How To Test a Water Heater Thermostat

Ran out of hot water? Here's how to troubleshoot (and replace if necessary) the controls and heating elements of most common residential 120, 208 and 240 volt electric water heaters. Please read this entire wiki before beginning the procedure. Click on any picture for larger, more detailed view.

Step 1  

With power off, remove the upper (and if provided lower) access panel(s).

Step 2  

Clear away any insulation blocking access or view of the controls (thermostat and high temperature switch) and heating element.

Step 3  

Locate the parts below:  

• High Temperature Limit Switch: Has a reset button There will be (4) terminal screws / wires connected to it. Usually the top two terminals have wires that go up to the field wiring compartment that bring the power to the rest of the water heater's controls and heating elements. 
• Thermostat: Has a graduated, adjustable dial. The dial may indicate letters "A" "B" "C" etc., "warm, hot and hotter" or as in the case of the one pictured, the actual temperature in degrees. 
• Heating Element: Has two terminals with a wire connected to each. It is usually located below the controls and supports the controls with an a clip of some type. 

Step 4  

Test to ensure power is off. Set the meter for AC Volts and insert the black probe into the black or common jack, and the red probe into the red or Volts jack.

Step 5  

Measure Voltage. Set the AC voltage range to the highest available. Touch the probes to the top terminals of the high temperature limit switch as shown in the image to the right. If desired, the range may be lowered to any desired - provided the range is greater than the voltage displayed at the highest range. If unable to prove power off, double check circuit panel.

Step 6  

Set the multimeter to Ohms or Resistance. Observe the meter indication. If an analog type, the needle or pointer will be resting at over the higher resistance values (left-most position) this is an "open circuit" indication. If digital, it may display "OL" or "1" ("1" without trailing or leading zeros). This OL or 1 indication represents a value greater than the meter is capable of displaying (similar to the way a calculator does) for "overload" or "infinity". Infinity in high resistance range is also called an "open circuit" or "Open Loop" (OL). Take note of this open circuit indication for this meter (when in a voltage or current range is selected and the meter displays OL or 1, the measurement should be taken again after adjusting the range upward). If you are unsure what your meter should read in an "OL stituation", just leave the terminals unconnected to anything and not touching each other and switch the meter on, it should then indicate the resistance of the air between the terminals which should be infinity in normal conditions.

Step 7  

Remove one wire from the heating element, it does not matter which.

Step 8  

Connect the black test lead to the jack marked "Common".

Step 9  

Connect the red test lead to the jack marked "Ohms" or "Resistance" if there are multiple jacks from which to choose.

Step 10  

Set the range (if provided) to R x 1. If the meter being used does not provide for range selection, it is likely an "autoranging" type. This simply means that the meter will automatically adjust the range up or down as needed without any intervention by the user. This feature is far more common on digital meters than analog types. Most analog meters that do not provide manual range selection likely support only a single range. These meters are much more accurate indicating low values (such as 0 through 500k or 1M ohms) than higher values (such as those above 1M), but will work well for this procedure. Pay close attention to the display of an autoranging digital meter when reading - there is a huge difference between 20, 20K and 20M ohms. A "K" indicates thousands, and an "M" indicates millions. The examples above would correctly be read as 20 ohms, 20,000 ohms (20K ohms or 20 kilo ohms) and 2,000,000 ohms (2 meg ohms or 2 million ohms), each is 1000 times larger than the previous.

Step 11  

Press the metal tips of the test probes together. The analog meter pointer should move to lowest values of the resistance scale (or fully to the right). The digital meter should indicate 0 or a "very low" value approaching zero. Locate the "Zero Adjust" knob and turn so that the the meter indicates "0" (or as close to "0" as possible). Most digital meters do not have this Zero Adjust feature. Once "zeroed", this needle position on the dial is the "short circuit" or "zero ohms" indication for this range of this meter. The meter must be zeroed if the resistance range is changed. Measured resistance values will be inaccurate if unable to properly zero the meter.

Step 12  

Replace batteries if needed. If unable to obtain a zero ohm indication on an analog meter, this may mean the batteries are weak and should be replaced. Retry the zeroing step above again with fresh batteries. Digital meters often graphically display the battery's level of discharge or simply an indication to replace the battery. Check the meter manual for help determining the battery charge state.

