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ECO Basics
Reset button on electric heaters

Thermostat mounting bracketECO red reset button basics: 
1. ECO will trip when either upper or lower thermostat sticks in ‘on’ position causing continuous heating. Once water reaches 150-180° F, the ECO trips and turns off electricity to water heater.
2. Loose wire or electrical connection inside thermostat -or- between parts will trip ECO.
Loose wires generate heat, causing high resistance. ECO senses heat from bad connection and trips. Not every loose wire will cause ECO to trip, only those that cause top thermostat to overheat.
3. Bad element shorted to ground near center of element will trip ECO.
When element is shorted to ground near center of element, the short does not draw enough current to trip circuit breaker. By design, a thermostat only turns off 1 hot wire, but a 240V water heater has 2 hot wires. If short does NOT trip breaker, then 120-volts flows to ground through half of element, continuing to heat water until ECO trips.
4. ECO can trip if factory-installed insulation and cover have been removed. Result: Thermostat exposed to cooler temperatures and misreads temp inside tank. Elements overheat tank.

5. ECO will misread tank temperature if it is not sitting flat against tank wall
Resources:
Add another thermostat to electric water heater
How to test elements
How to test water heater wires
How replace elements
Convert energy smart to regular water heater

Apcom style thermostat
100-150° high-low/ reset trip @ 170°
Upper thermostat: ECO reset button


Therm-o-disc 89T
90-150° high-low/ reset trip @ 170°
240Volts arrive at upper thermostat
✔ ECO reset tripped
Information
Red reset button or 'high limit control' or Energy Cut Off or ECO will cut off electric power to entire water heater.
ECO shuts off both hot wires in the 240V circuit. 
Diagram on left: 240V arrives at L1 and L3 screws of upper thermostat <> when ECO is tripped, L1 and L3 screws are hot but electricity stops there.
Push reset button firmly. If ECO resets and water heater turns ON and works, then watch over next few days if ECO reset trips again.
✔Causes: ECO tripped
Check if wire is 10 gauge and breaker is 30 amp.
1. ECO will trip when either upper or lower thermostat sticks in ‘on’ position causing continuous heating. Once water reaches 150-180° F, the ECO trips and turns off electricity to water heater.
2. Loose wire or electrical connection inside thermostat -or- between parts can trip ECO.
Loose wires generate heat, causing high resistance. ECO senses heat from bad connection and trips. Loose wires cause burning and melting and should be visible on inspection, and/or cause burned smell. Not every loose wire will cause ECO to trip, only those that cause upper thermostat to overheat. Action: Replace part connected to burned wire. Test thermostats  How to replace thermostats  How to replace element  Use multimeter to test wire for continuity

3. Bad element shorted to ground near center of element will trip ECO.
When element is shorted to ground near center of element, the short does not draw enough current to trip circuit breaker. By design, a thermostat only turns off 1 hot wire, but a 240V water heater has 2 hot wires. If short does NOT trip breaker, then 120-volts flows to ground through half of element, continuing to heat water until ECO trips.  Actions: How to test elements. How to replace elements.
4. ECO can trip if factory-installed insulation and cover have been removed. Result: Thermostat exposed to cooler temperatures and misreads temp inside tank. Elements overheat tank. Action: re-install insulation
5: ECO can trip if water heater has stacking event where repeated short draws of hot water can overheat tank. Action: read about stacking
6. ECO can trip if water heater is wet from flood, leak or tank failure. Look for signs of rust and dampness. The spring holding thermostat flat against the tank will rust so thermostat is not held tight against tank wall, and then tank overheats.
Thermostat must sit flat against tank.
7. Factory insulation that ordinarily covers thermostat is missing.
Other link: Test electricity to water heater

Water heater cover
Larger image
✔ ECO reset keeps tripping
Quick answer: Replace both thermostats, test elements. Check for loose wires.
How to replace thermostats
How to test elements

Causes:
1) Make sure thermostats are covered with insulation, protector, and cover. See image
Without insulation, Thermostat exposed to room temperature will misread temperature and cause overheating.
2) Make sure thermostat sits flat against tank wall. Wiggle thermostat. Check that bracket is not rusted and loose.
3) Bad upper or lower thermostat. Action: Replace both upper and lower thermostats. How to replace thermostats
3a) Upper thermostat wired wrong during replacement  See residential thermostat wiring  Wire colors vary
4) Bad element causes run-away heating. Action: Test elements. How to test elements
5) Many short draws of hot water can cause tank to overheat. Action: Read about Stacking
6) Make sure watt rating printed on end of element matches watt rating showing on label located on side of tank.
7) Use nose and eyes to inspect for burned parts and melted wire.
How to test wires
8) Loose, partially-melted wire will cause overheated wire. Overheated wire will cause ECO to trip. Inspect wires. Make sure wires are firmly under screw plate. Check that no wire insulation is under screw plate.
9) Label on side of tank will show 240Volt Non-simultaneous for residential dual-element water heater. Make sure water heater is wired correctly. Check water heater wiring
10) If water heater was flooded, then all parts must be dry. Flooded water heater
11) Undersized wire from breaker can cause overheated wire, but should trip breaker instead of tank ECO. For 240Volt: If breaker is 20 amp, wire must be 12 gauge, and maximum 3800 watt elements. If breaker is 30 amp, then wire must be 10 gauge, and 4500 or 5500 watts elements.  Match breaker and wire size




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