Can you be more specific about your location? What is the closest major city?

Are you hiring your own HVAC contractor for this new house? Did you get a copy of the load calculations? A new home must have a load calculation. I suspect from the sizing you have received so far no calculation was done. Did you get any sizing for the gas furnace?

You should seriously reconsider installing the duct work in unconditioned space. Post back some more details.

What Mike is recommending is to put the ductwork into a dropped ceiling, a chase, a soffit (many different terms for the same thing) below the attic. Especially in your area of weather extremes, doing this will lead to significant savings for heating and cooling. Running warm air through the cold attic in winter, and cool air through a hot attic in summer, makes no sense.

I'd like to add that outdoor ducts are a bigger problem than just warming or cooling the air in the ducts via conduction through the insulation. Ducts leak air. When they leak into unconditioned space, you have an uncontrolled power vent thing going. A good designer can often plan chases that look like they belong in the space rather than being stuck there because they are needed for ducts.

Echoing Mike's comment, you have proposals for 5.5, 5.0 and 4.5 tons. That is a 20% difference between the high and low. What's up with that?

Good mini splits work well, especially well at humidity control. I am a big booster, but there are drawbacks. I have cold floors in a raised house. In that situation, the high wall units work well for ceiling heaters as they approach the set point for heating and the blower backs off. Paddle fans and manually boosting the blower speed of the mini splits remedies that. They demand more maintenance than a typical ducted system. The blower wheels and "coils" tend to clog up and they need cleaning. Nat gas is a less costly fuel than electricity in most places so tying in the upstairs with the central unit will result in lower ongoing costs. You need some substantial heat in zone 4.

In regards to the different sizing I'm not sure why the differences. I have gotten three estimates, the first was the 3.5 ton with a 2 ton upstairs. No furnace size was given. I asked about the mini split and he gave me the option for the 12,000 btu carrier mini split (that contractor accounts for the 4.5 and 5.5 ton options).

The second estimate was for the 5 ton with 110,000 btu furnace. They have an up charge to use steel duct for the supply which makes me question them if they are doing supply ducts out of flex. They came recommended by a friend.

I had a third estimate but it was about 50% higher than the other two and was lacking any descriptive detail other than specifiying 95% efficient furnace with 16 seer ac.

For all the subcontractors I have given window u factor, house plan with orientation, insulation values in walls and attic, specified hard pipe with 2" insulation on all supply ducts. Seal all joints in ductwork with mastic.

I'll try and get a 4th quote and see what they recommend.

So I talked to a 4th heating and air contractor and after sending him my plans and specs he is telling me I need 6.5 tons of cooling. Looks like I'm getting a wider spread. I also reached out to another guy who is supposed to get back to me soon.

Have any of these contractors provided a load calculation to back up their sizing quotes? You should do your own calculation. Here is the link to HVAC calc. I have never used it, but everyone who has on this forum were positive about it.

mike_home: None have provided a load calculation. I am guessing maybe a couple have done one. I did one using loadcalc.net It seemed really thorough, but it was a free program so who knows. It shows needing 24,241 btu's cooling, with sensible load of 22,042 latent load of 2199, and and btu's of heating at 104,838

The other contractor I was waiting to hear back from called me today. He sells American Standard products. He had me with a 4 ton unit downstairs and a 1 ton mini split in the bonus. He quoted me for a 95% efficient with a 16 seer condenser and 16 seer American Standard mini split.

The other guy I am considering had me at 3.5 tons with upflow coil (not sure what that means) for the downstairs unit and a 1 ton mini split for the bonus. He figured Bryant products with 95% 2 stage furnace (bryant 925TA60100), 16 seer condenser (Bryant 116BNA042), and 21 seer carrier mini split. I prefer the Bryant over American Standard, he has 2" insulation on duct vs 1" for other guy, he is cheaper by $500, but he has not been as responsive as I would like which worries me a little.

5 tons seems to be the consensus with a couple outliers after getting 5 bids, I don't think I want to get anymore than 5 at this point.

Any thoughts on Bryant vs American Standard or any of the data I provided?

"It shows needing 24,241 btu's cooling, with sensible load of 22,042 latent load of 2199, and and btu's of heating at 104,838"

Your load calculation from loadcalc.net says you only need a 2 ton AC, but you need 110K BTU furnace for heating a 2200 sq. foot house in your climate. The numbers make no sense to me given that your climate is very similar to mine. The cooling load seems too low, and the heating load seems too high. You either entered the wrong values, or the free calculator is very poor at estimating.

"None have provided a load calculation. I am guessing maybe a couple have done one."

There is no need to guess. Either the contractor has done a calculation or he has not. Ask the question directly, and ask for a copy. Some contractors won't give you a copy because they don't want you to take their work and then buy form someone else. In that case ask him to show you a hard copy and have him explain it to you.

If you were buying carpeting for the whole house, would you expect the contractor to come in and measure every room, draw up a plan on where the seams would be placed, and tell you options on how to minimize waste? What would you think if you got quotes from 5 carpet contractors each of which had very different amounts of carpet and none of them were what you estimated? Wouldn't you be asking "show me your measurements"?

HVAC sizing is more critical than carpet sizing. If you buy too much carpet, you have left overs which you can either throw away, or put to some other use. The carpet that has been installed is not affected. If the HVAC is sized incorrectly, it will not function well, waste energy, on likely fail prematurely. All of that can be avoided by spending 30 minutes doing a correct load calculation.