I am replacing a 240V wall heater. I get 120V from black to ground but 0V from black to white and 0V from ground to white. Is this correct? The old heater worked but not really well. There was a thermostat in the wall single pole and I get the same readings there. It is hooked to a 40 amp circuit breaker. This was originally installed in the 1970s with aluminum wire.
You say the thermostat is "Single Pole". I take that to mean that there is one wire in and one wire out? Does the wire out happen to be white? If so, the reason you don't have any voltage on the white wire at the heater is because your thermostat is off. Set the thermostat so that it turns on and test again.
Alternatively, test from the wire on the in side of the thermostat (which is probably white) to black or ground and you should get 120V.
If they wired this device with only two conductors (black/white) for the 240V, then you have an open hot. And if they wired it with 3 conductors (black/red/white), then you need to test your second hot, but you most likely have an open neutral. Assuming you have an open hot, there are several possibilities:
Disconnected wire when removing the heater so you no longer have voltage going to the wire you're testing.
One of the two poles of the breaker are still tripped when you run your tests.
The aluminum wiring has worked loose and you now have a fire risk in your home (aluminum does this, which is why we don't use it anymore).