How to Replace Electric Water Heater Elements and Thermostats
Why this matters
Most no-hot-water and not-enough-hot-water calls on an electric tank come down to one failed element or one failed thermostat, both of which are parts on the truck and an hour of work. The callbacks come from two places: the part changed by guess rather than by meter, so the real fault is still there, and the element energized before the tank was full, which burns out the new element in seconds. This HowTo works from a reading sheet so every decision has a number behind it, and it puts the refill ahead of the power every time.
Electrical work on the heater's terminals is done where your license covers it and by a qualified person, as 29 CFR 1910.333 uses the term for electrical work practices (29 CFR 1926 Subpart K on construction work). If nobody on the truck is qualified for the energized checks at the end, those go to an electrician.
Isolate first, and prove it
Lock out and verify dead before the covers come off. Turn off the heater's double-pole breaker, lock and tag it, and test for absence of voltage at the upper thermostat's line terminals with a tester proven on a known live source immediately before and after the test, per NFPA 70E Article 120 (120.5 in the 2021 edition; check the current edition's numbering), with work practices at 29 CFR 1910.333(b)(2). Test both legs to each other and each leg to the tank. A heater fed from a disconnect beside it still gets the breaker locked, because the disconnect may not be the only source and a homeowner will close it.
Water and electricity. If the tank or the floor under the access panels is wet, find out why before working inside the panels, and keep the floor dry where you stand.
Scald. Draining water is at the heater's set temperature; hose it to a drain and keep hands off the hose end.
The reading sheet
Fill this in with the power locked out, the insulation and plastic guards pulled back, and the wires removed from the element terminals so the meter reads only the element. Read the element wattage and voltage off the heater's rating plate.
ELEMENT READING SHEET 50 gal electric, 2 elements
rating plate: 4500 W at 240 V each
-----------------------------------------------------
expected resistance R = V x V / W
= 240 x 240 / 4500
= 57,600 / 4500 = 12.8 ohms
check other way I = W / V = 4500 / 240
= 18.75 A
V / I = 240 / 18.75 = 12.8 OK
-----------------------------------------------------
UPPER LOWER
terminal to terminal 12.6 ohms OL
term 1 to tank OL OL
term 2 to tank OL OL
-----------------------------------------------------
verdict good OPEN, replace
thermostats (cold tank, power locked out):
upper, line to upper element: closed
upper, to lower terminal: open (upper calling)
lower thermostat contacts: closed
The example on the sheet is the worked case below. Here is what each line means.
Terminal to terminal, near the calculated figure: good. The element's resistance comes from its wattage and voltage. A reading a little under the calculated figure, as 12.6 ohms is here against 12.8, is normal for a cold element; a reading wildly off is not. Calculate it from the plate every time, because elements come in several wattages and a remembered "about 13 ohms" is wrong for a 3500 W element (240 x 240 / 3500 = 16.5 ohms).
Terminal to terminal, OL: open. The resistance wire has burned through. That element produces no heat. OL on a meter means over the range, effectively infinite.
Either terminal to the tank, any reading: shorted to the tank. A healthy element reads OL from each terminal to the tank or the ground screw. A reading means the sheath has cracked and the element is leaking current to the tank, which trips breakers, trips the high limit, or energizes the tank if the ground is poor. Replace it, and check the equipment grounding conductor is connected at the ground screw.
Which fault gives which complaint. The two elements do not run at the same time on a standard residential heater. The upper thermostat powers the upper element until the top of the tank is hot, then switches power to the lower thermostat and element. So an open upper element means no hot water at all, because the upper thermostat never satisfies and the lower never gets power. An open lower element means a small amount of hot water that runs out fast, because only the top part of the tank heats. Read the meter before you trust the complaint, but the complaint should match.
Replacing an element
- Confirm the lockout is still on and re-verify dead at the terminals you are about to touch, with the tester proven before and after. Someone may have been in the panel while you were at the truck.
- Shut the cold supply and open a hot faucet to break the vacuum.
- Drain below the element. For an upper element, drain a few gallons more than enough to bring the water level below its opening. For a lower element, drain the tank, since it sits near the bottom. Skipping this puts a tank's worth of hot water on the floor the moment the element threads clear.
- Remove the element with an element wrench, turning counterclockwise. The wrench seats on the hex of a screw-in element; a pipe wrench on the hex rounds it and can twist the tank spud. On an older flange element, remove the four bolts instead.
