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
The "traditional" approach would be to add a separate
compressor/evaporator for each of the 3 heating units.
I have seen that some high end remodels are using a single chilled
water unit to supply coolant to multiply blower units and that this is
Can anyone comment on the pros/cons and appropriateness of using such
a system residentially?
I am particularly interested in comparing initial cost (equipment &
installation), efficiency, noise-level, and reliability.
I will be hiring a reputable HVAC contractor for the installation.
One house and 3 furnaces = high energy bills. Water AC system will not
I'm not sure if this would will work, but you should look into a
ground-linked heat pump system. I think some are water systems plus
you might be able to use it to heat your house.
Not sure, but I think the OP is looking for a system like a "motel
system" that circulates chilled water to all of the systems and a fan
blows when a unit calls for cooling. He is looking for a system with a
heat exchanger rather than a air-based system like a regular AC.
Richard J Kinch wrote:
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.
"better" can have different conotations. for example, if you have
to add a 400 amp panel to handle 3 freon based dx cooling systems, a
single chilled water condenser might be 'better".
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.
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.
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
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.
What??? You want to slap in a system that isn't right and has lots of
compromises, but yet you want to purchase high-end equipment? That is like
saying you need a car for fishing and hauling the trash up the driveway but
you are going to buy a BMW.
You already know you are going to get a system that doesn't work right and
may not last long, but you want to spend lots of money? That is fucked.
IMO, using the existing warm-air gravity ducting with ac would be a
huge mistake. Most of that old style ducting is 10-12" in dia, and
while they may be able to handle the cfm requirements, they will not
provide the velocity that is required. Sure, you can still use the
wall and floor penetrations and grills, just replace the gravity
ducting with new product sized for the job.
a real problem with any engineer is they have no hands-on, real
world experience. Most 'engineers' couldnt engineer their way out of
a paper bag.
If you're worried about a contractor doing the job right, require a
'performance warrantee' from the installing contractor. Either they
deliver what they promised & sold you , or they dont get paid......
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