This 4 ton unit is for the upstairs. I currently have a 4 ton unit that does cycle on 100* days but not much. There is not enough insulation in the attic and the ductwork is in the attic. The the insulation will be increased after new ducting placed. The current guestament of the ducting is around 1000 CFM but will be redesigned to handle 1400 CFM.

I had 3 companies come out and give bids and all 3 said 4 tons. The only one that did a manual J said 3.5 Tons after insulation improvements. I bought the HVAC-Calc and used accurate inputs but did include upgrading the insulation in the attic and came out with 43,672 (39,548 sensible) BTU's heat gain.

This is the question: The window load is 22,933 with no shading internal or external except for the South facing windows that are shaded by a large overhanging porch. I never leave all the window shades open in the summer when the sun is shinning directly in so is it necessary to use the recommended "no internal shading" for the windows? It would lower the gain alot to label them shaded. The total gain would then be 35,983 (sensible 31,859).

If the average system has 70% sensible capacity the I'd still be at 4 tons with the shades closed.

This area is bedrooms and bathrooms. The hottest part of the day no one will be up there.

I am looking into the posibility of installing a 4 Ton zoned system that ties into the downstairs (currently a 4 ton package unit w/ only electric heat $$$). Using the unit to cool the upstairs at night by setting the downstairs stat up 10* then setting the upstairs stat back 10* to cool the downstairs during the day.

The house was built in 1953 w/o AC or heat so it's not easy to do anything but the complexity of extra ducting to zone it may not be feasable.

Thoughts?