Originally Posted By: Greg Owens
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the way I understand it is you must have a heating and air license or work under someone who does in order to disassemble any part of a furnace. you should not claim there is a crack if you can not see it. how much rust would be excessive in a heat exchanger I have seen a furnace that had rust deposits covering most of the burners and having the service guy (who would get a commission if he could sell a new furnace) find the heat exchanger to be solid. soot around the grills? could have been a furnace that was already replaced, bad filter maintenance causing dust to hit the heat exchanger and burn, a dirty or worn out electronic filter. flames jumping are a sign but they also could be from inadequate combustion air supply, or leaky duct work.
NACHI SOP 2.4 IIA
The inspector is not required to Inspect or evaluate interiors of flues or chimneys, fire chambers, the heat exchanger, the humidifier or dehumidifier, the electronic air filter, solar heating systems or fuel tanks
if it is older or looks neglected recommend they have it checked by a HVAC company.