Step 13  

Press the probe tips against the terminals of the heating element (one probe to each screw). Read the meter display. Look for a range multiplier (a "K" or "M" in the display) to be sure the value displayed is really ohms, not kilo ohms (K) or meg ohms (M).

Step 14  

Be aware that a "good" heating element will read a very low value (between 10 and 20 ohms depending on wattage of the element). Determine correct resistance value: According to the nameplate of a typical residential water heater, it is rated "volts 240" "watts 4500 / 4500" (a "4500 / 4500" watt rating simply means there are two heating elements and they are 4500 watts each). In the procedure outlined in this wiki, the electrical system voltage is 208. This means that the wattage for the elements are 3500 / 3500 as indicated on the lower portion of nameplate data decal.

Step 15  

Determine resistance in ohms. Since this is a purely resistive AC circuit, we can use Ohm's Law for DC circuits. To do this, we simply divide volts by amps. Since the nameplate only provides watts and volts (4500 / 4500 watts at 240 volts or 3500 / 3500 at 208 volts electrical info, the current in amps must be calculated before determining the resistance in ohms below:  

• Calculate amps, divide watts by volts: 4500 watts / 240 volts = 18.25 Amps, (The element under test in the pictures: 3500 watts / 208 volts = 16.8 amps)
• Continue with these results to calculate ohms, divide volts by amps: 240 volts / 18.25 amps = 13.2 ohms (The element under test in the pictures: 208 volts / 16.8 amps = 12.3 ohms). A "4500 / 4500" or 3500 / 3500 rating simply indicates the wattage value for the upper and lower elements since at no time are both energized at the same time. The examples above are for two 4500 or 3500 watt elements (very common values).
• A good heating element will read no more than about + or - 20% of these calculated values. If not, the heating element is suspect, should be considered for replacement (described later). The range of acceptable resistance is variable because resistance is determined by the material's resistivity, length & width and temperature. Changing any of these values can significantly increase or reduce measured resistance. Since the element material's resistivity, length & width are kept constant in the manufacturing process, only the temperature can be varied for the element under test.

Step 16  

Check for a grounded element. Prepare the meter by setting the meter to highest resistance scale.

Step 17  

Hold the probes at the end of the test leads together. The analog meter pointer should move fully to the lowest values of the resistance scale (to the right). The digital meter should indicate a "low" value very close to zero. Locate the "Zero Adjust" knob and turn so that the the meter indicates "0" (or as close to "0" as possible). The digital meter may not have this Zero Adjust feature. Note that this position is the "short circuit" or "zero ohms" indication for this particular resistance range of this meter. Always "zero" the meter when changing resistance ranges.

Step 18  

Press the red probe against either terminal screw of the heating element. Press the black probe firmly against the metal tank or the heating element mounting nuts or screws (not the other terminal screw). Scrape the metal to ensure a good contact. The meter should display the "infinity" indication as described above in the meter setup. If the meter displays a value other than a very high value (millions of ohms), preferably infinity, the element should be replaced, described later.

Step 19  

Repeat the procedure for the lower heating element.

Step 20  

Reconnect the wire at each heating element.

Step 21  

Set the bottom thermostat to minimum.

Step 22  

Set the top thermostat to maximum.

Step 23  

The steps below assume that there is warm water in the tank. If the tank is cold or very hot, it may be difficult to get the expected changes when dialing in different temperatures on the thermostat.

Step 24  

The rest of the steps will require power on for testing. Use extreme caution as the risk of shock injury is greatly increased if not fully paying attention. Ensure all wires are reconnected to respective terminals and there are no "accidental conductors" anywhere that could cause a short circuit or shock.

Step 25  

Restore power to water heater.

Step 26  

Remove red test lead from the "Ohms" or "Resistance" jack of the meter and insert into "Volt" jack.

Step 27  

Set the range of the meter to the lowest Voltage value that is greater than 240 Volts "AC" or "VAC". As mentioned earlier, common voltages for residential (and mobile home / RV) type water heaters are: 120, 208 & 240, with the most common being the 240 volt variety. When steps below discuss measuring " line voltage ", substitute the voltage for your particular water heater instead.

Step 28  

Check the top heating element's terminals for presence of line voltage by touching a probe tip to each terminal, as done in the resistance test earlier. Line voltage is likely 120, 208 or 240 in the U.S.

