HI,  Leaving the "heat up all the time to stabilize the humidity" isn't
a very good idea, but not for the reason given.  Given a constant
amount of moisture in the building and the outside air, raising the
temperature will make the humidity _lower_, not higher.

A better approach would be to _lower_ the temperature.  Then the
humidity would go higher, as you want.  In fact, your instruments
would probably be better off in an unheated building as long as the
temperature was fairly constant.  (You don't want them to get really
cold at night and then be exposed to warmer, more humid air while they
are cold.  Dew isn't good!)  Another approach is to constantly run
just the fan and the humidifier to raise the humidity without raising
the temperature.

I always keep a glass of water in the Mills to increase the humidity,
and I haven't had any rust problems in 15 years.  A wine glass from the
1991 MBSI convention is preferred.  Just top it off with water every
few weeks.  We tried wine but it made the rolls wander!  If I don't do
this in the winter, we get a little buzz on a couple notes when the
piano plays.  This also happens if we forget and use the wine!

Get an electronic humidity meter (less than $30) and put it inside the
piano.  Then find something like a large peanut butter jar (crunchy
preferred) and fill it with water.  Put it in the bottom of the piano
and refill it when it gets empty.  Then check the humidity after a
week.  If it is over 55% or so relative humidity, cover part of the top
of the jar or open the door a crack.  If it is too low, put in another
jar.  If you keep the humidity around 50%, you won't get any rust.

Craig Smith