cleaning an indirect water heater coil

Hi I have a customer with an 80 gallon indirect water heater.. with a new correctly sized pump it yields 65 gallons continuous hot water. its supposed to be around 300 according to manufacturer. I called manufacturer .. they said tank is 22 years old and the coil is properly full of sediment. they gave methods to clean. Iwould like to know, how many out there have experience doing this successfully. and with the tank being 22 years old how many think its worth homeowner to pay for 3 to 4 hours of flushing tank and cleaning coil

thx

Joseph Hardoon