AquaStar 240FX temperature surfing up and down

[diyhome]

have an old Aquastar 240FX tankless water heater that has developed the annoying habit of "temperature surfing". Once the running water is hot and you're taking a shower, the water will get cold for a while, then heat up again. This cycle continues. The time between hot to cold to hot again is maybe around 3 or 4 minutes.

Does anyone know what the problem might be?

It might be relevant to add that AFAIK the unit has *never* been descaled. When the unit came in to my possession, it was at least 10 years old, and had not been regularly serviced. A repair guy who fixed the unit after the igniter failed, recommended that I not start descaling the unit. He said that if it had not been regularly descaled in the past, that descaling might cause it to leak. Better to just leave it. The water (Los Angeles) isn't super hard but I do find limescale when I flush my coffee machine boiler.

Thanks!

 

[diyhome]

Also, sorry for the double post. My first post disappeared so I reposted this one. Then both showed up... Thanks!

 

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[Fitter30]

This style of heater has small water passages. When mineral deposits collect on heat exchanger tube surfaces it creates hot spots in the exchanger. These hot stops will warp the heat exchange and will creates leaks in them and their done. Sensors are coated gpm, water flow sensor and any other sensors won't read correctly. If you were going to try descaling not a company might try it but it could make leaks 20% chance it might work. If it doesn't work you will be without hot water. My recommendation replace it.

 

[diyhome]

Thanks @Fitter30 .