AC sizing help

csabNY

I have a 1800 sq ft split level home in the northeast (built in the 1960’s). I will be putting on a master suite addition of about 600 sq ft. I need central AC for all 3 areas. Essentially they are 3 separate floors, each stair case is 1/2 a flight.

I am getting conflicting quotes. 1 system with 3 zones vs 2 systems. Will one system be sufficient? If so, what size? 4 ton? 5 ton?

The new roof across the entire house will be spray foam insulated.

Not interested in a heat pump system. We have oil and baseboard heat.

thanks!

pecmsg

3 Stories
3 Systems!

Get interested in heat pumps! Much cheaper to operate for 1/2 to 3/4 of the winter.

EdTheHeaterMan

If you have the standard 1800 Sq Ft. 3 level home that was built in the 1960s and you add 600 Sq Ft in 2023, The standard air condition system will not work because there is no such thing as a standard split level 2400 Sq Ft Home that was mostly built in the 1960s with a 2023 addition.

If you had a 2300 Sq Ft split level then that would be entirely different. Too bad you made that addition 600 Sq Ft, and not 500 Sq Ft

Sorry about that. I guess you will need a contractor that knows how to do a manual J load calculation to determine your actual needs.

EdTheHeaterMan

All joking aside, Ron has a good point. There is more than one way to air condition your home. The newer Mini-split systems are excellent at providing up to 4 different inside evaporator units to one variable speed compressor. They use something called Inverter technology to run the compressor at different speeds based on the load. as it gets hotter outside the indoor units will all be requesting the outdoor unit to give full capacity to them. this is when the compressor will operate at full capacity. as the temperature drop outside some of the indoor units on the lower floors may not even call for cooling while the upper floors will require less than full capacity. The outdoor compressor will operate at a fraction of full capacity at those times. This is the most efficient system available.

Some of those mini split indoor units can be connected to ductwork in order to cool the entire floor from one indoor unit. This way you need only one outdoor unit and four indoor units. That might be the most expensive option.

Another option that may be less expensive would be to have one air handler in the attic of the existing upper level and have ductwork go to the new upper attic that will drop down to thru the ceiling of the two different attics. Some of the ducts can travel thru closets to the floor below. The lowest floor will need the least amount of cooling by design.

EdTheHeaterMan

Here is my duct design idea for the lowest price option. One zone for all 4 floors. Volume dampers in each main trunk to balance each floor as needed. Thermostat on top floor. Adjust lower floor as needed manually. Future option will. be to add motorized dampers for future zoning.