To replace the thermostat on an electric water heater, switch off the breaker and prove the power is dead with a tester. Then remove the access panel and insulation, swap the old thermostat for a matching one, set it, and replace the cover before restoring power. A typical tank has two thermostats and two heating elements. Replace the one that failed, or both if the tank is old.
These steps are for electric tank heaters only. If you smell gas, leave the building and call 911 or National Grid at 1-800-233-5325 before you call us. For gas, oil or tankless units, or if any wire looks scorched, call (617) 752-0573 for water heater repair.
Signs the thermostat is the problem
Elements, thermostats and the high-limit switch all produce similar complaints. Rule out the cheap items first.
- Lukewarm water. The lower thermostat or lower element is failing.
- Water too hot. A stuck thermostat that never shuts the element off. Treat as a scald risk.
- No hot water at all. Check the breaker and the red reset button on the upper thermostat before replacing anything.
- Reset button trips repeatedly. A shorted element or a bad upper thermostat.
- Hot water runs out fast. The lower element or lower thermostat is not working, or the tank is full of sediment.
Not sure it is the thermostat?
Call dispatch with the model number and what you see.
Power off, then prove it
Turn off the double-pole breaker for the water heater. Do not rely on the dial alone. Remove the upper access panel and use a non-contact voltage tester or a multimeter across the terminals. It must read zero before you touch any wire.
Water heaters run on 240 volts. That is enough to kill you. If you cannot confirm the power is off, if the panel is wet, or if the wiring looks burned, stop and call.
Replacing an electric thermostat, step by step
Have a Phillips screwdriver, a flat-head, a multimeter or tester, and the replacement thermostat in hand. Match the part to the manufacturer and model on the tank label.
- Kill the power. Breaker off, then test at the terminals.
- Open the panel. Remove the screws and fold back the insulation. Take a photo of the wiring.
- Remove the plastic cover. A protector sits over the upper thermostat. Lift it off.
- Disconnect the wires. Loosen the screws or pull the connectors, one at a time. Label each.
- Pull the old thermostat. It clips against the tank. Slide it out.
- Press in the new one. It must sit flat against the steel for the sensor to read the tank. Gaps cause wrong readings.
- Reconnect and tighten. Match the photo. Wires on terminals should be snug, with no bare copper outside the screw.
- Set the dial. Many manufacturers set the thermostat around 120 degrees Fahrenheit. The state table for residential service lists 120 minimum and 130 maximum.
- Close it up. Replace the insulation, the cover and the panel. Make sure the tank is full of water before power comes on.
Never power a tank that is not full
Electric elements burn out in minutes without water around them. After any repair, open a hot tap and let it run until water flows steadily with no air spitting. Only then restore the breaker.
Wait about an hour, then check the water temperature at a tap. Check again that evening. A properly installed thermostat should hold steady and the reset button should not trip.
Check the elements before blaming the thermostat
If the new thermostat does not fix it, the element is the next suspect.
| Test | Reading | Meaning |
|---|---|---|
| Voltage at upper thermostat | 240 V present | Power is arriving. Thermostat or element may be at fault. |
| Voltage at lower thermostat | None | Upper thermostat or high-limit is open. |
| Element resistance, power off | About 10 ohms (for a typical 4500 watt unit) | Element is good. |
| Element resistance, power off | Infinite or zero | Element is open or shorted. Replace it. |
| Element to ground, power off | Any reading | Element is leaking to the tank. Replace it. |
Disconnect one element wire before testing resistance. Skipping this gives false readings.
Call a professional if any of these show up
Scorch marks, melted wire insulation, a tripped breaker that trips again, water leaking from the tank or a rusty bottom all point past a thermostat. A leaking tank cannot be repaired, and the answer is water heater replacement.
Tanks over about ten to twelve years old are often better replaced than repaired, because the next failure usually follows soon. Also call if the temperature and pressure relief valve discharges. The state plumbing code (248 CMR) says its discharge pipe must run full size with no shutoff to within twelve inches of the floor, and that piping is not a place for a shortcut. We make these calls in homes from Newton to West Roxbury as well.
What Brookline owners should know before opening the panel
The Brookline Building Department says all work associated with wiring, plumbing or gas fitting must be performed by licensed individuals, and homeowners cannot pull plumbing or gas permits (per the Town's published FAQ). A like-for-like part swap on an existing appliance is maintenance, but if you are unsure that it counts, ask the Building Department before you start or call a licensed plumber.
Massachusetts 248 CMR 10.00 says a check valve must not be installed in the cold-water supply to a water heater, and that a closed system needs a properly sized expansion tank (per the state code). If your tank often weeps from the relief valve, the problem may be thermal expansion, not the thermostat.
Brookline tenants make up about 53 percent of occupied homes (U.S. Census American Community Survey, 2020 to 2024). If you rent, the heater is usually the landlord's, so tell them about a failing thermostat. State rules set 120 to 130 degrees as the residential range, which helps with scald safety. Mass Save also lists heat pump water heater rebates if the tank is near the end of its life.
Water heater thermostat questions
On an electric tank, many homeowners can, if they shut off the breaker, test for zero voltage and match the part. If any step feels uncertain, call a licensed plumber.
Test them. With power off and one wire disconnected, an element should read a few ohms of resistance. An open element or a short to the tank means replacement. If the element tests fine, suspect the thermostat.
Around 120 degrees Fahrenheit is common. The state code lists 120 as the residential minimum and 130 as the maximum. Hotter water raises scald risk.
Usually a shorted element or a failed upper thermostat. Resetting it repeatedly is not safe. Turn off the breaker and call for repair.
No. Gas units use a gas control valve with a thermocouple. If you smell gas, leave and call 911 or National Grid at 1-800-233-5325. Then call us.
If the tank is more than a decade old or rusted at the bottom, a new tank is often the better spend. We can look at it and tell you honestly.
Yes. Dispatch is open 24/7, and we offer same-day appointments when available. A leaking tank should be shut off at the cold inlet and the breaker.
Prefer a licensed plumber?
Call (617) 752-0573 and we will check the heater safely.