Step 29  

If there is no power, attempt to reset the high temperature switch. It is a button either red or black, located directly above the thermostat. It most likely has "RESET" printed on it. With a screw driver or pencil, GENTLY but firmly press in. If it is tripped, a mechanical "click" should be felt or heard. A tripped high temp switch is indicative of a thermostat that will not open. More on this, later.

Step 30  

After attempting reset, check the heating element for power again.

Step 31  

If still no power present, test the top left and right terminals of the high temp switch for line voltage with the probe tips.

Step 32  

If no power, trouble is an open circuit. Check the heater's "field wiring compartment" (usually located on the top of the heater), along the length of the cable that feeds the heater, and lastly inside the electrical panel. Remember, unless power is shut off at panel, this circuit is live at some point between the fuse or circuit breaker and the water heater. Tighten any and all wirenuts and connections in the wiring compartment all the way back to the terminals of the circuit breaker or fuses in the electrical panel. Replace open fuses or reset any tripped circuit breaker if off. Check for power at fuse or circuit breaker. A circuit breaker that immediately trips after resetting indicates a short circuit or less likely, defective circuit breaker.

Step 33  

Once power is restored to top terminals of high temp switch, test for line voltage at heating element.

Step 34  

If line voltage not found, replace top controls.

Step 35  

Set top thermostat to minimum.

Step 36  

Set lower thermostat to maximum.

Step 37  

Check bottom heating element for presence of line voltage.

Step 38  

If no power present, determine which wire is connects a heating element terminal screw to the bottom thermostat terminal screw. These will be the common screw terminals. The other screw on the thermostat and heating element will be the power screw terminals. Touch red probe to the power screw terminal of heating element and black probe to the power screw terminal of thermostat. Expect line voltage.

Step 39  

If line voltage not found, replace upper controls.

Step 40  

If line voltage found, check for line voltage at heating element terminal screws by touching each probe to the terminal screws.

Step 41  

If line voltage not found, replace lower thermostat.

Step 42  

If line voltage is found, wait for water to heat or perform the Ohms (or Resistance) checks on the elements once more with the power off. If line voltage is present on a heating element, it must heat the water unless the element has failed.

Step 43  

Return all thermostats to an equal value of your choosing, but should not be set higher than 140 degrees.

Step 44  

Replace insulation and access covers.

Replacing Elements

1. Ensure power to water heater is off.
2. Empty the tank to a point below the level of the element to be removed. If removing lower element or unsure how much to drain, empty the entire contents.
3. To quickly drain and fill the water heater, shut off the cold water supply to the heater. Open the nearest hot water faucet to relieve pressure and allow air into the tank as it drains. Connect a garden hose to the valve at bottom of heater and extend the hose to a floor drain or some other point that is lower than the drain valve (it will be impossible for the tank to drain "up hill"). Open drain valve at the bottom of the water heater. Close valve when empty.
4. Disconnect wires from the terminals of the heating elements.
5. The heating elements are secured by one of two methods. The first, is by bolts that are installed through holes in a flange around the element. Simply use a ratchet and socket or wrench to remove the 4 screws and remove the element.
6. The other is by threads that are under a hex shaped flange of the element. Usually, a 1-1/2" socket will fit perfectly. If a socket of that size is not available, use of channel lock style pliers or adjustable wrench will get the job done. Twist the element counter-clockwise until it is out.
7. Clean the tank surface around the opening for the element. Is is important that any gasket material, filings, rust etc., be completely removed.
8. Copy the nameplate data of the water heater to obtain correct replacement parts. It will help to bring along the old element for comparison to the new element.
9. Install the new element and gasket.
10. Connect wires to heating element.
11. Open the nearest hot water faucet and fill water heater by opening the cold water supply valve before applying power to heating elements. The heating elements must be completely submerged in water before applying power. Failure to fill tank prior applying power will result in instantly burning out the heating elements.

Replacing Controls

1. Ensure power to water heater is off.
2. The tank does not need to be emptied to replace controls.
3. Label the terminals and the wires by writing numbers on masking tape or use of color coded tape, etc. before disconnecting.
4. Pry the fingers on the sides of the control away from the tank slightly and slide the controls out. Excessive force on the fingers may damage them and prevent the controls from seating properly. A poorly seated control prevents it from properly detecting tank temperature, as it relies on firm physical contact with the tank for conduction of heat.
5. Controls are held and pressed against the tank by spring steel. No screws are used.
6. Copy the nameplate data of the water heater to obtain correct replacement parts. It will help to bring along the old controls for comparison to the new controls.
7. Clean the tank surface that the controls contact. Remove any rust, debris and dirt.
8. Slip the controls under the steel fingers.
9. Rewire the controls based on the labels applied on the old controls.

