If I were you, I think I would find an AC installer who does more
light-commercial work than residential work. I would lose the idea of
doing chilled water because there are issues with that. I would see if
anyone had any experience doing a zoned freon system. You put 1
compressor somewhere out of the way so it doesn't bother you. You run
a manifold to either balance the system or to have electronic switcing
to shut down systems that don't need cooling, and you put in 3 regular
heat exchangers (evaporators) in your forced air system. Except the
switching system, it would be pretty straight forward. There's no
reason you can't move freon rather than chilled water. In fact, it
might move more efficiently.

Oh, and hope and pray it doesn't need to be a 3-phase system. But it
doesn't seem like it should. It's not that big.

BTW, every old gravity fed coal system I ever saw had pretty big ducts.
So you probably have enough size -- maybe even too big. So check for
leaks and make sure your blower is sufficient.

1 issue you will still have is that AC is must better coming out high
than at floor level. So you might need some ceiling fans or something
to help out. This might explain the high cost of ducting.

Good luck with it.



blueman wrote:
writes:
blueman wrote:

Our 100+ yr old house currently has 3 separate forced hot air
gas-fired heating units.

We would like to add central air sharing the same ductwork and
blowers.

fantastic. after you do a room by room load calc and a room by room
duct calc, you will know if your existing ducts and blowers are
adequately sized for air conditioning.

We had a couple of HVAC contractors in for preliminary estimates and
calculations. Their sense is that while not ideal (the ducts
appear to be sized and located for the original system which seems to
have been coal-fired gravity hot air), the cost of installing new
ducts would be substantial in addition to the need to make a fair number of
holes in the wall. This will mean that even after getting the
calculations there will be some compromises in determining the tonnage
relative to the heat load and duct capacity.

We will still like to get relatively high end AC units so that we get
good efficiency, reliability, and noise performance.

One question though is whether we should hire an independent HVAC engineer
to do the measurements and write the specs or whether we can "trust" a
recommended "reputable" HVAC contractor to measure, spec, and install.


Can anyone comment on the pros/cons and appropriateness of using such
a system residentially?

yes. with a freon based, dx system, most anybody can work on and
provide parts for it. on a resi chilled water system, parts are
propriatary, can be outrageously expensive, and very few people can
work on such a system.



I am particularly interested in comparing initial cost (equipment &
installation), efficiency, noise-level, and reliability.

go with a high end freon based dx system.


Sounds then like this is the way to go and as mentioned above we
expect to pay for the high end since much of the cost in our project
is labor (we need to have a bunch of ductwork in the basement replaced
too).