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How water heater thermostats works

Gas thermostat located inside copper tube

Gas enters control valve. If thermocouple or thermopile is correctly positioned in the pilot flame, then small voltage is sent to gas control valve indicating that it is safe for gas to enter gas control valve. (If no current is produced by thermocouple or thermopile, then magentic switch closes off gas entering gas control valve until pilot is re-lit or defective thermocouple or thermopile replaced). The gas control thermostat protrudes into tank and reads water temperature. When water temperature drops below selected thermostat setting, then gas valve opens and lets gas flow to burner.
If water heater vent pipe is drafting correctly, then there is natural flow of air drafting upwards through combustion chamber.
The newly released gas is drafted up through the burner where pilot light or flame rod ignites gas. If burner is dirty, sooted or obstructed, then gas can flow around the burner causing un-burned gas to rise into vent pipe. Sooted and dirty burner and yellow flame indicate this problem, which is serious explosion hazard. Correct combustion of gas at the burner will heat bottom of water tank, and water is heated. Excess heat and poisionous by-product resulting from combustion process is vented up the vent and out of each home into atmosphere.
When bottom of tank is heated, water temperature rises inside tank until thermostat setting is reached, causing gas control to shut off gas to burner.
On modern gas water heaters, if incoming air supply is constricted  then the flow of hot combustion gas up vent is slowed and trapped inside combustion chamber. Result is overheating that will trip the FVIR system. Different water heater companies have different air intake systems and different FVIR prevention. Depending on overheating event and specific model of water heater, the gas control might need to be replaced, other parts might need to be replaced. Or the repair could be simple cleaning air intake screen, and resetting thermal reset on front of combustion chamber, and/or repacement of FV sensor.
Gas control valve thermostat
Gas control valve thermostat
Gas control valve thermostat ECO energy cut off will trip if water inside tank exceeds 170 F.
ECO is usually single-use and gas control must be replaced
How to test ECO White-Rodgers and Robert Shaw
Intellivent gas control ECO can be reset
If water temperatures fluctuate at tap, then Gas control can be bad -or- there is plumbing problem
Check temperature at tank drain valve and TP valve and compare with temperature at tap
Adjust temperature at tank
Crossover: single handle faucet has bad cartridge and other causes
Stacking: serveal short water draws cause very hot water  
How to replace gas control
How to replace gas control

Electric water heater thermostats

How to troubleshoot electric water heater
How to wire thermostats
Thermostat operation
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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 basic concept is same from brand to brand. Water heater manuals available on-line. Label on side of tank shows watts and volts of appliance.

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

Back of thermostat
Thermostat must sit flat against tank wall so temperature reading is accurate and water heater is protected from overheating
Residential water heaters can be wired 4 basic ways. Non-simultaneous which is found in most residential water heaters, simultaneous, redundant, and off-peak. The key is knowing how a thermostat works.

Diagram on left illustrates basic thermostat function. This thermostat is found on 240 Volt water heaters that have 2 elements. Check label on side of tank for voltage info. See diagram below.

Power from breaker box connects to L1 and L3 screws > some thermostats, the screws are marked simply 1 and 3.

Top half of thermostat is the ECO or energy cut off > The ECO is heat sensitive, so if temperature reaches dangerously high 150-180F, 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 red reset 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.

Inside the thermostat is 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 diagram, the bi-metal switch is connected to T1. When temperature inside tank is cool, the bi-metal switch rests against T2 so power travels from T1 to T2, and then on 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 travels to lower element and thermostat through the T4 screw.

In a 240 Volt water heater, elements must receive power from 2 hot leads to complet 240Volt circuit.
Both elements have 1 wire connected to L4 screw on upper thermostat. So both elements test hot for 120 volts at all times. When the thermostat switches and sends power to element, that action completes the 240Volt circuit, and element turns ON.
See basic water heater circuit

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Screw numbers on upper thermostats

Upper thermostat is made of two parts >> the top part is the ECO, the bottom part is the thermostat

How to wire 240V dual element water heater thermostat

Bradford White tank

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Non-simultaneous water heater wiring
This wiring is found in most residential water heaters. Check label on side of tank.
Both elements are never 'on' at same time unless tank is wired for other purpose (see owners manual). This is called 'non-simultaneous operation' since both elements are not 'on' at same time.

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

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

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

Water heater cycle
Larger image
NON-simultaneous: Household electric water heaters are wired for NON-simultaneous operation.

Both elements are not ON at same time.
Upper element is ON, or Lower element is ON, or both elements are OFF.
Off-peak water heater wiring
See page on off-peak wiring

Purpose of off-peak ... save energy during peak electric prices.  
> Upper thermostat operates normally to keep top 2/3 tank hot.
> Less hot water is available.

> Lower thermostat heats bottom 1/3 of tank only when ordered by off-peak meter or timer

> Off-peak ends, and tank resumes non-simultaneous operation
Off peak water heater sequence
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OFF-Peak Non-simultaneous operation.
Lower element turned OFF during off-peak.
Upper element keeps top of tank hot
Less hot water is available.
When off-peak ends, normal NON-simultaneous operation resumes.

Step 1:
Remove blue and yellow wires from upper thermostat
Move black wire from terminal T4 to terminal T2
Tighten screws very tight against copper wire.
ECO protected thermostat must remain at top of tank to protect against overheating event.
Redundant thermostat

Upper element is disconnected.
Lower element heats tank.

Both thermostats control lower element.

When upper part of tank cools, the lower element turns ON. Hot water rises.
When lower part of tank cools, lower element turns ON.
ECO-protected thermostat must be kept at upper part of tank to avoid overheating event that can damage or rupture tank, cause scalding, release of hot water through TP valve and distill chemicals out of the water.

Cold enters lower part of tank through plastic dip tube.

Simultaneous water heater wiring
Used in large water heaters when elements cannot keep up with demand, or in high demand situations.

Lower thermostat replaced with ECO protected thermostat.
Each thermostat is wired separately.

Each thermostat wired to a separate circuit breaker, since each element draws 4500 to 5500 watts.
Simultaneous water heater wiring Simultaneous water heater wiring

Upper and lower thermostats and elements work independently from each other.
Increased first hour delivery of hot water
Faster recovery time

4500Watt elements, then total wattage is 9000 watts.
Use 40 amp breaker and #8 gauge wire
or two 30 amp breakers and two 10 gauge wires

5500Watt elements, then total wattage is 11,000 watts
Use 50 amp breaker and 8 gauge wire or 6 gauge wire
or two 30 amp breakers and two 10 gauge wires

Oversizing breaker and wire runs cooler and is more efficient

Connect lower thermostat to timer and convert water heater into off-peak operation.
Water heater companies are integrating their products with future smart-grid. Except they are installing electronic thermostats. Consider whole house surge protection
Whirlpool's (American water heater) new 'energy smart'

The era of generic off-the-shelf parts is coming to an end quickly as companies try to boost sales by using proprietary parts that must be ordered from them.
SO buy extra parts and water heaters now.

water heater electric meter How to install electric meter on electric water heater

Additional links

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How to replace thermostats
Upper thermostats
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