Determining the size of an air conditioner or heat pump appropriate for conditioning of a home depends on a number of variables including the climate zone, the level of insulation, the amount and performance of windows and doors, the performance of air sealing, etc. It is not based strictly on the size of your home nor vaulted spaces.

I suggest you ask the builder for a copy of the calculations.

In the municipalities in which we build, mechanical contractors are required to submit heating and cooling calculations in order to pull a mechanical permit. The calculations are reviewed by the plans examiner. If yours does the same, you should be able to get a copy from the building codes compliance dept. if you can't get one from the builder.

Is the system cooling the home adequately?

It is not. They are coming to adjust the vents on the second floor. Half of the second floor is running 6-8 degrees warmer than the side with the thermostat

What is the "green zone:?

New England - all of the online estimates show it as the ”green zone”

You said the HVAC is a dual zone system. Does that mean one furnace and AC split into two zones? Are the two zones the first and second floors?

It is not possible to tell if a 3 ton AC is the correct size for your house with the information you provided. I suspect this is a installation problem and not a size problem. Possible causes are the zoning is not working correctly or the duct work is too small or not connected properly.

@Elmer J Fudd,

If New England is in a "green zone" I can only infer a green zone is one where the government will take more of your green.

@mike_home yes, it is a single furnace and a single condenser. Each level has its own thermostat. I have seen many similar size houses in my area with two condensers. Im not sure what other information i could provide. I am just trying to get a sense hecause the HVAC company is just coming and opening and closing different vents.

Balancing dampers are inexpensive and a much better alternative to messing with supply registers in an effort to balance air flows.

@Charles Ross Homes is that the right way to do it? its brand new construction and under warranty. I want to make sure it is done correctly and efficiently and not just the cheapest way possible

If you fiddle with supply registers long enough, you might be able to reduce the room-to-room temperature variations. You may find you need to adjust them seasonally for comfort. Balancing dampers are a faster and easier way to balance air flows and you can mark the handle "summer" and "winter" and reproduce the settings with reasonable accuracy.

You'll need to check your warranty documentation to see what room-to-room temperature variation is considered "normal."

Do you know if the duct work for the second floor is located in the attic? If it is then adjusting dampers and verifying the duct work is connected correctly should not be too difficult. However if the duct work is running in the walls, ceiling, and floors, then the dampers adjustments might be limited if the furnace is in the basement. I think this the reason the installer is trying to close off supply vents in order to compensate for the uneven air flow.

A temperature difference of more than 2 degrees on the same floor is unacceptable in my opinion. The goal of zoning is to have even temperatures throughout the house.

@mike_home the ductwork for the second floor does go through the attic. They will address anything more than a 2 degree difference. I am also finding it muggy upstairs, but i think that relates to the inconsistent cooling

Mike,

While the OP may have a zoning issue, what they've described is an air flow balancing issue. Once the air flows are better balanced (as determined by a reduction in room-to-room variation in temperature,) they can evaluate the "muggy" condition on the 2nd floor. If there is a problem with relative humidity on the 2nd floor, it may be attributable to zoning.

You could be correct. But then again a 6-8 degree difference seems more than a air flow balancing issue. I would not be surprised if the flexible duct in the attic has a kink or is even disconnected. The zoning adds another layer of complications.

In my experience, HVAC installers rarely balance air flows. They only do so when there's a problem. It could be a duct work issue as well. Let's see what the outcome of the contractor's effort today was.

"While the OP may have a zoning issue, what they've described is an air flow balancing issue."

Agree this is the first thing they should check and adjust.

You can go into the attic to evaluate the ductwork. Identify which ducts supply which areas. Check for leaks with the blower on. See if any balancing dampers have been installed. If you don't find any, ask the hvac contractor to install them. Mention this lack before they try to use the registers. Also identify return air ducting. This can contribute.

For those at the edge of their seats… they have been at the house 3.5 hours and had to call for backup. They are replacing the thermostat on the second floor, replaced some dampers and redid some of the duct work. its going to be hot, so we will know pretty quickly if it doesn’t fix the issue.

Good luck!