I am building an 1850 Sq. Ft. home with a full basement and it's ICF to the roof line.  My HVAC calculations are calling for a 2 ton unit to be installed.  I was wandering if anyone is curretly living in a house approx. this size with a two ton HVAC unit and if it has any problems keeping up with the demand.  Don't want to undersize.  Thanks!

I have an 1800sq.ft. conventional house with 2.5 ton so it is certainly in the ballpark. So much depends on things that you didn't tell us such as where it is and how did you come up with this number. Manual J?

I'm not exactly comparable, but here's my info anyway. I've got a 2000 sf (inside the walls) one story with conditioned crawl space. ICF from footing to roof with spray foam on the underside of the roof deck. The attic space is part of the conditioned envelope. I have a 3 ton Daikin heat pump and had no problem with cooling this past summer in SW Idaho. Summer temps in the upper 90s. Since I don't have power monitoring on the AC system I don't know what the actual output was, but I'm quite sure it was well under the 3 ton max output. The Daikin has a variable speed compressor and fans so it runs only as fast as needed to keep up with the heat gain, or heat loss in heat mode. The Manual J calculation showed I need less than 2 tons for cooling, but about 3 tons for heating. The particular Daikin unit I used only comes in 3 and 4 ton sizes.

Because of solar heat gain and internal heat gains from body heat, cooking, TV and computer running, etc., the heat pump doesn't kick on to provide heat until the outdoor temp drops into the mid to lower 40s. It usually doesn't kick in until late evening after the sun has been down for awhile.

Because it's variable output I don't have an over/under size issue like you can have with a fixed output AC or heat pump. You are better off, in AC mode, to be undersized than oversized. Ideally, at the max design outdoor temp for your area your AC should run 100% of the time. The design temp will be slightly less than the average max temp for your area, so the AC will not cool the house to the design temp when the outdoor temp is above the design temp.

Also be careful about using a larger than necessary blower. A too large of a blower can cause uncomfortable drafts, both in cooling and heating. With my setup I can hardly feel the air exiting the floor registers and the house is uniformly warm or cool, most of the time. Northern exposure, even with ICF, can complicate the heating/cooling system. In my house the two rooms with northern exposure, the master bath and master bedroom, are noticeably cooler in cold weather if the air handler is in low speed mode. This is especially true if the weather is sunny as the south side of the house warms up quite a bit with solar through the windows. The fan kicks into high speed only when the heat pump is actively pumping heat into the house. In high speed the temp is quite uniform throughout the house. The same situation is apparent during cooling season also. The north side is cooler because of solar heating through the walls and roof in the south side and before the house warms up enough to make the AC turn on.

I say you need to take a good hard look at the entire heating/cooling patterns in your house. Internal heat gains from life activities will be a significant contributor of heat. The northern exposure will cause significant heat loss on the north side, while solar heat gains, especially through windows but even through the ICF walls, will warm up the south part of the house. You may want to make provision to move air from one part of the house to another.

Here's a good program... http://www.cement.org/bookstore/profile.asp?itemid=CD044

Be very careful putting a three ton blower on a 2-ton coil, you move too much air and your coil wont get cold enough to remove the moisture from the air. Thats where the Latent heat is and your house could feel sticky because of humidity levels, oversize blower is not a good idea in my opinion.

Having a properly sized, or even SLIGHTLY undersized AC system is better than any oversized system.  Humidity removal should be your priority.  I would take some time to investigate a multiple speed air conditioner with variable speed blower, that will allow the system to move with the load, and provide a certain amount of humidity control.  With tighter construction, humidity will be a bigger enemy then temperature.

Since humidity seems to be an issue do people install dehumidifiers?

I live in SW Idaho, part of the northern clime but quite dry. Interestingly, humidity hasn't been an issue for me at all. I installed an ERV because it has an economizer mode that no other ventilator has. The usual thinking is you need to use an HRV in northern climates, but my experience says not so. I have it on an automatic timer/speed control and have it set to run at low speed for 20 minutes every 2 hours. That's the minimum run time available on the controller. I have humidistats in both bathrooms. The shower in the master bath is totally enclosed so humidity escapes only after I'm through showering. The ventilator will kick on for maybe 5 to 10 minutes. The stat is set at about 55%.

Whenever I happen to look at the humidity reading in the living room weather monitor it is never greater than 40%, except maybe when it's raining outside. I think then it got up into the 50s.

I have a 2200 sf ICF single story house with full basement (conditioned space) in northern Illinois (6500 heating degree days average and hot, humid summers.) I installed a 16,000 - 46,000 BTUH modulating, high-efficiency boiler and a 1-1/2 ton central AC/ system, based on a heating/cooling load analysis. These systems provide more than enough heating/cooling capacity for the coldest/hottest days of the year. Ideally, I should have a 1 ton A/C unit, but couldn't find one that small. To compensate, my AH has a variable speed fan that reduces the air velocity over the cooling coil to maximize dehumidification. As stated above, in a tightly built, well-insulated house, summer time dehumidification is more challenging than air cooling. Do a careful load calc and err on the side of a smaller blower and undersized condenser.