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Tankless water heater inconsistent water temperature

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.


Re: Tankless water heater inconsistent water temperature

Unfortunately the tankless water heaters dont get hot enough compared to the tank. My brother is an HVAC guy and says that if that happens in what your experiencing is unfortunately normal.

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