I am going to have an ICF home built in south central KS. Radiant floors will be in the basement as well as the main floor. I have been told that a geo-thermal unit would take forever to break even because of the ICF shell. So, what I was thinking, because of the house being super insulated with the radiant floors, can you have an air conditioner that is bigger than your heater? I feel that with the radiant floors I won't need a big heater. Is this a dumb question? if so, go ahead and laugh!!!

The heater and AC should both be sized for the actual load of the house. The calculations to figure what is correct is Manual J. Your HVAC company should use Manual J to size the furnace and AC properly for your house. When it comes to AC, bigger is not better. If Kansas gets as humid as where I live, a bigger than required AC will not remove the humidy from your house. The AC would run a very short period of time to cool the house, but not long enough to get rid of the humidy.

It's not a dumb question. There are still some HVAC contractors that size by rule's of thumb. The rules might give a close answer for a stick built house. However, the rule of thumb would produce much too big of an AC unit for an ICF house. MAKE SURE YOUR CONTRACTOR USES MANUAL J. Sorry for yelling, but it is a point that will really make you enjoy your ICF house much better in the summer. Also, it will save you money by buying the smaller right sized unit.

Manual J, got it!!!! thanks!!!

Remember the Manual J is based on your house design, i.e., type of walls/roof, # and type of windows/doors, type of insulation, orientation, location, design temps and other inputs.

Based on final plans, our Man J coincidently came out almost the same for heating and cooling, around 38,000 Bth each for just under 3,100SF. If we were to reduce the SF, decrease # of windows, adjust insulation or something else, we might need to redo the Man J to see the effect of these changes on the heating and cooling loads and subsequently sizing equipment.

Also, I would urge caution in assuming an ICF house is super insulated. Certainly it is well insulated and air infiltration is reduced (assuming the windows and doors are well sealed) which contributes to comfort, but you might check out Super insulated or Passiv or Zero energy homes to compare with.

Kind of an odd note was reading your question, I have recently built an ICF home as well, and yes, put in what its designed to use, ie the load....however sometimes your local code may require a certain size based on sq feet and you may have to do some convincing of the local inspector to allow a smaller unit.....

But one note, ICF homes are soo tight, its ridiculous, the vacuum effect in the home can be very strong..... One thing I found, at least in my home, you want to really want to instualte and seal up everything really tight.

basically, while your home is really tight, it draws more air at the few cracks in the house, ie the doors, windows and roof. IE the cold / hot spots around those areas seem to be amplified. Imagine a pressurized airplane cabin with a little hole in it

SO a suggestion, pay a little bit of extra attention to the quality and insulation of the doors and windows.... for me, some patiently and carefully applied weather stripping worked wonders, and I'm in the process of adding more insulation to the roof / attic

Overall, the ICF concept seems to work very well, I went form a 1200sq foot home to a 2200 sq foot home and my utility bills are basically the same, and considering I went form nat gas int he 1200 sq ft home to propane is pretty impressive