Technician arrived to perform a yearly friends and family club furnace maintenance on three furnaces and change a thermistor on one of the systems. Technician adjusted gas pressure, Checked amps, checked temperature differential, check static pressure, clean flame sensors, check filters and insured proper operation. Technician noticed dirty ductwork and no surge protection so estimates were made for the customer on getting the systems cleaned and upgraded. Estimates have been emailed to the customer for further review. All systems are heating properly at this time.