"Given that all these systems are 15 SEER with variable speed air handlers and txv's does it matter much if they are over sized?" -- Yes. Oversizing decreases efficiency (off/on cycles often cost more money as well).

What is the area (in sq. ft.) of the upstairs? They may have oversized it slightly since it is upstairs and heat will rise.

"When the air handler slows down, what exactly happens? Does the liquid freon remain under pressure after the compressor stops [I'm assuming that the compressor is constant speed on/off] and then slowly release into the evaporator coil as needed?"

By slow down, do you mean during an off cycle? After the compressor turns off, the coil will still remain coil for some time. During dehumidification mode, the air passes air slower over the coil, thus increasing latent (humidity removal) capacity.

"Given that all these systems are 15 SEER with variable speed air handlers and txv's does it matter much if they are over sized?"

Oversizing is oversizing; however, the variable-speed blower can increase humidity removal by slowing the blower down during a call for dehumidification.

"In other words, will I still get the dehumidification that I need, and also keep my power bills low if the units are oversized by 1/2 to maybe even a full ton?"

Oversizing SIGNIFICANTLY can run up power bills and hurt the ability to remove humidity. Being 1/2 ton oversized may require you to set your thermostat a bit lower, but I don't think you'll see any major issues. Remember, nominal capacities aren't always actual capacities. If your duct leaks, you lose capacity. Sometimes coil matchups change capacity from what the unit is rated at. (i.e. 22,000 btu's instead of 24,000 btu's).

It's really all guesswork from here without seeing a copy of the load calculation, if one was performed.

All those load factors are just a way of estimating how the system will perform. I can look at the running system today and see that the cycle times are not short. The compressor is running over 1/2 the time, and for 30 minute or longer stretches on a 94 degree day. The fact that it turns off at all tells me that the system has more than enough capacity for a 94 degree day. We seldom hit 98 around here, but have a lot of summer days in the 90's.

We're heating and cooling about 1250 square feet with the 2 ton upstairs unit. My question is will 2 tons suffice for the downstairs unit. The conditions are that it heats/cools the same 1250 square feet, plus another 300 square feet breezeway room. I think it probably can because the 1250 square feet of ceiling is below the conditioned upstairs instead of a hot/cold attic.

The existing system that we're replacing was 2 1/2 tons downstairs for the AC and 2 tons up. The existing 2 1/2 ton unit is 9 years old, but handles the cooling load well.

The house was built by a heating and air conditioning contractor who installed a $30,000 multi-zone micro boiler, high-efficency oil furnace for heat. When heating oil was $1 a gallon, that made sense !!!

Anybody want to buy a used oil furnace?

If the existing 2-1/2 ton unit handles the load well, then why drop to 2 tons? A load calculation is not estimating how the system will perform; it's calculating what size unit is needed to handle the heat load and loss of the home. If the home was built by a HVAC contractor, then I sure hope he sized his equipment correctly. : )

2 tons may work okay for the downstairs. If your cycle times were fine and humidity control was good with the old 2-1/2 ton, then I see no reason to downsize. I believe you stayed with the 2-ton unit upstairs with good results, right?

If it can cycle off on a day in the upper 90s, then that's pretty good. Might even indicate a slight oversize. That's usually around the design point where the system will run constantly.

Good luck. Hope this gives you some sort of direction. I think I'd stick to the 2-1/2 ton if it worked for you before. Plus, 1250 + 1250 + 300 = 2800. I guess you also have a finished basement. Do you condition that basement?

I think you're right. If we're that close, then maybe leave well enough alone, use the original calculations and install a 2 1/2 ton unit downstairs.

2800?

Yes, we have another 800 square feet of office space and another bedroom over the garage. The new 1 1/2 ton AC doesn't seem to be handling it very well. I think it's my fault -- I added an extra wall to create the bedroom and the return is inside the bedroom, but 3/4's of the conditioned space is outside the bedroom. That's a lot of air to cram under the door!!

So I'll have to move that return outside the bedroom, or add a second return outside the bedroom.