My Aquastar 170 natural gas tankless has been working faithfully since about 1985. My handyman recently descaled it, and since then the water output has been way too hot. It's been several years since the temperature control knob had any effect, but I've always managed to add enough cold water previously to get the desired water temperature at the faucet, but now if too much cold is added, the burners shut off. (I should mention that the temp sensor was bypassed years ago because the heater wouldn't work at all otherwise.) No doubt the themostat is shot, but of course the most essential parts haven't been available for years now. Does anyone know a workaround for a bad thermostat?