"George E. Cawthon" wrote in
:

Ha. Ha. By this reasoning only professionals should change
wheels on cars-- someone might put the wheel on backward and
with power the vehicle would just spin around. Seriously,
anyone that doesn't know what they are doing, should hire
someone who does, but reading a few books can often provide
all the knowledge anyone needs. Anyone that doesn't have
enough sense to check motor direction before installing,
shouldn't. There are damn few thing I can't do or fix
around my house and I'll do it better and a lot cheaper than
a professional. Those things I don't do because I don't have
the tools or the desire to learn are ac stuff and
complicated electronics in a gas furnace.

The problem for most people is, if you don't have the sense to know
something, how do you tell that you don't have it? :-)

My previous house had an old (like, 40 years) Williams 5-in-one furnace. Me
being too cheap to actually replace the unit, I chose to fix things as they
broke. After removing the non-functioning electronic aircleaner, replacing
the already removed by the previous owner humidifier, replacing the
thermostat, blower motor and gas valve (which failed open one night!), the
electronics failed. So, unable to find replacement parts that I was willing
to pay for, I replaced the electronics with some generic off-the-shelf
components. I also learned a lot about how HVAC systems work :-) After a
day or two of fiddling with adjustments, it worked fine for for several
years, and was still there when I sold the house. The new owner will
probably replace the whole system... he seems like that kind of guy grin.