Complete 2 heating maintenance pm check in Suwanee. Change filter, check safety and operations. Find upstairs air conditioner unit was not operating due to breaker was set on off, turn on breaker and system operating. Find 6 year old carrier furnace operating but oversized for house, missing return box. Find downstairs heating furnace operating with rusted heat exchanger, worn motor, gas valve and high CO level. Due to system age and poor conditions, suggest to upgrade system. Customer need consultation visit