And another possibility is the old cooling unit is clogging up inhibiting proper vapor/liquid---heat transfer.
Off level operation over time---accumulative---will cause crystals to form (like plague) and build up blocking the flow.
Improper cooling on GAS.....gas pressure, dirty burner/orifice, dirty flue, out of position flue baffle
Improper cooling on ELECTRIC......low AC voltage, heating element bad
Improper cooling on BOTH....blocked ventilation, faulty thermistor, off level or clogged cooling unit
You will only smell ammonia and/or see yellowish residue when tube has cracked from overheating (inadequate flow of vapor/liquid cooling fluid)