I installed a Cadet 240v elect with a King double pole thermostat from a double pole breaker. Followed the instructions for both the heater and the thermostat. When the thermostat is turned up, there are two 120v lines to the heater, but the heater does not heat up. When I test each line in the heater to ground I get 120v. If I test the two supply lines with a multi-test I get 0v. Shouldn't I get 240v somewhere? Any ideas or need more information?

I will guess your using a tandem or half size two pole breaker. If so, you have both poles of the breaker on only one leg or busbar of the panel. These types of breakers must be installed so each pole of the breaker is on each of the legs or bus of the panel.

That was perfect. Moved the breaker and instant heat. Thanks for the quick answer.