I am trying to wire an electric baseboard heater that requires 240v. I put in a double pole breaker and wired the heater. I am getting 123v on each leg but if I meater both legs I get 0 volts. What am I doing wrong? The heater won't get hot.

Sounds to me both lines are coming off the same leg. Have you ever installed a double pole before? How to Install a 240 Volt Circuit Breaker

Yes I have many times, for a dryer, welder, oven.

Are you sure it is not a tandem breaker?

I'm sorry, your first response got me to check the breaker. I had it situated so that it only got power from one leg. I moved it over one space and now I have 240v. Thanks for the help.