Performed maintenance today on furnace. Change customer supplied air filter 20 x 25 x 1” gas valve was set extremely high and furnace is too large for ductwork and system of house. Adjusted gas valve to lowest possible setting and it is not cycling off on high limit anymore however, it is still outputting far too much for current set up. Flexible line running inside furnace that is not safe or to code. Single wall flu should be upgraded to B vent and also enlarged to 4 inches exiting the furnace. Thermostat is old mercury style and recommend upgrading for accurate temperature readouts and energy saving features. Hard pipe flex line inside furnace