I'm installing a hot water heater at the cottage, there is a 240 circuit already in place that was wired for a dryer - so it has black, red, white and bare copper. The water heater has the standard black, red and green for 240v. I know the dryer wire black and red are hot and on the water heater black is hot and the red neutral, so can I hook up the black and red from the circuit to the black on the heater, then the white to the red on the heater, leaving the green to ground along with the copper from the heater? Or do I just cap off the red from the circuit, or do I need to run new 3 wire cable from the panel?

Cheers

Are you sure this is 120 volts? if so, cap the red wire from panel, connect the white from wall and connect to red, then black to black. Change to single pole breaker.
If the water heater is 240 volts, hook red to red and black to black and cap the white.
Breaker can not be larger than the wire it is protecting. Dryers are usually 30 amp breakers and #10 wire. Likely need a 20 or 30 amp breaker, nameplate will tell.

The red and black are hot in both cases. The white is neutral for dryer but is not needed for water heater. Cap off white, connect red and black wires together. Connect grounds. Breaker should be 30 amp.

Could have sworn original said 120 for water heater, my mistake?

i'm installing a hot water heater at the cottage, there is a 240 circuit already in place that was wired for a dryer - so it has black, red, white and bare copper. the water heater has the standard black, red and green for 240v. I know the dryer wire black and red are hot and on the water heater black is hot and the red neutral, so can I hook up the black and red from the circuit to the black on the heater, then the white to the red on the heater, leaving the green to ground along with the copper from the heater? or do I just cap off the red from the circuit, or do I need to run new 3 wire cable from the panel?

cheers
Thank you both, heater is 240. Hot running water on the way!