How to wire off-peak water heater thermostat

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How to wire off-peak water heater thermostats

Homeowners should check with plumber before altering water heater wiring

240 Volt potential exists between L3 yellow wire and each L1 black wire	Purpose of off-peak ... save energy during peak electric prices. > Upper thermostat operates normally to keep top 2/3 tank hot. > Lower thermostat heats bottom 1/3 of tank only when ordered by off-peak meter or timer Compare: Off-peak water heater thermostats Ordinary water heater thermostats 1. Lower thermostat replaced with ECO-protected thermostat identical to upper thermostat. ECO safety red reset cuts off power when water in tank reaches dangerously high 150-180°F 2. Upper thermostat: Red wire is moved L4 to T4. Color of wires can be different. 3. Black wire from lower thermostat does not connect to upper thermostat T4 screw 4. Black wire from lower thermostat runs to top of water heater for connection to off-peak meter, or off-peak timer 5. 240 Volt potential exists between L3 yellow wire and each L1 black wire at top of water heater See Off-peak operation sequence	Ordinary tank wiring
Off-peak water heater timer	How to wire off-peak water heater to EH40 programmable timer	How to wire off-peak water heater to timer Lower thermostat receives power only when timer orders > Upper thermostat receives power full time Compare box-type water heater timers

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Some thermostats, the L2 and T1 screws are empty slot or rivet since wires do not connect to either L2 or T1.

Brands of thermostat vary slightly, screws are located differently, some numbers are slightly different, but concept is same from brand to brand.

Each thermostat comes with instruction sheet.
When replacing thermostat, take digital photo of water heater and old thermostat to hardware store or plumbing store so quick identification can be made.

Residential water heaters can be wired different ways. The key is knowing how a thermostat works.

Diagram on left illustrates basic thermostat function.

Power from breaker box connects to L1 and L3 screws > some thermostats are marked simply 1 and 3. See image showing different thermostats

Top half of thermostat is the ECO or energy cut off > The ECO is heat sensitive, so if temperature reaches dangerously high 150-180°F, then ECO trips and water heater no longer has power. This prevents water heater explosion.
Viewing the diagram, power from L1 and L3 pass through ECO and go to L2 and L4 screws.

After power passes through ECO, it is available for heating water. To control heating, power arrives at the thermostat which begins at T1 screw > some thermostats have no visible screw, however wires do not typically connect to this terminal

Inside the thermostat is a bi-metal switch. Two different metals are fused together. Each metal expands differently when heat is applied. Since the two metals are joined, the metal that expands most causes the bi-metal switch to bend. The bending action opens and closes the switch.

In the illustration, one end of the bi-metal switch is permanently attached to T1. When temperature inside tank is cool, the other end of the bi-metal switch rests against T2 which sends power from T1 to T2, and then down to the upper heating element. When temperature inside tank is hot, the bi-metal switch disengages from T2 and clicks over to T4. When this happens, T1 sends power to T4, so the upper heating element is turned off and power is sent down to lower element and thermostat through the T4 screw.

Thermostats
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How to wire 240V water heater thermostat

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Both elements are never 'on' at same time unless tank is wired for other purpose (see owners manual).

Either upper element is 'on' or lower element is 'on,' or both are off.

Upper thermostat is main controller <> it turns on upper element until temperature is reached, and then sends power to lower thermostat.

Thermostat reads temperature through wall of tank
Check thermostat sits flat against tank to prevent overheating

Off-peak water heater

Upper thermostat with ECO red reset button
Screws identified by number L1, L3, L4, T2, T4

Dual element upper thermostat
Screw numbers include L1, L3, L4, T2, and T4

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