- Clean the spud threads and gasket face, and pull any loose scale out of the opening. A ridge of scale under the new gasket is the usual cause of a weep after the swap.
- Install the new element with its new gasket, matching the wattage and voltage on the rating plate. Never install a higher wattage than the plate states: the circuit and the heater's wiring are sized for the plate, and a larger element overloads both. Snug it with the element wrench to compress the gasket per the element's instructions; do not crank it.
- Refill before power. Close the drain, open the cold supply, and run the upstairs hot faucet until water runs without spitting air. Then check the new element for drips at full pressure. A dry element energized for even a short time burns out, and the warranty on it is void.
- Reconnect the wires to the terminals, tight, and replace the insulation and the plastic guard over the thermostat and element before the access cover goes back on.
Thermostats
Check them the same way, power off, cold tank. The upper thermostat has a line side from the breaker, an output to the upper element, and an output to the lower thermostat. With the tank cold, the upper thermostat should read closed to the upper element and open to the lower thermostat. The lower thermostat should read closed. A thermostat that reads open on a cold tank where it should be closed has failed.
The red button is the high limit, not a reset for convenience. The upper thermostat usually carries the energy cut-off (ECO), which opens when the water overheats. If it has tripped, it tripped for a reason: a thermostat with welded contacts, an element shorted to the tank, or a thermostat not clamped tight against the tank so it cannot sense the water. Find which before pressing it. Replacing a working ECO, or pressing it and leaving, puts the heater back into the condition that overheated it.
Replace a thermostat with the power locked out and verified dead, labeling each wire before removal. Seat the new one flat against the tank in its clip; a thermostat standing off the steel reads late and overheats the water.
Set both thermostats alike, commonly 120 F, the same number on both dials. Setting the lower higher than the upper does not make more hot water; it makes the bottom of the tank hotter than the top and wastes energy.
Restoring power and the finish test
With covers on, everyone's hands out of the cabinet and the tank full, remove your lock and close the breaker. Stand to the side of the panel, not square in front of it, as you close it. The finish test is three observations:
- The upper element heats first. With the cover removed, a clamp meter around one lead of the upper element reads near the plate current (about 18.75 A on a 4500 W element at 240 V). Taking that reading is energized work inside the cabinet and needs the PPE and qualification your shop's NFPA 70E program assigns to it. If nobody on site is qualified, that check goes to an electrician, and the plumber confirms the repair by hot water at the tap after a full recovery.
- The upper thermostat switches to the lower. As the top of the tank satisfies, current moves to the lower element lead.
- The lower element heats. The clamp reads near plate current on the lower lead. On a 208 V supply, both readings run lower, about 208 / 12.8 = 16.25 A, because the element resistance is the same and the voltage is not.
Then replace the access cover, and confirm hot water at the furthest faucet once the tank recovers.
Worked example: the lukewarm shower
Complaint: a 50 gallon electric heater gives about five minutes of hot water, then lukewarm. Rating plate: two 4500 W elements at 240 V.
Isolation. Breaker off and locked, tester proven on a live receptacle, upper thermostat line terminals read 0 V leg to leg and leg to tank, tester proven again.
Readings, as on the sheet: upper element 12.6 ohms terminal to terminal, OL to tank on both terminals: good. Lower element OL terminal to terminal: open. Thermostats on the cold tank behaved as they should. The complaint fits an open lower element: only the top of the tank was heating.
Repair. Cold off, hot tap open, tank drained to the floor drain. Lower element removed with the element wrench; scale cleared from the spud. New 4500 W, 240 V element and gasket installed. Tank refilled until the upstairs tap ran solid, no drip at the new gasket. Wires reconnected, insulation and guards replaced, both thermostats confirmed at 120 F.
Finish test. Lock removed, breaker closed from the side. The qualified tech, in the PPE the shop's program assigns, read 18.6 A on the upper element lead, then about 25 minutes later saw the current move to the lower lead and read 18.6 A there. Covers on. Hot water at the furthest bath after recovery.
Why the meter mattered. The same complaint comes from a lower thermostat that has failed open. Changing the element on a guess would have left that fault in place and the customer at five minutes of hot water.
References
- NFPA 70E, current edition, Article 120 for establishing an electrically safe work condition
- 29 CFR 1910.333, electrical safety-related work practices (29 CFR 1926 Subpart K for construction)
- Water heater manufacturer's installation and service instructions, part of the listing
- See related: Anatomy of a Tank Water Heater and Its Connections
- See related: How to Replace an Electric Tank Water Heater