I don't mind mucking around with it's guts, but I can't figure out what's wrong. Aquastar 240FX.
It's turning on, but not getting very warm. Or getting a little bit warm and then cooling off. The shower gets warm enough, but on full hot, it fluctuates a few degrees - almost like someone else it turning on something. The bathtub can get warm, but it seems to be tough to get a flow rate that it likes, and usually the water is too cool. It seems like it is slowly getting worse.
Since it started being inconsistent, I checked the flow & filter, and we attempted descaling. The descaling solution turned a bit blue/green, but there wasn't a lot of debris, so I think that's OK. The flow sensor also seemed to be in good condition, but I cleaned per their instructions anyway. The input filter had a couple little flakes of something, but those came off easily. After doing all that, it seemed a little better - for a day or two.
There are several replaceable parts, but it is had to tell what might help. I could replace the flow sensor, but it seemed "OK" (not sure if there's a way to test it). I could replace the thermistor, but if that's not the problem that's $70 down the drain. Or it could be something in the gas - I'm not adverse to cleaning or swapping parts, but not sure what's possible.