Originally Posted by
Sleepydoc
I think you’re reading the graph wrong - in second graph I posted, the orange is the heater power and the black/grey graph that you see stepping up is the water temperature. On the first two cycles, all 3 heaters come on immediately and the power goes up to ~380 watts, then right before they shut off you see the orange graph step down to 280 watts. In the 3rd cycle, you see a notch where the heater shuts off, then comes back on for a bit before they all get shut off by the Apex.
Historically, the thermostatic switches have been the primary failure point for heaters. They are typically a relatively cheap bi-metal switch that flexes completes a circuit to switch the heater on or off. With each cycle, a tiny bit of arcing occurs and eventually can cause the two contacts to fuse together in the ‘on’ position, meaning the heater never turns off and can over heat the tank.
The relay switch in a controller is typically better quality and less likely to fail, so I set the heater thermostats a few degrees above the set point for the controller. That way they are always ‘on,’ regardless of whether the Apex outlet is on or off. If for some reason the apex fails and doesn’t turn off, the heater thermostats will kick in if the tank gets too hot.
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