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I have a Cape Cod with 1000 ft2 on the first floor and an apartment in the attic. In a few years I may get rid of the apartment and build up a second story + attic. Both the first floor and the attic currently have window AC units. The heat is hot water radiators and the boiler is on gas.

Does it make sense to install high-velocity AC in the crawl space for the first floor and later, when I build up the second floor, add another AC system in the new attic to cool it?

Or is that going to be too expensive?

I'm thinking that a dual zone system would be nice anyway. Are two AC systems twice as expensive than a single dual-zone one? Or what is the ratio? Also, what are the prices of such systems?

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A dual-zone system is one heating element and/or AC coil serving two separate ductwork segments. The extra cost of a dual-zone system as opposed to a single-zone system is in the dual-zone temperature monitoring/control panels, and a system-controlled diverter that sends the air to the side of the system that needs it (or both).

The upsides:

  • One interior and one exterior unit to buy/maintain. Drastically reduces most routine maintenance costs and generally increases MTBF.
  • Easier to balance your home's ventilation; you don't have to worry about the relative heating/cooling capacity of two units versus the space they're expected to cover.
  • Dual-zone systems usually have a "master/slave" control panel arrangement allowing the "master" panel to control both zones while the "slave" panel can either also control both or only control the secondary zone, possibly with additional restrictions like max/min temperature that can be set on the master panel.

The downsides:

  • No backup; if something on the one unit goes, there is no heated/cooled air anywhere in your home.
  • Impossible to differentiate the electric costs of you versus an upstairs tenant.
  • Longer ventilation runs to the second zone, depending on home design; the second zone needs vents and returns run from the main unit to a usually distant part of the home. These longer runs will not be as efficient.
  • To supply ample heated/cooled air to both zones at once, the unit must usually be a bit beefier than a comparable single-zone system, to compensate for inefficiencies and maintain desired airflow when both zones are being ventilated.
  • You cannot have the heat on in one zone and the A/C on in another. The entire system must be set to heat or cooling, meaning if your tenant likes it substantially warmer or cooler than you do, the ability of the system to provide the proper temperatures can be limited at times.

A dual-unit system will have two separate single-zone HVAC units each controlling one area of the home (upstairs-downstairs is common in new construction, as is having a second HVAC for a new addition).

The upsides:

  • Having a backup system means at least some of your house can still be heated or cooled in the event of a failure of one unit.
  • Vent runs can usually be shorter, as the units can be strategically placed in new construction for the most efficient ductwork layout to each zone.
  • Each unit can be smaller than a single unit for the whole house would have to be, meaning the exterior units can be located in tight spaces or more easily hidden behind shrubs.
  • By hooking the second HVAC along with all upstairs circuits to a sub-meter, you can easily determine (and sever) the electrical costs of an income property.

The downsides:

  • More expensive to put in than a single unit, even a dual-zone. Two 1-ton units will cost more than a 2-ton.
  • Maintenance costs also increase; with two units the MTBF of a single unit in the home is halved, meaning on average you'll have to call the repair guy twice as often.
  • Less centralized control. There are some nice thermostats that can communicate wirelessly (usually as part of a whole-house automation/alarm system), but basically each unit will be its own completely separate system and to balance the temperature in the whole house you must go upstairs and down to fiddle with settings.
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If your living in an area that doesn't get below -20c I would look very closely at the geothermal units they have.

These units can be used for heating and ac.

They are basically a heat pump designed to reverse depending on the demand. Because they use the ambient air to pull heat and not an electric element they are extremely efficient. They sell both ducted systems and ductless. We have had them installed in a community hall I look after and so far they have been great. I live in North okanagan and we have seen temps down to -25 so far this year and they have had no problems.

here is a link.

Mitsubishi Electric

Edit; Answer to question.

Here is a links that will do better justice to the explanation.

How Heat Pumps Work

What It boils down to is, it is a lot cheaper to transfer heat than it is to create it.

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Not sure this is Geothermal??? –  Tester101 Dec 21 '10 at 12:57
    
It's not geothermal in the classic form. It is not a system that requires lines in the ground. It does work on the exact same principle where it pulls heat from the environment and transfers the heat into the home and as an ac it pulls heat from the home and transfers it back to the environment. –  Renshia Dec 21 '10 at 16:27
    
The one unit available can be set up as an ac/heating unit for the main floor, then add a second zone to the system for the upstairs apartment when ready. This will drastically offset the cost of the fuel he is currently using, because the unit does not use a classic heating element the heating cost are significantly reduced. One system will do the whole house so their is no need to buy separate systems. Also for almost the same cost of a straight ac unit he get the benefits of both. –  Renshia Dec 21 '10 at 17:05
    
These units are also energy efficient and in BC they qualify for a 1500.00 rebate. –  Renshia Dec 21 '10 at 20:01
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My understanding is that Geothermal takes advantage of the fact that the ground has constant temperature. So these Mitsubishi systems are not geothermal. When you say duct-less are you referring to the split systems? Also, can you help me undestand why these are more efficient? –  Peter Q Dec 22 '10 at 16:58
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