Went to a friend's mothers home. She was told by the co.. that has been doing her maintenance for 8-10 years her older systems need replaced. They are quite old, 1979 GE A/H with a 1990 York ODU, 3.5t and a 1986 York Champion1.5t system, (going by style and color for age). The only sign of maintenance were the filters and new valve caps. Read the ticket and said they were serviced and all in working order. She has a sales call schedule today by the servicing co.