Ground Source Heat Pump for backup heat

After building the solar thermal system, it would make sense to get rid of the forced air system all together. This will require a backup system that will heat the water tank in periods of low insolation. The most efficient system is a ground source heat pump for radiant systems, aka water-to-water heat pump. A 3-ton unit can be found on ebay for $3000. Problems have being reported after years of use of GSHP, due to the ground temperature changing over the years. To keep the efficiency of the GSHP high throughout the years, it is possible to use solar energy to recharge the ground loop. In my system, a secondary solar collector of about 120sqft would be used to heat the ground loop during the winter in periods of high insolation. The size of the solar collector is determined by the building that will hold it (a 14FT*14FT shed on the field that will hold the ground loop). To reduce cost, the ground loop will be buried at only 3FT, because we can rent locally 3FT Ditch-Witch units. Such a shallow loop cannot store heat seasonally, so the solar recharge must be used within days. The solar collector needs not be high temperature, so a pex collector, or even a pool solar heater, will suffice. I will use a pex collector build to fit the shed size. Because the GSHP is a backup, ground temperatures are unlikely to significantly change. It is not clear if the solar recharge will provide significant benefits. in the coming weeks, I will research on the backup system, solar assisted GSHP, and COP with solar recharge.