Cantherm 520's can't keep up with the heat - in hot water
I've got two ground loop geothermal Cantherm 520's in Carlisle, MA. They provider heating, cooling and domestic hot water. I love that I don't have a combustion heating system, or noisy compressors outside. The Catherms are about 25 years old. I'm pretty sure I need to replace them, but with what? I'd prefer not to fall back to fossil fuel.
The units can't keep up with cooling when it gets above about 80 deg F. They super-heat the hot water tank, which then trips the pressure valve and blows about 35 gallons of scalding hot water onto the basement floor. I'm routinely having to reset hi-pressure on the upstairs unit, but strangely not on the downstairs unit. The downstairs unit is forced air, while the upstairs unit runs liquid to blowers on second and third floors. Lots and lots of windows in this house.
What would I replace these units with?
Can the grounds loops be re-used?
What are my options?
All suggestions welcome.