I need some help I have a 42 inch cadet baseboard heater that needs 240 volts to run at full power. The heater is in a small water meter room I have it hard wired in but its only 120volts coming in and I noticed a 15 amp plug close by that is on a different breaker so I was wondering if i can wire a plug to the heater and plug it in while its still hard wired in to get the additional 120 volts so the heater runs at full power or can I just remove plug and hard wire it from there to thanks
You can run a 240V heater on 120V if you really want to, but it'll be weak tea, operating at 1/4 of power.
You could attempt what you hope, but it will require completely redoing the circuit wiring, to have 2 hots and a neutral in the same cable, at least up to the point where a 120V-only subcircuit branches off. All the conductors must be in the same cable. You cannot grab one hot from this cable and another hot from another cable.
Also, the fixed loads on each half of the circuit must account for less than 50% of circuit capacity. Otherwise you can't have any receptacles on it at all. This is per pole, so if you have a 6 amp heater on both poles obviously, and a 5A sump pump on one pole, that pole can't have any receptacles.
You must use a 2-pole breaker with 240V between the poles, otherwise you create a serious overload hazard (and also the heater won't work). An example of a bad breaker to use is a double-stuff that fits in one space.
Doing this type of wiring correctly is hard, and easy to get wrong. It's harder than using punctuation, for instance.