Heater- repair or replace?

I have been getting rust spots on the bottom of my pool under the inlets. I suspect that it is coming from the heating tubes (it is the only steel that I can find in the system). I get a loud knocking sound from the heater when the system is on. I replaced the ESC temperature control board last year. What do I need to order to replace the heating tubes? Or due to the age of the system should I look into replacing the entire heater?
thanks for your help.
 
Time to replace it. The cast iron header is rusting internally and at that point, get rid of it. Yeah sure some DIY person will fiddle around and try to replace the header but if the bolts break in getting it off you're done for. Plus you can upgrade to something new which will be a bit more efficient and have a digital readout and many more pluses. It stinks but it's time.
 
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