I'm in the early planning stages of a timber frame hybrid, and I'm considering using a combination of AirFloor heating linked to a geothermal heat pump. I like the prospects for low heating cost, environmental friendliness, and this would eliminate the need for ductwork in the frame. Has anyone seen something like this done before? Any special considerations I should be taking into account?

I checked out the airfloor web site. It still appears that you will need ductwork to feed the floor air inlet?
Have you seen one of these systems in action??

Thanks, Drew. No, I haven't seen one of these in action yet, but an architectural firm near me uses these, so I should be able to see one soon. It does require a feed duct, but this comes up into the center of the floor from the basement and ends there, so no ductwork is necessary in the walls or ceiling.

We have been installing "air slab" systems since 1985. The design is our own and based on data from the Agricultural Technical Institute study of swine breeding facilities design. It uses ABS corrugated culvert and drainage pipe for the in slab ducting. Normally we integrate it with hydronic in the same slab. This allows the system to act as a back-up heat source. So far they work well and eliminate the need for AC.

The intake air usually is collected at a high point like a plenum in the chimney. The return air usually is ducted to floor level of the main living floor. On a slab on grade, they are one in the same.

Interesting! How do you interconnect the ducts (what pattern, spacing)? Do you run the hydronic system over the ducting?

The simplest system would be a rectangular basement. You run two 10 or 12" plenums parrallel to the footer in the long direction, one on each side of the floor. Turn up you supply riser on the end of one plenum, and your return on the opposite end of the other. That way the flow is balanced through the system. Then you run 3 or 4" diameter tubes from plenum to plenum on 12 to 16" centers. Drill holes in the plenums with a hole saw and use standard flex pipe couplings to snap into the holes. I like to foam around the fittings to seal them.

I try to lay out the system so the pipes and plenums are flush on top, then backfill with sand to wthin 2" of the tops of the tubes. Next you roll out #10 wire fabric over top of the tubes. Tie your PEX directly to the wire fabric. When you pour your floor, the flex pipe ends up embedded about half way in the slab.

The amount of insulation you put under the slab is a little controversial. I recommend 2" EPX witnin 48" of the edges, and 1" everywhere else. This allows the floor to act as a cooling unit in the summer, without allowing significant heat loss in the winter. Don't forget to insulate the tops of the footers and edge of the slab.