Taroz wrote:
Hello! I just learned that I will have the replace the central A/C in
my two-floor house. The unit is about 10-year old, and yesterday night
I noticed a fairly large stain in the ceiling of a bedroom. I checked
above it (where the internal "piece" of the A/C is) and there was
water underneath it. This "piece" (I don't know what exactly it is)
lies just next to the gas heating system.

The water is condensation from the A/C evaporator coil which is located
in the air handler (that "piece"). The evaporator coil condenses the
moisture from the air. This coil is designed to allow the moisture to
drip into a pan. There should be a drain coming from the air handler
(probably made out of white PVC plastic) that drains the water outside.
The PVC line can become clogged with algae and muck and needs to be
blown or flushed out from time to time. In some installations there
will be a second 'safety pan' under the air handler, and that second pan
should have a float switch that turns off the A/C when the safety pan
gets full. It sounds like you don't have the safety pan and switch. In
any case, you might want to find the end of the line outside and take a
water hose and flush out the line from the outside. You must be careful
if you do this procedure. You can flood the pan and cause further water
damage. If this procedure doesn't let the condensate water drain out
then you need to call a professional back. Most likely that drain is
plugged.

Today I called a contractor (which apparently has been in the business
for about 30 years), and he confirms there is a serious leak. He also
found one in the external unit, which is also a bit rusty. He checked
with a "weird" tool that started beeping like hell when a leak was
found. He thinks I should replace the whole thing (without being pushy
about it), and gave me a quote of $3800 including taxes, installation,
two years of free service, a 10-year labor and parts warrantee, and a
programmable digital thermostat. I think it's a 12 SEER, but didn't
ask the brand.

The beeping is probably from a refrigerant leak detector. This of
course is a different 'leak' than the one that is staining your ceiling.
If your evaporator coil is leaking that is a bad sign, and probably
means that a replacement system is in order. If your current system is
over 12 years old then you should think about replacing it.

It is very hard to give an opinion on pricing over the internet - there
are too many variables. However, if I were you I'd first see if I can
fix the plugged drain myself.