"...northeast summer sun." What state do you live in?

AIRSOME,

Pennsylvania. We get our share of 90+ days in JN / JY / AU.

Any input??

Thanks

The 1600 sq ft first floor is handled by a 3- ton 2-stage heat pump w/ 2-stage variable speed LP gas furnace

The larger 1900 sq ft second floor is handled by a 4-ton 2-stage heat pump w/ 2-stage variable speed LP gas furnace ...

so you have 3 tons on 1600 sq ft, and 4 tons on 1900 sq ft.
and you have load calcs? I see a 500 sq ft per ton sizing here, and in my hot humid climate short cycling & shorter life of system would be a given from day one, sick house syndrone would follow in 3 to 5 years.
given that you have 2 stage units you could hope that the installer would set the fan speeds to the lowest settings
which gives you a better sizing for the highest cost.
but then again hoping that the installer even knows what dip switch settings are, much less how to set them properly would be asking too much.

I think that your ac company did poor load calcs, and installed the largest most costly equipment he could sell you. Once you question or change insulating methods he can slow down units to perform, but still has gotten top dollar out of you for oversized units. Now he will just make it work.

according to what I have know for years and the load calcs I am now doing you could have installed a 2 ton system & a 3 ton system..or even better a 5 ton zoned system for lesser cost & same performance.

I'll be interested in reading your ac company's fix on this one.

best of luck

What he said. But go with two systems. Don't force 5 tons into small spaces.
Humidity in attic will be a problem. It will be exacerbated by warm humid air rising to attic and condensing invisibly into tops of walls. Consider how to dehumidify, where to run drain lines, how to manage water collected in auxiliary drain pan when coil ices up. Seal all penetrations to attic.
You will have problems with ice dams if roof insulation is not applied correctly.
These are normal problems.
Get more quotes. Dealer should measure every room and ask many questions. If he does't, he doesn't care.

A local builder in Dallas, built a 36oo sqft house, that uses 4.5 tons. A 1.5 ton system is the basic system. Secondary is 3 tons. Both kick in if exceptionally hot. Usually the 1.5 is all that runs.

The house is ICF construction, with 8 inches foam in attic rafters and 6 inch foam in attic floor. Hot water radiant floor solar heat and AH's have hot water coils. The house has 3 AH's for the 2 condensers. Chill water and solar heated water. They call the house NET ZERO energy usage. This is a misnomer, because they expect the windows to be open past the time they should be closed. At any rate, the utility bills are very low.

Foam is expensive, but will pay for itself faster than most improvements. I estimated a cost of $3.6k for my house for the attic floor only. Maybe another $600 for the ducts.(Materials only) I would expect to pay for the insulation within 3-5 years. (Tiger foam)(1750 sqft)

This is from Manual J 8th edition version 2 ACCA

Improper Practices 2-4
Do not use "rules of thumb". The idea that the required equipment capacity equals the floor area multiplied by some magic number has resulted in many customer complaints and legal actions. Heating and cooling loads depend on individual circumstances. Floor area to tonnage ratios for the U.S. housing stock can range from less than 500 sq ft per ton to more than 1,200 sq ft per ton. Efficient single family detached homes with a normal amount of well distributed glass typically fall in the 700 to 1,200 range. Limted exposure dwellings with concentrated glass (that produces a time-of-day peak)may fall into the 500 to 800 range. Homes with exceptional features can be all over the map in this regard. Just rotating a home on the site can change the ratiio by 100 to 400 sq ft per ton.

I edited out the replacement equipment paragraph...

Comfort system performance is only as good as the accuracy of the load calcuation. Efforts to "adjust the load" to provide a "safety factor or to produce a solution that is compatible with the " I have been doing it this way for thirty years" syndrome only produces designs that generate customer complaints about poor temperatureand humidity control, or more serious problems, such as mold and mildew.

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and Bob...please- this isn't a DIY or best case house we are talking about, but a homeowner trying to get his house right.. save your diy projects for your indulgent threads on that topic.
As he is trying to decide to spend xxx dollars insulating his roofline and the timeline for payback your project does not project the numbers he will be paying someone to do his job as yours is diy.

I have yet to see a foam insulation company do a job that has a payback of less than 20 years, and I do these projections often.

depending on the pitch of the roof the cost of the foam insulation is a consideration. having a complete thermal barrier at the attic floor is a much more conservative approach if detetmined to use foam, although this thermal barrier can be achieved with out foam with attention to sealing of ALL penetrations into the conditioned space
(wire, plumbing, recessed lights and knee walls where ceilings change heights).

I would recommend the latter, sealing and insulating well with conventional insulation for same performance at a lesser cost and faster payback.

the load calcs are only as good as the inputs and the information provided to the person doing the load calcs. I would question the information that has been provided by taking the house plans, a copy of the current load calc to another more experienced hvac company for comparism.