Isotemp Spa water heater lifespan

[Ian M]

I installed an Isotemp Spa water heater about 10-12 years ago. This season it would not remain full of water, but partially filled with air, yet I could not find any trace of a water leak, even using paper towels, and the water pressure pump would cycle every 15 mins, so I kept it off until I needed water. Yesterday I decommissioned it, disconnecting it from the freshwater circuit. When I drained it there was very little water. With it disconnected from the freshwater circuit, the water pressure remained constant, indicating the Spa as the cause of my season's water woes.
Does anyone know the estimated service life of an Isotemp Spa, and is this consistent with the normal equipment failure and replacement cycle?
Every winter I drain, disconnect it and leave it empty. May some components have rusted through?

[rslifkin]

I can't think of any way it would have drained on its own with the water system pressurized, but not had an external leak (including through the pressure relief valve).

When you re-commission the system in the spring, run plenty of water through the water heater to make sure you get all of the air out. Then check for leaks and see how it behaves.

Have you had any signs of air in the cold water side of the system?

[sailormed]

I do not know if this is relevant to your issue but this is what happened when my waterheater started leaking.
There was a pinhole leak in the engine coolant loop in the water heater. When connecting the boat to wall water the engine coolant overflow reservoir would fill up and overflow filling up the engine bilge because of the backflow from the wall water to the engine coolant system via the pinleak.
The waterheater was installed in 2004, leak started in 2021.

[Ian M]

Thank you for your replies. Very helpful.
At the beginning of the season, after the issue arose, I ran a hot water faucet to purge air from the lines. As the season is relatively short, I did not want to pull the plumbing apart, as getting access to the tank connections is quite involved, and I decided to leave it for offseason maintenance.
Later, I found a more efficient solution. Before starting the engine I purged the air from the hot water tank. After the boat was idle for a week, there was a lot of air.
The only place I could find a trace of a leak was at a connection to the hot water temp control valve - line connecting the Spa's cold water to the valve, and it was a just a very light dampness. To test the leak I wrapped paper towels around the supply line. They did not become noticeably wet.
Perhaps this is the cause, or there is another pin hole so small and the leak so slight that the water evaporated and did not collect into a noticeable pool.
From other posts I read advice about troubleshooting leaks in the pressurized water system, on the suction side and the pressurized side.
There is no change in the level of the engine antifreeze, indicating the engine cooling lines in the heater are sound.
At this point I am inclined to replace the 40G tank with a new one, as sooner or later the +10-year old current tank will fail. Before connecting it, I will monitor the pressure in the cold water lines to ensure there are no other leaks.
The boat has a Groco PJR-A with a pressure storage tank and pressure gauge, making it easy to see if the pressure remains constant. I have carefully studied the Paragon Trouble Shooting Guide.
Thanks for your advice.