"Infinity 20" probably 8yrs old now. Recently started making a humming noise whilst in use. The noise continues 15secs or so after hot water tap has been turned off
Sounds like the diaphram in the gas control valve which is not replacable. You would have to replace the gas control valve and I would also suggest replacing the thermocouple. However since the heater is 8 years old you may think about replacing the entire hot water heater. Depending on how much water has gone through it in these past 8 years it will probably rust out soon and then you will be dealing with a flood.
Answers & Comments
Sounds like the diaphram in the gas control valve which is not replacable. You would have to replace the gas control valve and I would also suggest replacing the thermocouple. However since the heater is 8 years old you may think about replacing the entire hot water heater. Depending on how much water has gone through it in these past 8 years it will probably rust out soon and then you will be dealing with a flood.
Good luck,
Marty