Heater problems

My spa heater is no longer working - my symptom description is not going to be very technical unfortunately.

After lighting up it runs for about 30-45 seconds, and then it starts to make what I can only describe as a moaning/groaning noise.
This picks up in intensity until it begins to make a harsh clunking sound which happens for about 6 seconds and then the gas shuts off - the blower continues to run to clear the heater out, but then it's done and does not try to cycle back on.

The heater is old...I bought it used for approx. $800 more than 7 or 8 years ago. So I've definitely gotten my money's worth out of it.
But replacements are pricey.

Would like some opinions on whether it is worth trying to have it serviced.

Thank you!
 
Clunking is usually a sign of water leaking into the burner area and boiling. You likely have a corroded heat exchanger. If you are handy you can open it up and take a look.

Heater more than 10 years old i would plan on replacing.
 
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