in the Heating & Air conditioning forum.

For line voltage heating systems, I believe you need a 'heat-link' which I've read is only available in Europe. Your transformer/contactor solution may have merit, but also has technical issues, and no, you cannot just stuff them into the wall.

Condescending but not that helpful. Thanks anyway.

jbw1984-

I thought Randy's advice was sound. It appears to me that he is trying to keep someone who is admittedly not up on electrical stuff from burning down his house. When you ask for advice in the electrical forum, people are going to warn you off potentially dangerous things. It's not like when someone asks for advice in the petunia forum.

I should clarify something here. I am not likely to attempt to do this by myself. I did not say anywhere in the post that I was. I am cognizant of the general risks of hacking at the electrical wiring in my home (and was so cognizant before posting here).

What I'm trying to do is understand whether there is a solution here. I'm not going to call in a pro if i don't know ahead of time that something worthwhile can be done (not that I wouldn't listen to a pro's other ideas); and I'm not going to permit someone to alter the electrical installation in my home without understanding to my satisfaction what they're doing.

This is a volunteer forum and I understand that. If someone with actual information is willing to share an informed opinion, such as whether you see specific risks with this setup or can think of a general alternative worth pursuing, I would appreciate it, and I'm grateful for any time anyone puts into considering this. If there are risks or problems, I'd like to know what they are so that I can investigate ways to mitigate the risks or eliminate the problems. Thanks.

There are devices that will control a line voltage unit with a regular low voltage thermostat. You can find them at the home centers and any HVAC guy worth a plug nickel would know how to find one and use it.

jbw1984-

i'm glad you're going to get some assistance if you think you need it. When you say stuff like "I...intend to install," and "the only place for me to put it," and there is no reference to anyone but yourself, someone might assume you were going to do this yourself.

Here is a sample relay - a Honeywell. It's for 120v - but there must be other 240 volt.

If you have a line voltage thermostat - it must be for a single room? Or a very small area?

Honeywell HVAC switching relay

This is what you want for 240 controlled by a low voltage thermostat http://www.airsupplyamerica.com/mm5/merchant.mvc?Store_Code=ASA&Screen=PROD&Product_Code=RLY-RA832A1066

• "flip a switch sending 240V over to the heating appliance"

It would really help us if you told us what kind of "appliance" this is and what its ratings are.

• "If I am using this kind of relay with a transformer, the only place for
  me to put it is inside of the cavity behind where the thermostat will be
  located on the wall, since that is where the wiring is. But I think
  transformers give off heat. Is this setup going to create enough heat to
  distort the operation of the thermostat?"

This just a BAD idea - and not just because of the heat might cause a problem with the thermostat. Why is the only place the switching relay can be located inside the wall? It should be located at the "appliance" - then you run low voltage wire inside the wall to the location where you want the thermostat.

I installed a Honeywell R8845U when I upgraded my thermostat. It looks like it has two NO contacts that could be used to switch 240volts - but it would need a neutral connection for the 120volt transformer. But you need to know the ratings of your "appliance" - you may need something with a relay/contactor that can handle a larger load.

It seems that line voltage stats might have been used for electric baseboard at some time you can use a Aube RC840t-240 relay