We have a water to water 4 ton unit that heats our indoor pool.  I then take water out of the pool with a circulation pump, run it through pex and heat our finished cement basement floor along with about 1/2 of the main floor that has tile.

With no additional heating (only heating the pool) the geothermal runs about 5 hours a day.  Right now the heat pump is running about 10 hours a day and I have 14 hours a day of off peak (cheap electricity rates) as well as all weekend.  Basically this is heating our house to about 15-20F (depending on wind) without the natural gas furnace kicking in.

So I am thinking of ways to get more heat out of the pool and we came up with three ideas.

1. Our HVAC contractor suggested just adding a second "AC" coil in the furnace and plumb hot side that comes out of the geothermal before it hits the pool heat exchanger.  He said if the furnace fan was off almost all the heat would go to the pool and if it were on it would pull some of that heat off depending on blower speed.  We could set the T-stat to turn on the blower on with the furnace and use the stage two set point to kick on the natural gas, if needed.

2. Add another coil similar to above but instead circulate pool water through it, our HVAC guy didn't like this idea because the pool is only 90F and he said the amount of heat would be minimal and didn't like the idea because of the coil failed it could potentially pump 16k gallons in to the furnace (he said smiling)  Also the control would be more or less manual, potentially controlling the circulation pump off the central t-stat.

3.  Add a radiator type setup not really connected to the furnace, again circulating pool water through it with a simple fan and thermostat.  This is my favorite option and until I just read the other thread about the “MultiAqua” fan I couldn't think of an easy way to do this.  Would something like that work?  Again I am assuming the BTU output would be quite a bit lower since the water temp is lower, but heck if I can get another 10 tons of heat a day out of the pool I could avoid running the natural gas furnace.  Initially I had thought of using an old car radiator, but placement and control and “looks” would be an issue, again the MultiAuqa might solve the control and looks part.

4. Leave it alone and let the natural gas act as my "heating strips".  This is what my wife wants to do.  ;)

Thoughts?

I think I like your HVAC guy.

I like 1) Provides warmest source of heat w/o turning mechanical room into an annex to your pool. I assume he'd install it such that air hits geo coil before being heated by gas.

If you run continuous fan during cold you'd get much more geo heat (Assuming I understand your setup) Geo coil in duct would steal the heat before it got to the pool, so pool would call more often, perhaps continuously. Stop continuous fan in milder weather and most of the heat theft would occur only when air is moving through coil (during calls for gas heat)

How would this change affect summertime operation?

We currently run the fan on low, (45w) all the time off peak, it helps even out the heat with the rooms on the south and north side if the house. So the extra coil would just add some warm air all the time and once it got warmer I could just bypass the geo coil. As you said if the pool needed more heat I could shut the fan off or bypass that coil.
 
I am not sure how to do this automatically (without getting really complex), and honestly that is one of the reasons I am leaning more towards a standalone system. If we ever moved I could just remove the stand alone system and be done, having to manually move valves depending on the time of year (things I think are fun to do) might not be appealing to buyers.

As it is in summer initially I run the field and the furnace coil in parallel and as more house cooling is called for the field is basically shut off with all the cold being dumped to the house. Sometimes when the compressor is off I circulated the field through the furnace to get some limited cooling. This is all done with manual switching.