Arrived on site to perform Fau maint on 30 year old Payne furnace. Cycled system for heat. Verified proper heating sequence of operations. Tested and verified heating components. Indoor blower motor is in good condition, appears to have been replaced at some point. Blower wheel is dirty and rusted out. Temperature rise across heat exchanger is above rated system limit. Temperature rise is rated for 30-60 degrees, system rise currently at 65 degrees. Pulled off burner cover to inspect burners and found that all dimples are separated on heat exchanger chambers. Went over this information with homeowner and advised what the possible concerns are, and informed that water testing the heat exchanger was the only accurate way to ensure that it is not compromised. With homeowner approval, went ahead and water tested heat exchanger. Heat exchanger failed water test, and has a crack in the middle chamber where water is leaking through. Verified findingS with field supervisor. Heat exchanger is c