How big should an a/c units be for a house that is 2200 sq. ft down and then 1000 sq ft upstairs. It will have two units (zoned for up and down) and curious what the sizes should be for each one and does getting one 'bigger' than needed help cool better?

Do not buy a larger unit thinking it will help with cooling. It will actually hurt because to large of a unit does not run for long periods and it will not remove all the water from the air. This will make the house cold and clammy.

More info needed for size.
Where do you live?
What is the average summer temperature?
Does the house have many windows/doors?
How are the walls and attic insulated?
With this info I can give you a ball park but you need to do a manual J to get the exact size. Also manual D will be necessary for duct layout.

You need a load calculation done. Manuel J

Hvac1000... Thanks for the quick feedback. Here's some quick answers to your ?'s:

Downstairs Windows - 11 ; Upstairs Windows - 6 (All low "E" windows)
Live in Oklahoma - Average Summers 90's (quick guess)
Insulation - R-32 blown in attic; R-13 in walls

This is new construction and my understanding is that the ds unit will be a 3.5 and not sure what the upstairs unti will be. I did upgrade to a 14 seer as I plan to be in the house a long time and think it will pay off. That and upgraded to a 90% furnace. Does 3.5 sound about right and any guess on how big the upstairs should be?

A guess is 18,000 BTU. But a 24,000 might be better since the upstairs still has a heat load larger than the first floor.

I stress to you this fact.
You are spending some big$$$$ on a home. Spend a bit more and make sure it is done correctly.
Tell the HVAC / builder that you demand a manual J and a manual D. This will really narrow down the sizes for you and also cover the heat requirements.

All utilities are expensive and we have seen nothing yet. As time goes by the rates we pay today will look like a bargin. By having the above J and D done you are assuring yourself of maximum energy savings by not over size the equipment and the duct will be sized to keep all rooms the same temperature and air velocity to insure your comfort.

Comfort is what HVAC is all about and if done improperly you will be very unhappy with the results and to correct defects in the system design will cost you a fortune to tear out walls etc to put in the right size duct.

If you have the $$$ you might also consider a higher SEER for the A/C. 16 and 18 SEER are becoming more popular as the utility prices go up.

All above referenced sizing is a GUESS on my part and are not to be used as a recommendation.

BTW you might have the HVAC company pull 8 conductor cable for the thermostats and make sure they use at least 18 Gauge wire. You will probably only need 5 conductors but with all the new energy efficient control that will be available in the future having the extra wires already in the wall will make the install of newer controls a snap when the time comes. The 18 gauge recommendation is because when you use a smaller wire over a long run (big house) there can be a lot of voltage and capacity drop. This is not good for the thermostat or the computer boards that come on all new equipment. The bigger wire is a very cheap upgrade. I would ask the contractor what gauge of wire they are going to use first. If it is smaller like 20/22 (5 conductor ) wire then make sure they use 18 (8) conductor wire.

Do it right and enjoy your new home.

I would pull an 8 cond/18 AWG and a CAT 5 cable. Having the latter will allow installs with communicating stats that use RS-485.

Tell the HVAC / builder that you demand a manual J and a manual D. This will really narrow down the sizes for you and also cover the heat requirements

At what point should they do these checks? This house is a standard plan they have so hopefully this is something they have done before to determine the size A/C it needs already.

[QUOTE=hvac1000]
Comfort is what HVAC is all about and if done improperly you will be very unhappy with the results and to correct defects in the system design will cost you a fortune to tear out walls etc to put in the right size duct.


Sadly enough a personal friend has a really nice house that they just bought have run into this. It was a nice house and only 3 years old, but they have to rework the ducts upstairs as the are too small! I want to make sure and avoid this!



If you have the $$$ you might also consider a higher SEER for the A/C. 16 and 18 SEER are becoming more popular as the utility prices go up.


Wish I could.. I already upgraded to the 14 seer and to go up to a 16 would cost too much for me...



BTW you might have the HVAC company pull 8 conductor cable for the thermostats and make sure they use at least 18 Gauge wire. You will probably only need 5 conductors but with all the new energy efficient control that will be available in the future having the extra wires already in the wall will make the install of newer controls a snap when the time comes. The 18 gauge recommendation is because when you use a smaller wire over a long run (big house) there can be a lot of voltage and capacity drop. This is not good for the thermostat or the computer boards that come on all new equipment. The bigger wire is a very cheap upgrade. I would ask the contractor what gauge of wire they are going to use first. If it is smaller like 20/22 (5 conductor ) wire then make sure they use 18 (8) conductor wire.

Do it right and enjoy your new home.

Really good idea. I am trying to build into the house upgrades like this that cost little now and could pay off down the road. I'll make sure and ask them about this. Any other thoughts on things related to HVAC on the new house to try and ask about or consider upgrading?

Check out ERV/HRV's

Energy and Heat Recovery Ventilators (ERV/HRV) (http://www.toolbase.org/Technology-Inventory/HVAC/energy-recovery-ventilators)