Performed furnace maintenance on 19 year old Goodman unit. System is heating only. Cycled system for heat. Verified proper heating sequence of operations. Temperature rise across heat exchanger is high, and is basically at max allowed range. Static pressure on system in fan only mode is high as well. Ductwork is not set up correctly, and is all very undersized which is causing a lot of pressure and heat retention inside the unit. Blower wheel is in good shape, as well as blower motor. Indoor secondary motor control within range. Tested and verified heating components are workinG accordingly. Wiped down heating compartment as well as blower wheel compartment. Pulled blower wheel out to inspect heat exchanger. Heat exchanger has hot spots throughout chambers. Informed homeowner that due unit location there is a 2x4 in front of heating compartment that would not allow us to pull heat exchanger out to thoroughly and properly inspect it. Went over findings and recommendations with homeowner