Central Air Zones

We're doing some renovation and installing central air. I want to put the upstairs and downstairs on separate zones. My HVAC guy says that the only way to do zones with air conditioning is to have one zone be the Master zone (which decides whether the cooling is on or off) and one zone be a slave zone (which cannot actually start the cooling, but can only open and close dampers).
This seems absurd to me: first of all, it seems next to useless in residential cooling. I can see some applications where it would be useful, but wouldn't many applications be something like I want to cool the upstairs primarily during the day and the downstairs primarily at night (bedrooms downstairs, living space upstairs)? I don't want to turn cooling off upstairs at night, but I want it to be 5 to 10 degrees warmer than downstairs, and vice versa during the day.
Also, doesn't this seem like it would be a simple electrical or switch to the air conditioning unit? I mean, yes the dampers need to be wired to the thermostats but couldn't they also both be wired to the unit so that it will turn the unit on if the ustairs OR the downstairs thermostat wants it on? That seems like a no-brainer, but I don't know much about the design of central air units.
Thanks for any help.
(sorry about the fake email -- I've gotten way too many spam from posting in groups previously)
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useful,
cooling
the
Zoning can be done. It certainly is not cheap and is and always has been complicated. Adding motorized dampers and such is a complexity that most residential installers know nothing about.
If you want a real challenge ask him about adding in a fresh air supply.
As others have said 2 sets of duct work, 2 compressors, 2 to-stats and 2 air handlers is the way it is done for most homes. I live in Phoenix and have never seen a dampered a/c system in a home, commercial yes.
Since heat rises the upstairs is always warmer in the summer. So that unit will run more. I once saw a home done with a 5 ton down stairs and a 2 ton up stairs for the master bedroom. There were no doors upstairs just division by halls and the stairs. In the summer time the upstairs unit ran 24-7 and the down stairs cycled not exactly what the owner had wanted.
I suggest that you start contacting more contractors. Look at some 2 story new builds in your area for some ideas.
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If your installer doesnt know about different theremostat zoning I wonder if he did a load calculation ond sized your house right. You need to get someone else out, a real pro.
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control
I have never lived in New York but I have heard that it gets cold as well as hot and humid. My parents lived in Iowa and had a full basement. They had to run a dehumidifier all year long to keep the humidity down in the basement. Their duct work ran under the joists in the basement. There were manual dampers that we opened when we went to use the basement. But that was only in the winter for heat. Summer time the basement was comfortable with just the dehumidifier. Are you getting a scroll compressor? they are quieter.
From your description you may be making more of this than necessary.
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I would put in 2 separate systems.......

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the
only
and
Me too.
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That may be true with what he sells, but I have a 4 zone system and all zones are peers. (No master, no slave)
The system's computer board reads all 4 thermostates and decided when to turn what on. Sometimes it will simply move air from one zone to another - for example, when heating, if one zone is too cool and one is to warmer than required, it will move air from the warm to cool zone.
On Sun, 13 Mar 2005 22:16:21 -0800, "PrecisionMachinisT"

