Water Heater Not Getting Hot - What It Means and How to Fix It

No hot water is a household emergency. Whether you have a 40-gallon gas water heater or a 50-gallon electric, the diagnosis process is different — and so are the fixes. This guide covers both. Work through your applicable section first: find your water heater type (gas burner at the bottom, or electrical panels on the side) and start there.

What Does “Water Heater Not Getting Hot” Mean?

Your water heater is running — or trying to — but the water coming out of your faucets is cold or lukewarm. The root cause depends on your heater type:

Gas water heaters fail due to:

1. Pilot light out or won’t relight
2. Failed thermocouple or thermopile
3. Failed gas valve
4. Dirty or obstructed burner assembly
5. Sediment buildup reducing efficiency

Electric water heaters fail due to:

1. Tripped circuit breaker or GFCI
2. Failed upper or lower heating element
3. Failed thermostat (upper or lower)
4. Tripped high-temperature cutoff (reset button)

There’s also a non-failure cause: demand exceeds capacity. A 40-gallon water heater serving a household of five people during back-to-back showers will run out of hot water. If your hot water “runs out” faster than it used to, sediment buildup reducing usable capacity is the likely cause — not a component failure.

How to Fix It

Gas Water Heater Diagnosis

Step 1: Check the Pilot Light (5 minutes)

Look at the bottom of your water heater. Most gas water heaters have a small window where you can see the pilot flame. If you see nothing, the pilot is out.

To relight (standard standing pilot models):

1. Turn the gas control knob to “PILOT” position
2. Press and hold the knob down (or press the separate pilot button if your model has one)
3. While holding, press the ignitor button repeatedly until you see the pilot light
4. Continue holding the knob down for 30–60 seconds (this heats the thermocouple)
5. Release slowly. If the pilot stays lit, turn the knob to your desired temperature setting (usually 120°F for anti-scald safety)

If the pilot lights but goes out immediately when you release the knob, the thermocouple has failed (Step 2). If the pilot won’t light at all, check your gas supply (are other gas appliances working?).

Newer “intermittent pilot” or “direct spark” models don’t have a standing pilot — they use electronic ignition. If these don’t ignite, you’ll usually see an error code on a status LED.

Step 2: Replace the Thermocouple (30 minutes)

The thermocouple is a safety device — a metal probe that sits in the pilot flame and generates a small electrical voltage when heated. This voltage holds the gas valve open. If the thermocouple fails, the gas valve closes for safety even if the pilot is lit.

Part number guidance: Most thermocouples are universal. The most common replacement is the Rheem SP11354 (30-inch thermocouple), compatible with Rheem, A.O. Smith, Bradford White, State, and most other brands. Measure the length of your existing thermocouple before purchasing.

How to replace:

1. Turn off gas at the shutoff valve near the heater
2. Disconnect the thermocouple from the gas valve (it threads in, usually 3/8-inch)
3. Unclip the thermocouple from the pilot assembly
4. Thread the new thermocouple into the gas valve (finger-tight plus 1/4 turn — don’t over-tighten)
5. Clip into the pilot bracket
6. Restore gas and relight

Thermopile note: Some gas water heaters use a thermopile instead of a thermocouple. The thermopile generates more voltage and powers a digital control. Thermopile replacement is identical to thermocouple but you need to match the connector type.

Step 3: Check the Gas Valve / Thermostat

The gas valve and thermostat are usually integrated on a gas water heater — the large “box” at the front with the temperature knob. If the thermocouple is good but the burner still won’t stay on, the gas valve itself may have failed.

Signs of a failed gas valve:

• Thermocouple tests fine (generates proper millivolt output) but pilot won’t stay lit
• Status LED (if present) shows an error code (check the label on the heater)
• Burner comes on but gas pressure is too low to maintain flame

Gas valve replacement on a water heater is a doable DIY job for someone comfortable with gas work — but if you’re not, call a plumber. The valve is a critical safety component.

Common gas valve part numbers:

• Honeywell WV8840B1109 (used on many A.O. Smith and Rheem models)
• White-Rodgers 37C73U-173 (Bradford White)
• Resideo WV8840C1500

Step 4: Flush Sediment

Over years of use, minerals in your water supply (calcium carbonate, etc.) accumulate as sediment at the bottom of the tank. This sediment insulates the burner from the water, reducing efficiency and eventually causing the water to sound like “popcorn popping” when heated. Severe sediment can reduce a 40-gallon tank to effectively a 25-gallon tank.

How to flush:

1. Connect a garden hose to the drain valve at the bottom of the tank
2. Turn off the cold water supply inlet valve
3. Open a hot water faucet somewhere in the house (prevents vacuum)
4. Open the drain valve and let the tank drain fully
5. Open the cold supply briefly to stir up and flush remaining sediment
6. Close the drain valve, refill, relight

Electric Water Heater Diagnosis

Step 1: Check the Circuit Breaker (2 minutes)

An electric water heater typically runs on a 240V, 30-amp dedicated circuit. Go to your electrical panel and look for the water heater breaker. If it’s tripped (in the middle position), reset it by switching fully to OFF, then back to ON.

If it trips immediately: a heating element has failed and shorted, which means a short circuit. Replace the element (Step 3).

Step 2: Press the Reset Button (5 minutes)

Every electric water heater has a high-temperature cutoff (also called an ECO — Emergency Cutoff) located behind the upper access panel on the side of the heater. It’s a small button, usually red.

