In article , TimR wrote:
I put a meter on the thermostat. The stat has four terminals, two
marked line and two marked load. With breaker off and stat off, I
read 28 ohms across load. That sounds reasonable, it should pull 8.6
amps if my math is correct, and the 20 A breaker should be fine.
Breaker off and stat on, 26.7 ohms appears across both load terminals
and line terminals, that sounds okay to me too, .8 and .7 ohms from
line to load across the contacts.

With the power on, I read 242 VAC across line, 0 across load, 120 from
each line terminal to load terminal, 120 from each line terminal to
ground. I didn't expect that. I thought a thermostat this old would
be single pole, and so one load terminal would read zero. That's with
the thermostat turned off, if I turn it on it trips the breaker
immediately. Can this be a double pole stat? or am I just
misunderstanding the readings?

Have you checked the rating plate on the heater to make sure it really is a
240V heater? Sounds more to me like it's a 120V unit connected to a 240V
circuit, which of course would explain the instant breaker trip.