Upon arrival customer stated that heat had stopped working and was not coming on. After inspection of 8 year old furnace found that system had a very clogged (20x25x1) filter that made the high limit switch activate and shut system off. Replaced customer supplied furnace and everything started up and began to heat. Checked over all operations of system, blower wheel is clean, blower motor capacitor tested good with no issues, inside system cabinet was clean. Amps on inducer draft motor and blower motor tested good with no issues. There is no drain pan on system or flood protection on equipment. Cleaned flame sensor and made sure pressure switch tube port was clean. The system is undersupplied on return duct work. Duct system is old. Recommended gas line repair, adding sediment trap, adding wet pan and in line flood protection, replacing condensation pump, and speaking with install supervisor about duct modifications, customer declined recommendations at this time. Gave customer estima