Furnace maintenance. Cleaned and serviced furnace. Tested inducer motor, pressure switches, flame sensor, ignitor, gas valve voltage, blower motor amperage and capacitor. Checked thermostat and heat exchanger. Blew out drain line. Replaced furnace filter that customer provided. Inspected hot water heater and found the exhaust vent is a little flat. Flue is not back drafting at this time. Sent estimate on replacement of water heaters.