Tell the HVAC guy he is full of bovine scatology.

It is a lot more work to perform an accurate Manual J on an older house with all sorts of unknowns for insulation and infiltration.

A manual J is easy to do for new construction since the value of all the insulation is known.

A big problem area for any manual J calculation remains assigning infiltration losses.

You have to make sure the builder follows good practices.

New windows and doors should have a specified infiltration value, but you still need to pay attention to all the details during construction.
Many are now called out in the model building codes.
Things like gasketing between wall bottom plates and the floor (not just unrolling fiberglass a few inches wide and crushing it with the bottom plate), and being sure to correctly foam gaps around windows and exterior door are important.

Even making sure the rim joist gets insulated.

Who supplied the plans for the new house?

Do it yourself. $49 for a short term license and well worth it.

Don't hire anyone who won't do a manual J. If they cut that corner, they will cut others for sure. Use the results you get from doing your own load calc to compare the results of those done by the HVAC contractors.

Here is a link that might be useful: HVAC-Calc

brickeyee-its our plans done by an architect acquaintance. we are trying to decide whether to contract it ourselves or hire a GC. I've been getting bids for different things and am on the HVAC now. Two of the contractors came back with HVAC systems of 13 SEER 2.5 ton upstairs and 1.5 ton for the master downstairs and 2.5 tons for the rest of the downstairs...the square footage is only 3500. I was thinking that might be overkill.
The guy I referred to earlier literaly told me the manual j is a load of $#@# and getting more than 13 SEER is a waste of money and a way to rip people off...maybe i should skip him over:-)

Creek side-thanks for the link, that is what i've been looking for...i will do that for sure!

When I call the different recommended HVAC places they should do this for me also, right?

Yes they should. Once they confirm you are a serious potential client, they should perform a Manual J. They will do if from the plans, which is easier than coming out to an existing house and examining and measuring everything.

6.5 tons for 3500 sf in a new modern tightly constructed home? Where are you, Death Valley?

nope, just the south:-)

"its our plans done by an architect acquaintance."

Mot a very thorough job.

In NC (at least my jurisdiction) it is code to do a manual J. That being said, my HVAC contactor did the manual J (I've seen it) and then ignored it. He actually put 4 tons upstairs where the load called for just over 2. (This was redone down to 3 as Carrier doesn't make a 2.5 Infinity)

The manual J is imperfect. The latest version doesn't accout for a radiant barrier in the attic which reportedly can save 15% on a/c bills (and would be the determining factor for sizing in the South). Infiltration for your house will likely be better than average if you are building your own house or watching it closely.

I personally foamed all penetrations into the attic and I bet that made a significant difference in infiltration. You wouldn't believe the holes in fan housings etc.

Now if they did the Manual J after the blower door test, then I'd trust some numbers (although the blower door test is itself imperfect). Our infiltration was 50% of the energy star standard before I had finished a few things. It really isn't possible (I don't think) as the units are hooked up by then. Although, that maybe only because they test the ducts at the same time as the blower door test. I just know when we were at that point, the units were already installed.

Anyway - the manual J is an absolute requirement. Just know that it is imperfect.

The Manual J overstates the heat load somewhat. Contractors usually overestimate the heat load to an even higher degree. Some do the Manual J then add a fudge factor for good measure. The end result is mostly over sized AC and heat pump installations in the US.

See the link.

Here is a link that might be useful: Proctor Engineering