I've heard there are online programs. You need to know your house construction pretty well to get an accurate calculation. Below is an example of how one exterior wall would be rated. Then you also figure windows and doors after that. Walls: (Each exterior wall, on each floor, must be done individually) Any windows and doors in a wall must be included with that particular wall. A rough drawing of each wall can be supplied, if easier, showing windows and doors. All measurements, in feet and inches for walls, windows, and doors, must be included for the length, width, or height. What direction does the wall face? O North O Northeast O Northwest O East O East Northeast O East Southeast O West O West Northwest O West Southwest O South O Southeast O Southwest O A: Wood frame, with sheathing, siding or brick Wall cavity O None O R-11 3 ½" O R-13 4" O R-19 5 ½" O R-27 7 ½" O R-30 8 ½" O R-33 9-10" Select exterior insulation O None O R-1.3 ½" Asphalt board O R-1.8 ½" Beadboard O R-2.5 ½" Extruded poly board O R-2.7 ¾" Beadboard O R-3.6 1 in. Beadboard O R-3.8 ¾" Extruded poly board O R-5.0 1" Extruded poly board O R-8 Sheathing O B: Masonry, above grade Select insulation value O R-5 1" O R-11 3 ½" O R-19 5 ½" Wall construction  no added insulation O 8" or 12" block O 8" block and 4" brick O C: ICF Insulated concrete form, above grade Select ASTM R-value O R-12 to R-14 O R-14 to R-16 O R-16 to R-18 O R-18 to R-20 O R-20 to R-22 O R-22 to R-26 O R-26 to R-30 O R-30 to R-36 O D: SIP Structural Insulated Panels O 4 ½" panel O 6 ½" panel Exterior finish O Stuco or siding O Brick O Split logs on both sides O E: Logs Circle the log thickness: 6" 7" 8" 9" 10" 11" 12" O F: Block or brick, extends to 5 below grade Select insulation value O R-5 1" O R-11 3 ½" O R-19 5 ½" Wall construction  no added insulation O 8 or 12 inch block O G: Block or brick, extends over 5 below grade Select insulation value O R-5 1" O R-11 3 ½" O R-19 5 ½" Wall construction  no added insulation O 8 or 12 inch block O H: ICF, extends to 5 below grade Select ASTM R-value O R-12 to R-14 O R-14 to R-16 O R-16 to R-18 O R-18 to R-20 O R-20 to R-22 O R-22 to R-26 O R-26 to R-30 O R-30 to R-36 O I: ICF, extends over 5 below grade Select ASTM R-value O R-12 to R-14 O R-14 to R-16 O R-16 to R-18 O R-18 to R-20 O R-20 to R-22 O R-22 to R-26 O R-26 to R-30 O R-30 to R-36

One fellow permitted me to look over his shoulder while he did the Manual J and D calculations for my house on his computer. He had never seen the house, but I brought him a set of plans. The plans and my answers to a few questions formed the inputs for the computer. Now I can say without hesitation that part of every proper installation is those calculations. Experience may enable a contractor to get many installations to be close to the ideal without the calculations. But by all means do the calculations. And install accordingly.

A load calculation is a detailed study, most frequently done with the aid of computer software. While the items you mentioned are important in evaluating system replacement, many contractors will ultimately size units based on a 'rule of thumb', such as so many tons per so many square feet. Many will replace with what is currently installed. Others will guess and cite their 'years of experience'. When doing a load calculation (manual j), such things as dimensions and orientation to the sun, as well as geographical location, insulation, infiltration, etc. That's why an identical house around the corner can have a different heat gain/loss profile. \ In evaluating heat gain, it needs to be expressed in terms of the sensible (that which you can feel) gain and the latent (humidity) gain and equipment needs to provide both capacities. The ductwork needs to be evaluated, especially with respect to returns (should have on both levels). If upstairs is a big problem, should consider additional attic insulation and radiant barrier, and this should be reflected in the heat calc. In discussions I have read, most professionals do not recomment an attic fan. V

mike_home, I used indoor winter design temp of 78F and summer 70F; for outdoor I used winter 18F and summer 91F. For wall and attic insulation values, I used R11. All windows are double pane. tigerdunes, I gave the wrong AC model number, sorry, it should be 24ACC624A003. I think you are right about oversizing. Now I remember most of the time High-stage heat ran was when recovering from set back. In the few mornings when the outside was 10-15F, the furnace ran about half time High- and half time Low-stage. I was debating between 60K or 80K BTU. After getting a professional calculated Manual J of 120K BTU (as a part of energy audit), I picked the higher one:( I like your suggestion of a 3-ton AC, which will be consistent with the 5-ton total cooling from the professional Manual J. Considering I have an 80K BTU furnace downstairs, do you think 40K BTU will be fine? Will the blower generate enough CFM for the 3-ton AC? Thank you both for your advice! Jeff