His attitude about sizing doesn't seem terribly professional, does it? Then again, it's possible he did not bother to analyze and size the system, b/c the previous owner just directed him to "replace what's there."
This attitude is very typical. They'll just use a rule of thumb, maybe based on little more than the SF of the floor. Typically, the result is units that are too big for the requirement (additional capacity doesn't cost them very much--a 3 ton unit and a 4 ton unit may only be a few hundred dollars different), and reduces the chances of a callback. Oversize units keep the spaces cool enough, but don't remove sufficient moisture--it's easier to tell the client that all is well if the house is just constantly clammy, you can't tell them that things are fine if the temperature won't get down to the set point.
Oh, my. That heat load calculator asks for a lot of info that we can't lay our hands on. Especially insulation - except for the attic, which has about 16-18" of loose insulation, there's no telling what's in the walls, floors, ceilings. Even the garage under the Frog is drywalled.
And knowing the insulation level is important--you can be sure the guys who gave you the estimate never checked it.
I'm glad the garage ceiling has drywall, that's now required between a garage and habitable areas (it gives you time to get out in case of a garage fire).
If you want to go to the trouble, you can often tell about the insulation by looking beside the electrical outlet boxes. Turn off the power, remove the cover, and (sometimes) there will be a gap between the drywall and the box. You may see fiberglass there. You can also insert a bit of coathangar wire bent into a hook on the end into that gap and try to snag some insulation, to see what kind there is (again-- circuit breaker OFF). Use the same wire to see how deep the walls cavities are. If the cavities are 3.5" deep (2x4 walls) and the insulation is fiberglass, then it is an R13 wall (it >might< have additional insulation as sheathing). Between the garage and the FROG, you may be able to get a peek into the cavity to tell if it is insulated by removing a light fixture in the garage, or by checking around any receptacles in the ceiling (usually used for garage door openers).
Hopefully, they did insulate the garage ceiling.
If you've got a hip roof over the FROG, I'm guessing you have knee walls (about 42" high) all around the room, likely with a small door you can open to use the area behind for storage. If so, you can tell everything you need to know about their heat loss by going into that back space and seeing how the knee walls are insulated.
If all of this is more trouble than you want to go to, you could ask the next person who comes out to do an actual heat load ("Manual J") calculation for that room--for heating and for cooling. If they charge you a couple of hundred dollars
and they actually investigate all the factors (insulation, shading by trees, window types, etc) it will be probably be well worth the money. IMO, a reputable HVAC company would do it for free as part of the bid--how else can they know what type of equipment you need? They can't.
Not only the least expense upfront but also when repair is needed you can buy another for the price of a service call. The smallest units should do the job on such a small space and cost around $200 for a better brand.
