On Fri, 18 Jul 2008 07:24:59 -0700 (PDT),
wrote:

I bought a new condo 4 years ago. Apart from regularly changing air
filter, I have not done any maintenance on water heater, furnace or
ac. Is there some 'bare minimum' I should be doing? Everything works
great, so I am not sure what I need to do.

You don't say what kind of furnace. You'll get much better results
here if you ask better questions.

You don't say what kind of water heater. That might not matter much,
but you didn't know that when you didn't say.

I cut open my electric water heater after 8 years, never flushed, and
there were 2 or 3 tablesspoons of tiny white rocks in the bottom.
Clumped sediment, not really rocks. At that rate it was going to take
more than 200 years to have too much sediment. Plus some people tell
stories about opening the drain and not being able to get it to close
again. I myself would do nothing to your water heater, but a
prudent person might ask around and find out more about crud build-up
in your town or your n'hood, and more about valves that won't shut.
Some cities may have real problems with sediment, but I don't know
that.

I'm in Baltimore. You don't say where you are, so people can't give
answers that relate to where you are.