He should first check to see that he has 240 volts on that circuit at the
panel

OK, thanks for input - Problem is resolved!

Because heating circuits are not clearly labeled on fuse panel, I did not
know which one the problem heater was on. Today I pulled all heater fuses
(They are in double-fuse holders with hinged cover to prevent removal of
individual fuses). I now did not have 110v to ground at the wall
thermostat. We then plugged the fuseholders in one at a time until the 110v
re-appeared. So we found which circuit the heater was on..

I tested problem circuit fuses by using resistance meter across the brass
forks of the fuse holder and it showed continuity for both pairs. But, when
I removed the fuses, one was bad! This despite it showing no sign of having
blown - fuse strip was still intact - problem must have been internal.

Anyway, a new fuse solved problem - I guess next time I should remove each
fuse from holder, but still don't understand why testing across the "prongs"
showed no problem!

Existing main panel is Amalgamated Electric Cat No 200-4240 combination
service entrance/panel 200A with 19 double fuses and one large fuse pair for
range. There are two small sub panels with 6 breakers each that serve areas
that were re-wired at one time. There is another breaker panel in the garage
also fed from the main panel.

Would it be worthwhile considering upgrading main panel to a breaker panel?
Rough cost?

Thanks for all the input guys!