Arrived on site to perform FAU maintenance on 16 year old Lennox furnace. Cycled system for heating. Verified proper heating sequence of operations. Temperature rise across heat exchanger is within range. Static pressure on unit is high, unit does not have any supply plenum and is operating on only 1 return. Indoor blower wheel is dirty. Indoor blower motor amp draws are in range, and secondary motor control tests good. Inducer motor amp draws are getting closer to allowed maximum range. Wiped down blower and heating compartment. Tested and verified heating components are working accordingly. Pulled off evap coil compartment to access and inspect heat exchanger. Heat exchanger has mild hot spots throughout chambers. Evaporator coil is dirty. System gas line does not have a sediment trap Before going Into unit. Unit also does not have any condensation safety switches for safety against condensation leaks in the AC mode. Homeowner just replaced his filter. Went over all findings and reco