I'm mystified. A friend of the wife's is having combo heat a/c units installed (Freidrich). The installer could not make the units work on the existing bb heat circuit. This AM I checked voltage; 120 to gnd on both bk. poles, and 240 between them. At the other end, 120 to gnd and ZERO between legs. Any one have any ideas?? . Also there IS 12-2 at the locations but the nameplate on the unit calls for 27.9 amps. Am I gonna need to pull 10-2? on a 30a bkr? More fun, it's a Zinsco.

Would there happen to be a SINGLE pole thermostat in the circuit somewhere ahead of where you are testing as well as possibly another heater in the circuit after the thermostat still connected????..
I have seen this a few times where they use a single pole stat to break one of the hot legs of a heating circuit, which works effectively BUT when you test from any point in the circuit after the stat you will see 120V to ground, that is with the heater still connected across both legs... That will allow you to see 120V to ground on both legs but no 240V, as long as the stat is open.... Close the stat and you see 240 across both legs..

A.D

I agree with rewired on the 120v thing. I also think you are pulling in a #10 if the "minimum circuit ampacity" on the label is 27.9. The breaker is sized to "maximum overcurent device" from the label. It will probably be bigger than 30

Are both poles of the breaker on the same phase?

Rewired and gfretwell, thanks; I'm an idiot or got too much sun over the weekend, I should have looked for the stats. The bb units were gone when I got there and I never looked at the walls,DUH.