Can I use this product with a Modine geothermal unit?

Hello LoopyChicken, No. The REHP80 heat pump water heater cannot be used to heat or supply energy to a geothermal system. It is a self contained unit designed solely to heat domestic hot water using ambient air. Modine geothermal units are used for space heating and cooling and operate on a separate ground or water loop. The two systems cannot be directly connected or integrated, though they can operate independently in the same building.