I have to replace a unit in an existing 4 unit condo building. I am the general contractor. After doing the calculations it looks like a 1-1/2 ton unit would handle the load without a problem. It also looks like the low setting on a 2 ton unit is even closer to the exact heating and cooling load. Ideally there would be a two speed 1-1/2 ton unit but it seems like there are only single speed units at that tonnage. My geo contractor suggests the two speed 2 ton unit but they are a bit more money and I'm not sure the second stage would ever come on.

Which one?

The 2 stage 2 ton unit operating in first stage would likely operate more efficiently that a single stage 1-1/2 ton unit, especially if you have 2 tons of heat exchange piping in the ground. But it ultimately comes down to determining if the additional up front cost of the 2 stage unit is worth it. To be honest, without knowing the specifics of your units, there isn't really much difference in capacity between a 1-1/2 ton unit and a 2 ton operating in first stage assuming 2/3 full capacity in first stage (2 ton x 2/3 = 4/3 ton = 1-1/3 ton). This is really splitting hairs when it comes to HVAC load calculations.

There never is an "exact heating and cooling load" and IMO, tenants are more likely to complain if doesn't quite keep up on design day or when they have a party. Also note that people will (we hope) turn the system down or off when they are gone for awhile and extra capacity allows it to catch up quicker.

He's a waterFurnace and Climatemaster guy and the smallest they make are 2 ton units. I think I can make other improvements in the property with the savings from a 1-1/2 ton single stage unit.

Modine and Trane seem to have some nice models. Does anyone have experience with them?

What are the loads? If you are thinking 1.5 ton I wouldn't be suprised if a 15K or a one ton would do.

I have 16868 cooling and 16264 heating. Made the design temps a little more realistic (or extreme) than usual. 15k would probably work in that with the calculator I used there is a shared heated wall and I did not have an input for that. Also the condo upstairs is heated. maybe i need to look at another manual j calculator.

If your calculation failed to account for conditioned space adjacent and above then stop, do not pass Go, do not collect $200...

I use RHVac...in cases of adjacent conditioned spaces one simply does not include the separating wall, ceiling, or floor as an input.
Perform or demand a proper Man J before sizing.

Don't mess with the design outdoor temperatures unless the site's particular microclimate demands it.

RHVac seems nice but I am a GC not a HVACC. Its $500. I redid the calcs using http://www.loadcalc.net and your suggestions and it came out 14023 cooling and 14544 heating.
I built the building and used 4-1/2" polyurethane SIPs. 1000sf condo, 1 wall and the ceiling is conditioned. the slab is r10 on the perimeter and r5 underneath.

But I am afraid. A architect friend, way into modeling, built some houses to passive standards and seemingly undersized his cooling. The residents can't get the house below 80 degrees in the summer.

Maybe 15k is a better bet.

The PH standard is known to be problematic for cooling in a hot / humid environment. A once-in-a-blue-moon 80*F interior is OK in Europe, but daily occurrence here in the States gets old fast.

There's a certain tendency on the part of rabid PH adherents to dismiss reality that takes the form of long hot days and nights. I call that "Passive House Stockholm Syndrome".

I talked with the owner of that house last night. There was a pushby the architect to get passiv certified and they had to keep the unit HVAC unit small to get certified. 18k BTU for 2700 sf. Too small I guess. Too passive aggressive, it seems. But in every other way a great house.

Nothing intrinsically wrong with 18k Btuh / 2700 SF since the load pencils out. We routinely see 1200 - 1600 SF / ton on well designed (and by NO means anywhere near PH standard) detached homes in North Florida.

My criticism of PHers comes only when they try stupid stuff such as a single ductless minisplit wall unit for a two story home, claiming that they can condition outlying rooms (especially those on the 'other' floor, the floor without the mini head) with a weird combination of ERVs and transfer ducts.