Hi -- I built this house in 1978 ---started with a Trane 5-ton air-source heat pump. After going thru 3 compressors, in 1988 I installed a 2,000 foot ground loop ( 4, 500" loops connected outside), using 1" Poly Butyl pipe, with a small Bus Brown Trencher I helped design. The ground loop uses 2 Grundfos circulator pumps in series. I installed a Command-Aire 511 4-ton split system heat pump that lasted until 1998. That total installation cost was $3,600. In 1998, the compressor failed. I replaced that system with a Command-Aire GSSD-48 4-ton, with a Copeland Scroll Compressor, P/N ZR42K3-PFV-230 (3.5 ton). It has been faithfully pumping away for some 21 years, with my only maintenance being a new module for the programmable blower motor twice at a cost of around $200, which includes a spare (they are excessively pricey now), and a new motor contactor. So as a retired Mechanical Engineer with a limited amount of days left, I need to make a decision as to what to do with the existing unit before it makes the decision for me. This summer, it has not been dropping the A-coil temp enough to take the moisture out of the air, which could be due to a lack of R-22, or the scroll is wearing out. I am considering replacing the compressor with a new one, which is available for $772 delivered. But should I also replace the TXV valves and the switch-over valve? What about the heat exchanger? If it has been good for 21 years, will it be OK for another 2o? I plan on putting new cartridges in the circulator pumps, since they have been going for 31 years! Your advice would be appreciated. Thank you.