I have a very old spa.

It has a 120V circuit for everything, the pump and heater.

I had to do some repair on it yesterday. The heater was not heating,
which, as I realized, was caused by a crimp that worked itself
loose. Which I recrimped.

It seems to work, but still, the low power heater is very slow to heat
the spa at 120V.

The heater is switched on and off with a power relay (contactor).

The question that I have is this. Would it be sensible to add another
circuit to this spa, supplying 240VAC, that would power the heater
only. The old control system and pump would still be on 120VAC, but
the contactor would switch 240VAC going to the heater.

I would use a GFCI breaker on that 240V line, just like I have on the
120V line.

The only issue that I have with this possible change is that instead
of one breaker disconnecting the spa, there are now two. It may
present a possible hazard to someone attempting to repair the spa, as
they could turn off one circuit and forget to turn off another,
leading to possible electric shock.

Any thoughts?

i