One unit. Dual zones. New HVA C contractor.

....or a mini-split. That's what we did. Our house is small- 1400 sq ft- and we heat and A/C with two small Mitsubishi Hyper-heat mini-splits. The main house has a single 12K BTU unit with one head, and it keeps the whole house cool in SC. We have an open plan, and very good insulation and sealing. In the master suite, we have a 9K unit, which is overkill, but it works well. We have a timer set up which turns off the 12k unit at night, and turns on the 9K unit. Our power bill for an all-electric home was less than $70 for July, our hottest month. Mini-splits will heat and cool in all but very cold climates, and are cheaper than ducted systems. The efficiency is amazing, with some units now topping 30 SEER.

Please describe the HVAC system you plan to use and the climate.

Well the plan was to have central AC and forced heat.. Every builder we went to said they would do 2 units.. 1 for the upstairs and a 2nd unit for the 1st floor that would also feed the basement unless we wanted a completely separate unit for basement.. This last builder suggested that we put a separate unit to control the master suite which includes master bedroom, master bathroom, and closets.. This equates to 1 half of the second floor living space.. I guess we would have to speak with his HVAC guy to see exactly how we would do it, but the builder said it would be more a luxury for me as I would be able to heat/cool my bedroom/bath/etc exactly to my liking and not to the comfort level of the other residents (kids).. I highly doubt we will go this route and will probably just do a 2 system setup and have the 2nd floor on its own unit and the 1st floor and basement on a second unit.. Was just curious if have the master suite set up with its own system was a new "norm" or what.. Doesn't sound like something I would do..

Ren's question was important and so is the house design. Do you have lots of glass? Southern climate with west facing glass? Northern climate with lots of north glass?

If youre building envelope is built with attention to air-sealing, code minimum insulation and well designed fenestration then its possible you could be very comfortable and have very low energy costs with a single zone ducted unit and a mini-split or two. Tough to know with so little info.

Whatever you do dont let your builder and HVAC sub do the design and sizing with rules of thumb. Get yourself an energystar certification or at the very least, a third party Manual J and possibly D.

I'm with Holly. Zones do it....you don't need separate units. And if geothermal is an option, I'd investigate it. If you're spending that much on hvac, you might as well minimize your operating costs over the long term.

Is the base design 2 furnaces in the basement with AC coils?

For a large complex house I usually use a high efficiency condensing boiler in the basement and place air-handlers near the spaces being conditioned to reduce duct runs. Variable speed fans add to the efficiency and comfort.

But perhaps your climate does not justify that level of efficiency and control.

how big is this area in sq ft?

when building new...this is the time to get it right.
IF you have a hvac co that understands zoning
then make the master suite on a separate zone.

size system for whole house manual J
design & size ductwork man S&D
install system & duct design correctly.
now is the easiest time to do so.

I'm not a fan of separate units for different floors.
homeowner ends up paying for multiple units
more cost upfront, more cost to operate.
the one system per floor is hvac co's way of
covering their butts...it isn't to your advantage
at all.

zoned systems work, when designs are incorporated
and installed according to design.
several zones per system.
you could have each floor on a zone,
and even break the floors down into seperate
zones controlled by their own tstat.
at night put less hvac in kitchen living section
and more to bedrooms for example,
zoned systems are to adjust to how you live
in the house.

shop hvac co's that understand & do zoned
systems...one unit multiple tstats...not multiple
systems.

best of luck.

OP - climate matters.

We have a similar layout with a zoned system upstairs. Slept at 62 last night (central NC) while the little one slept at 74. When he was an infant, he slept at 78 in the summer 72 in winter.

For winter, we typically sleep in the high 50s (no heat at all) and he sleeps at 66 - now that he is 4.

Would never have a small child again without zoning the bedrooms. Of course it is better. Probably should be the new norm but it is expensive and nobody is used to it.

A lot depends on your personal preference of temperature how you sleep. Zoning added $1000 which is much cheaper than an extra unit.

As an aside, we didn't have a seperate unit for the basement. It is set up to be zoned but we have lived for 3 years with just a damper. No a/c needed but there is some airflow for dehumidification. The winter need is pretty constant - but I may add zoning in the future

Here in Houston it is very common to have two different systems for first and second floor.

As a stay-at-home mom, I set the thermostat for the first floor at a comfortable temperature, while the upstairs is set higher on hot days and lower on our few cold days while the kids are at school. On cold evenings, my husband and I sleep without heat downstairs, while the kids sleep with low heat upstairs.

I would never build or buy a house a two-story house on just one unit. Once the kids are gone, we will use minimal heat and cooling upstairs.

Also, when one unit goes out and it is 102 degrees outside, we can stay in our home with a degree of comfort until the unit is repaired. :)

Bird - no one was talking about a 2 story house with 1 unit. That wouldn't meet modern code in NC without a zoning system

David, I must thank you for pointing out my idiocy. I'm sure that just made your day!

:)

And I have been in a recently built house in the midwest that had one ac with no zoning options for two floors. Perhaps because it was military housing...

For me in the deep south, four zones, one unit (heat pump), all electric house, 4000 sf, two story, foam insulation, July electric bill $150.