To access:

1. Turn off the breaker for the water heater
2. Remove the upper access panel (2–4 screws)
3. Peel back the insulation
4. Press the red reset button firmly until you hear a click

If the button immediately trips again, the upper thermostat has failed and is allowing the water to overheat. Replace the thermostat.

Step 3: Test and Replace Heating Elements (45 minutes)

Standard residential electric water heaters have two heating elements — upper and lower — each typically 4,500W at 240V. The upper element handles recovery (reheating from cold), while the lower handles most of the continuous heating. Either one can fail.

Tools needed: Multimeter, 1-1/2-inch element socket (specialty socket — available at any hardware store), channel-lock pliers.

How to test:

1. Turn off the breaker
2. Access the element through its access panel
3. Disconnect the two wires from the element (label them)
4. Set multimeter to resistance (ohms)
5. Touch probes to the two element terminals
   • A good 4,500W element should read approximately 12.8 ohms
   • A good 3,500W element should read approximately 16.4 ohms
   • Reading of ∞ (infinite/open): Element has burned out — replace it
   • Reading of 0 (zero/shorted): Element is shorted — replace it, and check why the breaker tripped

Common element part numbers:

• Rheem SP10874: 4500W/240V screw-in element (fits most Rheem/Ruud/GE models)
• A.O. Smith 100112013: 4500W/240V universal element
• Camco 02162/42H: 4500W premium titanium element (lasts longer in hard water)

To replace: Close the water supply, open a hot faucet to release pressure, drain a few gallons (enough to get below the element level), unscrew the old element, install new with a fresh gasket, reconnect wires.

Step 4: Replace Thermostats

Each element is controlled by its own thermostat. The upper thermostat is master — it energizes the upper element first, then switches power to the lower thermostat once the upper portion is hot. If the upper thermostat fails, neither element gets power.

Testing: With power off, use a multimeter to check continuity across thermostat terminals at room temperature. A good thermostat should show continuity (close to 0 ohms). No continuity at room temperature = failed thermostat.

Common thermostat part numbers:

• Rheem SP5988: Upper thermostat, 240V (fits many Rheem/GE models)
• A.O. Smith 9000547015: Upper thermostat
• Camco 07083: Universal upper thermostat

Parts You May Need

When to Call a Pro

Call a plumber or water heater technician when:

• You smell gas near a gas water heater. Leave the house, leave the door open, call the gas utility from outside.
• The tank is leaking. A leaking tank cannot be repaired — it needs replacement. Water heater lifespan is 8–12 years. If yours is leaking and over 10 years old, replace it. Emergency plumber calls for a tank replacement typically run $800–$1,800 including a new unit.
• You’re not comfortable working with gas components. The thermocouple is a safe DIY job. The gas valve is borderline. Anything involving the gas supply line itself should be done by a licensed plumber or gas fitter.
• The electrical panel shows signs of burning or melted insulation near the water heater breaker. This is a safety issue that requires an electrician before the water heater is repaired.
• Multiple elements and thermostats have been replaced and the heater still doesn’t work. At this point, internal wiring or an intermittent connection is likely. The cost of diagnosis may exceed the value of an aging tank.

Average repair costs:

• Thermocouple replacement: $75–$150 (or DIY for $12)
• Gas valve replacement: $300–$600
• Element replacement (electric): $150–$250
• Thermostat replacement (electric): $150–$200
• Full replacement (gas 40-gallon): $800–$1,400 installed
• Full replacement (electric 40-gallon): $600–$1,100 installed

Frequently Asked Questions

Q: My water starts out hot but goes cold quickly. What’s wrong?

A: This is a capacity issue, not a failure. Either: (1) your household demand exceeds your tank capacity, (2) sediment has reduced usable capacity, or (3) on electric units, the lower heating element has failed (the lower element maintains the bulk of the stored hot water; if it fails, only the upper portion of the tank is reliably hot, giving you 10–15 minutes of hot water instead of 30–40). Test and replace the lower element.

Q: My gas water heater pilot stays lit but the burner never fires. What’s the problem?

A: The most common cause is a failed gas valve thermostat (the temperature-sensing component). The pilot flame heats the thermocouple, which confirms to the gas valve that the pilot is present — but a separate thermostat inside the gas valve determines when to open the main burner port. If this thermostat reads incorrectly, the burner never opens. Replace the gas valve.

Q: How do I know if my water heater is gas or electric?

A: Look at the bottom of the unit. A gas water heater will have a burner assembly and a pilot assembly at the base, with a gas line connecting to it. An electric water heater will have access panels on the side covering the heating elements and thermostats, with no gas line. Electric heaters also need a 240V circuit (large breaker, often 30A).

Q: Can I set my water heater thermostat higher to get hotter water?

A: You can, but 120°F is the recommended maximum for households with children or elderly occupants — scalding occurs within seconds at 130°F+. If you want “hotter” water without increasing the tank temperature, check that no mixing valve on the water heater is reducing the outlet temperature. Some newer code-compliant installations include an anti-scald mixing valve at the heater outlet.

Q: My electric water heater trips the breaker immediately when reset. What does that mean?

A: A heating element has failed short-circuit. Instead of burning open (like a blown fuse), the element’s internal resistance dropped to near zero, drawing far more current than the breaker allows. The breaker trips to protect the wiring. You need to replace the shorted element before the heater will operate. Locate which element is shorted by testing with a multimeter (Step 3 above) and replace it.