[jeff5may]

I agree with Steve on improving the envelope of the home first. If you haven't done a blower door test yet, you REALLY need to! It is much more economical to seal up a leaky home that leaks air everywhere than to insulate without sealing leaks first. Yes, insulating is important as well, but if the walls and ceiling leak badly, the insulation loses its effect whenever the wind blows. This air barrier may or may not count as a vapor barrier, which is also important in the insulation project. Airflow carries heat with it, and heat not lost counts as energy you don't need to provide.

Downstream in the aquifer may or may not be downhill on the ground. Check with a geologist or water utility in your area, they will definitely know what exists underground. It may or may not be against the law to dump water back into the natural source: again, the pro's in your area will know the details. Also remember that cold water is heavier than warm water, so if you pump cold water back in, try to inject it deeper than your source well.

The effect of recirculating cold water through your well may not show up immediately. Usually it takes a few seasons of use to happen. What happens is after a few years of use, your source water temperature may change (for the worse) a few degrees. This trend will slowly continue with use until the ground can make up the difference. If you are heating and cooling throughout the year, these modes cancel each other out to the extent of duty cycle. For example: if you extract 2000 tons of heat in the heating season, but only add back 1500 tons during cooling season, the net loss of 500 tons will have an easy time being made up by the Earth. If you only heat with the system, there will be 4 times more heat to extract from underground.

Making your own evaporator is not all that difficult physically. The main difficulty in building is making everything not leak. The more difficult aspect lies in the design of the heat exchanger. Making absolutely sure the materials and configuration will handle your loading requirements (in a worst-case situation) on paper before you build anything is super important! Some fail-safe elements must be included in the control system as well. In your application, when something goes wrong, the heat exchanger will try to freeze the water flowing inside. This event is usually catastrophic, so failure mode analysis and control is absolutely necessary. Making sure your design can handle whatever the world throws at it before it is built can mean the difference between a unit that tries to flood your basement (while you aren't there to stop it) and one that does what it should for years.