I have called every HVAC guy within 50 square miles of me (the good ones anyways) and asked if they do a Manual J calculation for a replacement HVAC system. Only 1 does it and apparently he knows he is the only one as he charges $350 just to do it. If I buy his system then he takes that off the price of the install but he is by far the most expensive contractor on system. I do not know how to do this calculation, but I've heard the rule of "500 sq. ft per ton is what you follow" isn't the ideal. I looked at some software online but it's either really expensive or I am not smart enough to use it. lol How do I get this calculation done or is it's importance overstated? Our old system has performed well but we are looking at going with a 15 seer heat pump (air) to replace our old 10 (maybe 8) seer heat pump. New duct work and electric furnace will be part of the deal. If we are happy with the performance of such an old but run down system, I can only imagine what a new higher seer will do with tighter ducts and the like. As with everyone money is as tight as ever so I can't just be paying guys for fancy math, so any help is appreciated. How do I figure this on my own?

You could just base the new system size on the old system. Not necessarily the same size - smaller is OK if it wasn't maxed out.

It's not really all that hard to do your own. Heating and cooling loads aren't a function of the square footage of the home- it's a function of the construction R-values and window U-values, and the orientation of the house relative to the sun etc. The old schooler rules of thumb don't leave you too hot or too cold, but more often that not that approach will oversize the equipment by a large factor- 2x oversizing is typical, 3x oversizing common.

A pretty-good heat loss freebie is downloadable here:

http://www.taco-hvac.com/products.html?current_category=370

The instruction vidi lives here:

http://www.taco-hvac.com/products.html?current_category=370&show_vid=1

$350 isn't an outrageous amount to ask, since it involves a lot of measuring and poking around to get it right- time is money.  I've known some HVAC ciontractors who would take the Manual-J off the installation price if you opted to let them install it, but not all. Most would just have it as a line-item in the total bill, or billed separately.  If YOU take the time to measure everything and enter it into the Taco tool, play around with the infiltration factor and design temperature numbers a bit so you know it's not too wrong you'll probably hit pretty close.  I haven't vetted this tool against others (just downloaded it myself), but it has the look & feel of some of the decent tools. 

Don't be surprised if it comes up with numbers significantly smaller than the ratings of your existing heat pump, and try to trust the tool when that happens.  If your old system was 4 tons, and the tool says 2.5, go with 2.5 (3 at the most.) Most of these tools have some amount of oversizing factor built-in, and if it's saying 2.5, it could easily be closer to 2.   Going with multi-stage or variable speed scroll compressor heat pumps (and variable/multi-speed air handlers) you can get away with some amount of oversizing without losing efficiency, but with single-speed versions (or more than 2x oversizing) you'll take a hit in both comfort as well as efficiency.  In that situation a variable-output 2.5-3 ton will run long, efficient duty cycles, and will be quieter and more comfortable.

Like jonr says, you could use the ratings of your old system as an absolute upper-bound assuming it kept up even on the hottest/coldest days, but think multi-speed/variable-speed if you want to max out on comfort.

Its more than fancy math its the experience and knowledge to properly identify the construction of your home. its worth it in the long run as most contractor over size equipment and under size duct work. I would say pay the money you will be ahead in the long run. He is doing 3 to 4 hrs of work no one else is doing. Its like the diagnostic fee you pay the auto mechanic. You could also find an engineer to do it as a consultant. You will have a more efficient and more comfortable house if you do.

$350? I do a block load on every house before quoting a system, they take 15 minutes to do on an average two story 2,000 sq foot home. If you want a room by room load with Manual D thats a different story. I wouldn't expect anyone to give you their numbers until you pay them but all should do one before quoting a system size for you. What area of the country do you live in? It is fairly common here that 70% don't do a Manual J(used to be 90%) but more and more I hear my customers saying i am not the only one taking 15 minutes to measure the house and look at insulation and windows.

do you have a floor plan of your house with insulation values and window dimensions? if you want a block load send me and e-mail i will do it for free.

Pay him and be happy.

DIY heat loads...we talked about this. Truly ridiculous when trying to size of mixed radiant floor or hydronic retrofits.

Get your free heat loads where you find your free lunch.

Not evaeryone can afford to do it right, but if you can afford a true professional (even if he know it) there is no good argument for spending good money without knowing watt you are getting. (ma

True professionals i.e. profitable businessmen, don't work for free.