I want to replace my electric water heater with a gas tankless water heater. My current water heater like most electric hot water heaters takes 220 and the tankless gas water heater uses 120. The room I am installing in has no outlets. Can I use that 220 line and get a couple out 120 15 or 20a outlets out of it. Maybe with a sub panel type set up or if I can't do that if I can at least wire the new 120 water heater without running a new line. Thanks in advance.
In the panel, move the white wire to the neutral bar, and move the black to a 20A breaker. Fit a 15A or 20A duplex receptacle. And you're done.
You can use up to about 19.5 amps of power off that receptacle, since the water heater won't take much.
If you want two full circuits good for a total of 30A@120V, in the main panel, leave the black wire on the 30A breaker, still move white to neutral. Then fit a subpanel, of any size. Black to a hot pole, white to the neutral bar, ground to a different ground bar that is isolated from neutral. Only every other row of breaker spaces will work. Fit two 15A or 20A breakers and use each one for a different circuit. If you place the breakers in the wrong row, don't bust out more knockouts, switch the black wire to the other hot lug.
I would swap out the 240v double pole breaker for a 120v single pole and change the outlet. It is ok to use larger wire as most water heaters are wired with3 wire 10 awg this is small for a sub and the sub would need 4 wire. If wired 4 wire #8 I would put in a small sub, but if 10awg just convert it to 120v you need.