Without pulling my calculator out, the heater draws about 17a. The max you
are supposed to put on a 20a line when used continuously is 80% or 16a. So
just the heater is an overload. You can probably get away with it (my 23a
water heater was hooked up for 30 years with #12 wire until I happened to
notice and correct it) but I wouldn't recommend it.

My old refrigerator drew 23a to start and 3a to run. If yours is
comparable, it will trip the breaker everytime it and the heater run at the
same time. Even my new fridge draws 13a to start and 2a to run, so that
will probably trip it also.

I would be inclined to run a new 30a line for the heater and use the
existing line for the fridge.

Can the heater be run on 240? Then it would only draw 9a and would only
need a 15a (or 20a) circuit which would probably be more useful in the
futuere. 240v is a good idea for anything that heats electrically.