Three phase uses no more or less electricity. The electrical cabling might be less expensive with three phase. The motors are simpler since they don't require starting circuits so there is less to go wrong. I suspect that your choice of equipment may be rather limited, but I will leave that for the pros to comment on further. I hardly think it is worth the compromise if that is the case.

It is hard to say if revamping your existing ducts to add zones will be straight-forward without seeing the system. I suspect it will be difficult because you need to add return air for every system and you probably have one central return now.

If you want to split off some areas, consider mini splits from Mitsubishi, Toshiba or Sanyo. Look at their web sites. The most common type are ductless wall-hung units, but you can also choose ceiling and floor ductless. In addition, there are short-ducted models (that can supply more than one room). They will make a sense in a refit if you need extensive re-ducting than if you don't. Installers that are unfamiliar might not be crazy about installing them. One thing to consider is installation of one central unit to cool most of the house and mini splits in areas that are either little, or infrequently used.

Correction:

I meant, little, or most FREQUENTLY used.

ionized,

Are you sure you need that correction? lol

Three phase equipment is more limited. You may not be able to move beyond 13 SEER.

In you climate, the savings on energy offered by higher SEER efficiencies would make me strongly consider using single phase equipment.

How large are the units? (BTU/hr of heat and/or tons of cooling).

It can be very difficult to find smaller 3-phase equipment.

Most is commercial and larger than needed for residential use.

I just replaced one of out rooftop packages at work.

It was the smallest unit generally available at 5 T with gas heat.

don't take hvac advice from home inspector,
unless he/she has training to make these
statements.

have several hvac companies outline your options.
inspection of ductwork & layout of house is the
only way to accurately answer the question of ductwork.

but adding more units means each will have to tie
ducts into new supply plenum for new unit.
each new unit would also need return air plenum
& filter back grill.

rather than adding more units check into
zoning. multiple thermostats for the two
units. this is less costly than adding more
units & modifications to ductwork are not always
done well and comfort suffers.
a true zoned system has motorized dampers
that open & close as tstat calls for more
or less air. these dampers are controlled
by an electronic board.
true zoned systems are a wonderful thing
when installed correctly.

make sure that the hvac company has experience
with succesful zoned systems.

also look to increase SEER above minimum efficiency
13 SEER.
do you have a/c & electric heat or gas furnace?
if electric heat look into heat pump options
to save in utility costs in winter time.

best of luck.

Wow, thanks for all the feedback!

Right now we have two units and 10 tons total, I am assuming that means each unit is 5 T? So it sounds like replacing them is possible.

The heat source is heat pump, the electric strip "emergency" heat is inactive at both units.

Is it possible to get higher than 13 SEER with 5 T electric 3-phase units?

We will get some companies that are good at zoned systems out to look at our house.

We own a recording studio built into an old house that has zones and dampers etc. and it has been a constant problem. Huge HVAC challenge there because you need to keep all the equipment in the tracking room really cool but have silence (no A/C) in the booth and the sound insulation complicates things and it has been a nightmare! I am going to have to look harder to find a good company to work with...

I did not want to take inspectors word on anything but use his info as a starting point. I feel like I already have a clue and some questions to ask thanks to you guys. I am going to start looking for HVAC companies that specialize in zoned systems...

The fact that you are doing process cooling vs simple comfort cooling is a big deal.

I recommend calling a few mechanical contractors that do both residential and light commercial work, and tell them you situation and ask to someone with experience come out to look at your project.

juliekcmo- what exactly is process cooling? Google is no help. Having 3-phase units is radically different? I need different search terms to learn more.

I will take your advice and find someone who does residential and commercial work, thank you!

My parents home has 3-phase for the AC unit. At the time the house was built, it was a farm house with several acres of land which was all eventually subdivided. When we bought it, it has a 'water-tower' AC chiller in the basement. Great for Legionnaires Disease but not much else. It was replaced with a Carrier small commercial unit, 5-tons. That unit ran great for 25+ years.

In 1950ish in the New Orleans area, anything 3 tons or over got 3 phase equipment and, obviously, power. It seems like that is what the POCO decided to do. That comes from personal experience owing two houses that had 3 ph and statement from an experienced AC contractor/electrician. Water-cooled chillers were typical too.

shannon,
your cooling is not simply keeping people comfortable. You are cooling because of a job, or process. That studio equipment generates more heat than a typical residence. And your acoustic requirements add constraints that are not found in typical residential applications. Most outdoor residential units are say 80 decibels. And they are usually located for shortest line set length and indoor ducting and venting ease. Your situation may call for having the unit in a different location to avoid the noise.

Your job involves the heat load from electrical equipment, and has specific sound requirements that involve vibration control of motors and equipment, and possible specialty items in the ductwork. It is essential you get someone in who can work this all into your job.

also, depending on how much heat you must manage from the equipment, you may need low ambient control to run air conditioning when the outdoor temperature is lower than 45-50 degrees. This is true for applications like cell towers and computer rooms. A contractor will be able to assess the wattage of your equipment to know what amount of heat it gives off.