Tips

• A "clamp on" type ammeter allows the troubleshooting process to progress quickly and easily. Since both voltage and resistance checks need to be performed to determine if the heating element appears as a load in amps or not, testing with a clamp on type ammeter combines these two test results in one procedure without the need of removing wires, zeroing, range changes or swapping leads on the meter. Turn the top thermostat up and the bottom down - and simply clamp the meter around one of the wires connected to the top heating element. Do not disconnect any wires and of course, the power must be on. Note the amount of current displayed on the meter, and then turn the top thermostat down and the bottom thermostat up, check current in amps as done above, at the bottom element. Both amp measurements should be pretty close to equal (within 10 % or so). The difference may be due to the heating of the element, which changes the resistance (discussed in the steps above). A change in the element's resistance will change the expected current in amps. If one reading is significantly lower than the other or is a very low value approaching zero, that half of the water heater has the problem (either a faulty control or heating element). If zero on both elements, the high temperature limit switch is likely defective, assuming there is power present to the water heater. Use the multimeter's voltage and resistance functions to help narrow down to the defective device. 
• Use the wiring diagram supplied with / affixed to your water heater to answer specific question whenever possible. If unable to locate a wiring diagram, contact your water heater's manufacturer or check these A. O. Smith diagrams for a representation of the most common residential applications. Figures "C2" and "A6" are the most common, followed by the once popular "A7" which is only used when the electric company provides a lower rate per Kilowatt Hour when used during "off peak" times. This requires a special or second electric meter, and are seldom newly installed. The "S1" figure will be the least likely type employed in typical single family dwellings.
• Become familiar with the operation of your multimeter before undertaking this project. Different meters allow measurement of voltage and resistance differently. Some have specific jacks to connect probes for particular measurements, while others have two jacks that are used for all measurement types. Regardless of meter, be sure to select the correct measurement and use the correct jacks prior to touching the probes to live circuits. A meter set to measure resistance but connected a live circuit could cause damage to the meter or injury to the user.
• Owners of 120, 208 and 240 volt water heaters will substitute that value wherever the line voltage term is used. Water heaters that operate on other voltages can also have their voltage substituted when the line voltage term is used as well.
• This is the perfect opportunity to clean the water heater. See How to Flush a Water Heater for detailed step by step procedures.
• If unable to perform these checks, or would prefer to have others do it, contact a professional. An electrician should be called if the tank is an electric type and not leaking water on the floor. A plumber however, should be called if it is a gas water heater, part of the heating system ("SuperStor"), or if the tank itself (electric or any other type) has failed (leaking water) requiring complete a replacement. Most plumbers are not equipped to quickly diagnose a "no hot water" trouble on an electric water heater. Plumbers often will disconnect the old and reconnect new water heater's electrical supply, though in most areas, is considered a code violation.

Warnings

• Voltages present when power is on, are lethal. Please use extreme care while working on energized circuits.
• Conducting a voltage test with the meter incorrectly set up to test resistance will damage the meter, and may lead serious burns or shock. Learn how to use the multimeter's ranges and test jacks by reading the manual.
• Performing tests with power on should only be done when required, otherwise shut off power.
• Look for a range multiplier (a "K" or "M" in the display) whenever reading measurements from a meter. Be sure to be sure the value displayed is really simply as reading the number displayed, and not the 1000 (kilo or K multiplier) or the 1,000,000 (meg M multiplier). Get into a habit of looking for a multiplier indicator whenever taking measurements with any meter.
• Replacing parts must be done with the power off. Heating elements must be replaced with water tank drained to a point below the level of the heating element. If not, water will begin to leak when the element is loosened from the tank.
• A high temperature limit that trips more than twice, indicates a thermostat that is failing to open, keeping power applied to the heating element longer than needed to heat the water to the desired temperature. If the procedure above is followed, the defective control will be determined and can be selectively replaced. A failed thermostat in the closed position will cause water temperature to rise significantly, greatly increasing the likelihood of scalding and burns.

Things You'll Need

• Screwdrivers
• Multimeter capable of measuring AC volts and resistance in "Ohms"
• Garden hose
• Ratchet & sockets, channel lock type pliers or adjustable wrench