I will start with background because I believe our situation is unusual: I am sorry, but this is a rather detailed and long post. [B]The essential question is in bold below./B]

We re-purposed a 5700 sq foot solid brick warehouse with 14 ft ceilings into our home. 2500 sq feet is living space, the rest is "garage/hobby" space and is not heated or cooled.

We have a geothermal system with radiant concrete floors in the 2500 sq ft "house" portion. The water chiller cooling and backup heat is remotely located, away from the pump itself in a large soffit like attic space above the bathroom. This is a conventional gas furnace air handler system, not the usual heat strip. The house has two total zones, one for the air and one for the floor. The air zone can operate both AC and heat. The system has a storage tank for the water and the zones are solenoid controlled with a single circulation pump.

Existing Heating:
The floor is the primary heater (water to water ground source), but the air handler will kick on and use the water based heat exchanger to heat the air if there is a temp drop that the floor can't catch up to immediately. (perhaps a 50 F drop overnight that levels off in the 20s F) The gas system is supposed to kick on in severe weather ( perhaps a 50 degree temp drop overnight with sustained below 0 temps) and it has, once. We also use gas manually in the first month of fall when having the thermal mass of the floor warmed up doesn't make sense yet. We could use the heat pump and water coil, but I like to experiment and see cost differences in gas vs the heat pump. Short bursts of gas are cheaper than longer bursts of electric heat pump.

Existing Cooling:
Cooling is exclusively done using the air handler and water chiller coil. It was able to keep up even in the worst heat wave in St Louis history without a hiccup.

Here is the future expansion plan:
One 432 sq ft. corner section of the "house" was never finished and does not have duct work. It is a peninsula in the structure in that it has two exterior walls and one wall that connects to the unheated garage space. One wall connects to the rest of the house and is, of course, heated. We will be turning this into our master suite. In this section the floor has its own loops that are isolated from the rest of the structure because the floor was poured at a different time, so zoning that will be simple. (there is room in the zone controller)

I am trying to figure out some options to cover both supplemental air heat (backup and short term needs as described above) and cooling using the existing water to water heat pump. ( It was sized with this expansion in mind, so the capacity is there.) I plan to create a new zone for this section because it has very different heat losses compared to the rest of the house. The floor has a much higher ratio of exterior exposure than the rest of the house and for cooling has a much worse exposure profile as well.

I considered extending the existing duct work and heat exchanger, but that would require a ridiculous number of electric dampers to properly create a zone, plus two fire dampers as the duct work would have to enter and exit a garage. The duct work is exposed (no attic)and fat chance of messing with the look of it anyway, says the wife.

I thought of adding a second remote system identical to the one I have with smaller exchanger and tiny gas furnace, but I dont think there are 5000 BTU air handled furnaces.

Chiller coils are available in many sizes for all sorts of industrial uses, so finding that isnt an issue. What I am trying to figure out is the gas backup system to use in the space. It need not be integrated with the chiller coil but it would be better to have just one blower and both systems connected to it since the chiller coil will need to do double duty. Separating them would very much complicate wiring and plumbing as well.

Space and working conditions:
There is no attic or basement. Part of the bathroom will have a substantial soffit over a potion of it that can accommodate a standard small house furnace/blower, etc., but shouldn't need anything that large. The garage space next door shares a wall. the garage ceiling is 14 feet high, so suspending a system and using fire dampers for 2 registers and a return is also an option. My cold air returns are actually on the ceiling because 90% of the time they are used in the summer and I plan to do this here as well. The heat pump is in that garage space so getting heat pump water and controlling cables is a snap.

I want to avoid a heat strip, electric heat is terribly expensive and I we really don't have room in our 200 Amp electrical panel, which is why we have a gas backup for the heat pump in the first place.

So for any of you that made it though that mess (hopefully a complete description), do you have any ideas or suggestions? The guy that installed my system originally was only supposed to be the well driller, but ended up doing the rest of it (long story). He was not that knowledgeable about the zone system and the gas backup, ( someone else did the control system) so asking him is not really an ideal approach.