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Thanks for the reply.
Our situation might be a little different than most. We're located in New York City, so we have the basement and first floor of a 55' x 20' attached house. The total area to be cooled is about 1500sf. We're going with a 2.5 ton unit because there are not many windows and half of the area to be cooled is underground, and the long sides of the building are attached. Two units would be unfeasible -- as it is, fitting a single condeser (and its noise) in the back yard (20' x 40') is a burden -- adding another condesner would really be a problem.
The subcontractor seems to have no problem with the idea of putting in motorized dampers (and says it wouldn't be too expensive sicne he would only need one damper per floor) and has already taken into account a fresh air intake from the roof. All I really want is to have dual master control of the cooling unit so that either floor can turn it on. The motorized dampers will take care of the rest.
Any further suggestions would be appreciated.
thanks.
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sounds like the unit is still too big. with zoning, a smaller unit will often work because both zones do not need cooling at the same time. Get someone to do a real load calculation, measuring sq ft of windows, walls ceiling, wall & floor. If they just go by sq ft of floor they are not doing an accurate job. My 2000 sq ft house has just a 2-ton unit in hot humid Myrtle Beach South Carolina. It is based on my windows and insulation levels, not some rule of thmb. A good contractor should be able to do more than just install a slave damper when zoning.
stretch
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Gee a 2.5 ton to cool 750 sq upstairs and dehumidify a 750 sq basement. I bet no load calculation was done and if you place is well insulated you will find you are way oversized. Oversize and you will be humid in summer. You got yourself a hack. I live in a simialr heat load area, 1200 upper and 600 basement and 2 ton is oversized for me, but im in the shade and well insulated, you need a real pro to do a load calculation in writing and show you a real zone system, no you dont need 2 systems as you basement probably just needs mainly humidity removal
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[original post is likely clipped to save bandwidth]

First, doesn't sound like a load calculation was done! It probably will cool the place fine but never likely to dehumidify well.
Zoning AC is a problem in that fixed capacity systems don't take kindly to variable loads and air flow. Variable capacity systems exist. It could be that your contractor understands one of the zones can't supply enough load for the system he is planning.
It might be worth looking into Mini-splits or Multi Evaporator, Variable Capacity, Mini-splits. They are not common in the US yet, are common in Europe. They are a bit expensive but don't need the ductwork and it's costs.
gerry
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Get a written copy of his load calculation, if he actualy did one.
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Thanks everyone for the helpful ideas. I think now that we probably don't need multiple zones. However, I have become concerned (once again) about the load calculations done by our contractor.
Unfortunately, it's too late to change sub-contractors (he's already putting in the ductwork) however, I could change the unit size, if necessary. I've already questioned him about the load calculation and he did take into account the fact that we have 12foot ceilings and three large south-facing windows. However, I believe that we adequately cooled most of the area with about 17500 BTU (two window units) last summer and that we are only really adding about 600sf of area to be cooled (north facing, two windows), so 24000 BTU should probably do it. I'm just not sure. Can anyone point me to some do-it-yourself load calculation instructions. I would just like to get in the ballpark so I can talk to him about it.
Any further help would be appreciated.
thanks.
wrote:

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Hi Beartums, hope you are having a nice day
On 11-Mar-05 At About 08:33:36, Beartums wrote to All Subject: Central Air Zones
B> We're doing some renovation and installing central air. I want to B> put the upstairs and downstairs on separate zones. My HVAC guy says B> that the only way to do zones with air conditioning is to have one B> zone be the Master zone (which decides whether the cooling is on or B> off) and one zone be a slave zone (which cannot actually start the B> cooling, but can only open and close dampers).
B> This seems absurd to me: first of all, it seems next to useless in B> residential cooling. I can see some applications where it would be B> useful, but wouldn't many applications be something like I want to B> cool the upstairs primarily during the day and the downstairs B> primarily at night (bedrooms downstairs, living space upstairs)? I B> don't want to turn cooling off upstairs at night, but I want it to be B> 5 to 10 degrees warmer than downstairs, and vice versa during the B> day.
B> Also, doesn't this seem like it would be a simple electrical or B> switch to the air conditioning unit? I mean, yes the dampers need to B> be wired to the thermostats but couldn't they also both be wired to B> the unit so that it will turn the unit on if the ustairs OR the B> downstairs thermostat wants it on? That seems like a no-brainer, but B> I don't know much about the design of central air units.
there are zoning systems that do exactly what you want. I would suggest you call another contractor who knows zoning systems.
-=> HvacTech2 <=-
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Hi PrecisionMachinisT, hope you are having a nice day
On 13-Mar-05 At About 09:17:49, PrecisionMachinisT wrote to All Subject: Re: Central Air Zones
P> > >> .. You're never alone with a schizophrenic.
P> We love your sig line.
The program I use generates these automatically.
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