500 sq ft per ton is often used as a guideline, so at first glance 7 tons does seem on the high side for a home with less than 3000 sq ft, but as you rightly say there are several variables. These include insulation, house orientation and shading, size, type and orientation of windows etc.
The only sure way to size an A/C unit is to perform a Manual J calculation. You have agreed to pay well for this and I would absolutely insist on having it. I would then check that all the parameters have been entered correctly in the calculation.
An oversized unit will not only cost you more but will not function correctly and could lead to much more serious problems later!

if you REALLY need 7 tons, get 2 systems. one for the bedrooms and one for teh living area. my house is ~2400 sq ft and i have a 3.5 ton up front and a 3 ton in the rear. if your house is 2 story, you will want 1 for the first floor and one for the 2nd. the nice thing about running 2 systems, or even 1 large zoned system, is you don't have to heat/cool the whole house unless you want to. we basically shut off the unit to the front part when we go to bed, only cooling the bedrooms at night. during theweekend days we cool the bedrooms but keep them a little warmer than the front since no one is really back there.

of course, teh drawback is higher up front costs and depending on how you use it you would have higher monthly electric costs.

no matter what you do, stand your ground and FORCE them to run teh Man J and show you the results. if you are in a hot area, tell them up front if you want to be able to cool to 70 inside on a 100 degree day, otherwise you may be unhappy!

if the unit is much over size, you will not get proper humidity removal, and that will lead to mold.

I thought you could only realistically expect your home to be 20 degrees cooler than the outside with average insulation and A/C.

Good Lord! 6 1/2 tons for 2400 sf? We have a 2400 sf two story, and 3 1/2 tons does just fine, even when it is in the 90's. I can't imagine having almost twice as much capacity. The unit would cycle all the time.

Who is doing the Manual J calculation? It should be done by a qualified HVAC person. If the GC is doing it, I would have questions.

rachel, yes that is realistic. 20 degrees is the temp difference that a/c is designed to achieve. but many folks are not realistic. over 40% of hvac systems are oversized.
that is the reality.

emily, the orientation is ideal, the minimizing of east, west facing windows is also ideal. insulation is good.
these factors when properly entered into load calc will reduce size of unit.

I really doubt that he did a load calc for you, or if he did I would question the inputs he used.

best of luck

What are you paying $400.00 extra for? It was unclear from the message.

If you are paying $400.00 extra for that type of insulation, than that's a sweet deal.

But if you meant you are paying $400.00 for a Manual J, then I would be a little offended if I were you. I've never heard of a jurisdiction that didn't require a Manual J as part of the HVAC permit process. Every HVAC contractor I've ever worked with does a manual J before he evens submits a proposal. It is standard operating procedure. You should definitely find out if it is a required part of the permitting process...I would bet that it is.

We were told that most ACs are oversized because the HVAC guys never want to have to redo the system and people don't complain about too cold. :) Dh used a program and did a manual J himself and then compared it to contractors'. Many contractors were reluctant or declined to show us the manual J. They just use the standard 500 sq ft = 1 ton formula. Needless to say we didn't use those companies. Our house has 3200 sq ft + unfinished bonus space, unvented attic, foam, and we used a 5 ton zoned system (thermostats in different areas of the house). It is working well and I can keep it set at 78 when the temp is 100 outside and the interior is very comfortable. Our manual J actually called for 4.5 tons but since that wasn't available, we went to 5.

I agree with jwm...if you are paying only $400 extra for foam insulation you got a major deal!!

We have a 4600 ft2 home and have 2 systems. We have insulated with the open cell foam around the entire envelope. Our hvac is not on yet and I am so 'stoked' about what we have done. The house is comfortable even in 90 degree heat. The biggest thing that I have noticed is that the temp does not change readily unless the windows and doors remain open. We have also done geothermal hvac and are expecting big savings. Our AC contractor did the manual J and I did ask to see them, per the advice of these forums. We were able to reduce our tonnage some, but not by as much as the insulation guys said we could. Also, the geothermal units only came in whole tons.

We also are adding a 'fresh air' vent system that allows fresh air in to the house through the ducts, otherwise, nothing gets through!

Thanks everyone for the input!

The $400 is what the builder says the 'J manual calculation' costs. This is an extra cost as his A/C person doesn't usually do it, says only one person in town does it.

The insulation is costing $12,500, which covers the closed cell in exterior walls, open cell under roof deck, cellulose around interior bathroom walls. (I wish $400)!

txgal06 - DH is of the same mind as your DH, get an online program and calculate ourselves (actually he's nominating me to do it)!

I wish I could trust the builder and his subs to take some pride in producing a well built, energy efficient home!

So far with our experience I definitely think there needs to be a whole lot more quality control and monitoring in residential building. (Forgive me for ranting)!
Thanks again!

Wow!

IMHO, you really need to get a professional HVAC contractor involved.

Take a look at the link below for some interesting observations about unit sizing.

Here is a link that might be useful: HVAC Sizing Article

I forgot to add this: Be sure to look at the sidebar to the article linked above. They link it from the bottom of the page. I have also pasted it below.

Here is a link that might be useful: Sidebar To Above Lined Article

in my area load calcs cost about .10 cents per 1000 sq ft
prices vary, but the highest I have heard of is 15 cents per sq ft...and this was for large complex homes.

emily..he didn't do a load calc.

best of luck.. and look for a professional hvac company
this guy does not know what he is doing.

I went with foam insulation unvented attic, 2 story, high ceilings, hot summers, now over 4000' when done over 5,000(unfinished bonus room). downstairs unit is 4 tons upstairs 5 ton. both were highest efficiency Trane units with variable speed handlers. NO problems. Also have all vents high and returns low -- with returns in each bedroom and study, that can all be closed off. System is solely AC as I have radiant heat with a boiler. You need to consider total design NOT just size of units.

Your are well into commercial units for 7 tons, and even 6 tons in many cases.
A residential above about 5 tons is a big deal.

It really sounds like the Manual J is WAY off.

"I've never heard of a jurisdiction that didn't require a Manual J as part of the HVAC permit process."

Well, our jurisdiction doesn't require an HVAC permit, so there is definitely no requirement for a Manual J. We had one done though.

something stinks in the wood pile

The majority of the country does not require a manual J for permits. Around the Chicago area a lot of the villages and towns do require one.

No Manual J around here, either. Half the counties in this area don't even have a building code. In some, no building permit is required.

After requesting a copy, plus a meeting with the guy who did the J manual calculations, our builder informed us that they had spec'd the wrong house!

The builder was planning on having the duct work installed next week! We have asked him to hold off on that until we have the J manual results which we are getting independently through a Trane dealer. (not the builder's Rheem guy)

The Trane rep did a walk through of our whole house and unvented attic and took a copy of house plans. Should have results next week sometime.

our builder informed us that they had spec'd the wrong house!

hope that is true, but it sounds like cya to me.

best of luck!

Spec'd wrong house? That's scary. I'm very curious to see how much cooling you really need. Good job with getting the Manual J. I have 3.5 tons doing around 2800 sq. ft. (+ additional 700 or so in the finished basement) and it does just fine and makes the house very comfortable. My home was built in the mid 80s. It can hold 72* during the high heat 90++ degree days. Good luck, Emily.