Geothermal is more plumbing than anything else if installed with gauges and valves... troubleshooting is pretty straight forward. Its pumps and valves, plumping. I would call a refrigeration guy for the geo units before I called any HVAC guy..... or make sure the HVAC guy does refrigeration, its not a furnace. They are actually pretty straight forward. If you are not in a colder climate you ROI is harder to get to compared to air source. Ours makes all our potable hot water(in winter) So roi is much faster.
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Using a standard gas furnace as our air hander allows us to switch to 4 tons of hot water to the potable hot water indirect water heater, if the 80 gals of 115 degree water doesn't last. It then switches back to the water to air coil furnace shuts off. The way we currently have it we can't keep up in the summer. There is a timer when the unit switches between hot and chilled water. That lag time is a killer. We use indirect in the summer, we have less hot water demand anyway as we are not washing cars and trucks as much.
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its going on 28 years, however with some changes and improvements we can get it to make all our potable hot water and be even more efficient in the summer. I just can't figure out why I didn't think of it earlier.