How does a house get too humid in the Winter?? I've had a hydronic
system in the houses/apartments I've lived in for the last 25
years. There is no such thing as "too humid" during heating
season.

Here in Canada, with current codes, and high efficiency furnaces or
electric, houses are so well sealed that in winter, and with all the
windows closed, there is very little air exchange (and so air
exchangers are a health necessity). We've got plants, and aquariums,
and pets, and people breathing ... anon anon anon. Or, it can get humid
to the point that the condensation on windows begets black mold/mildew.
Now, in an older leaky home, like my last one, this doesn't happen. And
so by too humid, I mean considerable condensation gets dumped on e.g.
windows causing in turn mold/mildew.

There is such a thing as "too humid" during the heating season.