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This heater is powered by 3 voltage rails: 5VDC, 24VDC and 110VAC, but the power supply is the cheapest non-isolated type, and the 24VDC power supply had failed for this heater that I found in the street of SF abandoned. The 24VDC is used to power the 2x 24V relays, which are for connecting the heating element to the 110VAC, and its voltage regulation diode has failed. If my calculation is correct, the maximum power dissipation for this diode is about 2.3 watts if the load is not connected to it, but the onboard diode is rated only at 500 milliWatts, so it's designed to fail. A 3 watts+ 24v zener is needed or it will fail again.

(110-1-24)/884= 0.096A * 24= 2.3 watts for 24 volt zener (recommended 3W - 3EZ24D5)
(110-1-5)/884= 0.096A * 5= 0.48 watts for 5 volt zener (recommended 1W - 1N4733A)

Note 1: the 5V zener has a 500mW power dissipation with a lead length of 8 mm, so it can be failing too if the load is disconnected for whatever reason.

Note 2: the 24V zener failed probably due to the fact that the diode lead had inserted all the way, so limiting its power dissipation. If a 1/2 watts diode is used, a 1 watt 24v diode should be more than enough, and so a 3W diode may not be needed.

Note 3: If you don't have a 24V zener, it's relatively easy to make one. For example, if you can find 2x 12v zener, it's 24v.

Note 4: The calculations are based on the ideal worst situation assuming the loads are non-existence, and the maximum power spike only occurs on half of the AC cycle, but on the worst worst case, the maximum power spike is about 4W for the 24v zener.

Note 5: A heater may not be a life support device. Someone could be frozen to death if this heater is used as for main heating in the winter. Don't buy fancy heater with a lot of control. Get the simplest type for more reliability.