THE SHORT VERSION
- One tap or all of them? If only one shower or faucet runs cold, the problem is usually that fixture’s valve, not the water heater.
- Electric tank: check the breaker (and any disconnect switch near the heater). If the power is on, the red reset button behind the upper access panel may have tripped. Reset it once with the power off; if it trips again, call a pro.
- Gas tank: make sure gas is reaching the heater (does the stove work?), then check whether the pilot is out and relight it by following the label. A pilot that won’t stay lit, or a safety shutoff, is a job for a technician.
- Tankless and heat pump: read the code on the display first. Tankless units also need a minimum flow, power and gas; heat pump models may be in Vacation mode or out of their temperature range.
- Stop and call if you smell gas, see scorch marks or water near the controls, or a reset or relight fails twice.
Two questions before you touch the heater
Is it every hot tap, or just one? If a sink runs hot but one shower doesn’t, the heater is working. Moen’s troubleshooting for a shower that isn’t hot enough checks the hot supply valves, then the handle’s temperature limit stop (which caps how far the handle turns toward hot), then the cartridge inside the valve. A.O. Smith also lists a failed mixing valve in a shower control, which can make water seem cold “even though there is plenty of hot water in the tank.”
No hot water, or not enough? Water that is hot for a few minutes and then runs out is a different problem, often a failed lower element, a broken dip tube, sediment or an undersized heater. See running out of hot water.
Electric tank: power first, then the reset button
A.O. Smith’s 2026 electric-tank manual lists the likely causes of no hot water: no power, a burned-out upper element, a tripped Energy Cut Off (ECO), reversed connections on a new install, a broken upper thermostat, or a hot-water leak bigger than the heater can keep up with. You can check the first and third.
1. Check the breaker and any disconnect
Electric tanks run on a 240-volt double-pole breaker, often labeled “water heater,” and some also have a disconnect switch near the tank. Make sure both are on; a tripped breaker often sits between on and off, so switch it fully off, then on. If it trips again, leave it off: Oregon’s State Fire Marshal lists repeatedly tripping breakers as a reason to call an electrician. If the breaker is on and the heater still has no power, A.O. Smith says to have the wiring checked.
2. Check the high-limit (ECO) reset button
The ECO is a safety switch on top of the upper thermostat. If the tank water gets too hot, it cuts power to the elements and stays off until someone presses its red reset button. Rheem’s service bulletin explains the mechanism: heat from the tank wall makes a metal disc snap and push out a pin, opening the contacts. A.O. Smith says a trip “in most cases” means an element or thermostat problem, so treat it as a symptom.
Turn the power off at the breaker
The ECO sits next to live 240-volt terminals. A.O. Smith’s manual says to confirm the power is off with a non-contact circuit tester before going further.
Remove the upper access panel and move the insulation
The panel is the upper of the two covers on the side of the tank. Fold back or lift out the insulation pad behind it. Don’t touch the wires or terminal screws.
Press the red button firmly
A metallic click means it had tripped and is now reset. No click means it hadn’t tripped, and A.O. Smith says the upper thermostat should then be checked by a qualified person.
Put the insulation and panel back, then the power
A.O. Smith’s manual stresses that every cover is secured before power is restored. A full tank of cold water can take several hours to heat; A.O. Smith says to troubleshoot further only if there is still no hot water after two hours.
How the elements and thermostats share the work
Most electric tanks have two elements, each with its own thermostat, and usually only one heats at a time. Rheem explains the order: the upper thermostat heats the top of the tank first, then passes power to the lower one. So a dead upper element or thermostat means no hot water, while a dead lower element leaves some hot water that runs out fast. A.O. Smith notes the lower element usually fails first because it does most of the work.
Gas tank: gas supply, pilot and the safety shutoffs
A standard gas tank needs three things to heat: gas at the control valve, a lit pilot (or working igniter) and no safety device tripped. Check them in that order.
Is gas reaching the heater?
- Try another gas appliance. If the stove won’t light either, the problem is upstream: a utility outage, a meter shut off or an empty propane tank. Call the gas company or propane supplier.
- Don’t turn a shut-off meter back on yourself. San Diego Gas & Electric, for example, tells customers to call it to restore service and relight pilots.
- Propane that ran out needs a leak check before the gas goes back on. A brochure from Mississippi’s State Fire Marshal says many states require the supplier or a qualified technician to do it.
- A closed valve on the heater’s gas line may have been closed for a reason. Ask before opening it.
Is the pilot lit?
Look through the sight glass near the bottom. Many newer gas valves also have a status light: on A.O. Smith’s atmospheric models, no flashing means the pilot is out and one flash every 3 seconds is normal. If the pilot is out and you don’t smell gas, relight it by following the label. A.O. Smith’s label says to wait 10 minutes with the knob off, try up to three times with 10 minutes between attempts, then call a technician or the gas supplier. Details are in pilot light problems.
Lights, then goes out: the thermocouple or thermopile
A thermocouple (or the larger thermopile on many newer valves) is a small rod that sits in the pilot flame and turns its heat into a tiny electric current. That current tells the gas valve the pilot is burning; if the flame goes out, the current stops and the valve shuts off the gas. Bradford White’s service manual lists a weak or poorly connected thermocouple, a pilot flame that doesn’t surround it, and a faulty magnet in the gas valve as causes of a pilot that goes out when you release the knob. All are parts a technician replaces.
Flammable vapor shutoffs on newer tanks
Many gas tanks, including the A.O. Smith, Bradford White and Rheem models cited here, are flammable vapor ignition resistant (FVIR): if gasoline or solvent fumes reach the burner, a flame arrestor keeps the fire inside and a thermal switch or vapor sensor shuts the heater down. Bradford White’s service manual says to find out why its switch opened before resetting it. A.O. Smith says a heater that stopped and won’t relight should be checked for blue or black discoloration on the flame arrestor; after a vapor ignition, the whole heater is replaced.
Gas valves also have their own high-limit shutoff. On A.O. Smith’s atmospheric models it is built into the gas control, flashes 4 times after a relight, and is not resettable: the gas control valve is replaced. Turn the knob off and call.
Power-vent gas tanks: power, codes and a blocked vent
A power-vent tank has a blower on top that pushes exhaust out through plastic pipe, and an electronic igniter instead of a standing pilot. That adds three causes of a cold tank:
- No electricity. The blower runs on 120 volts, usually through a plug-in cord. Rheem’s troubleshooting starts with an ON/OFF switch on the blower that may be off and a blower that may be unplugged. A power outage stops it too.
- A lockout code. The control flashes a code when something fails. On A.O. Smith’s power-vent models, three trials to light the burner end in a lockout. Its manual says to reset a soft lockout by sliding the ON/OFF switch off, waiting 10 seconds and switching it back on, and warns that the heater will lock out again if the cause hasn’t been fixed.
- A blocked or restricted vent. A pressure switch shuts the heater down if the exhaust can’t get out. Look at where the vent leaves the house: Baltimore Gas and Electric (BGE) tells customers to keep exhaust vents for high-efficiency gas appliances, water heaters included, clear of snow and ice. Clear snow away from the outside opening by hand; don’t take vent pipes apart.
If the same code returns after one reset, or the code points to a vapor sensor, gas valve or temperature limit, call a technician. Rheem’s manual says repairs to the gas control, burners and vent connectors belong with qualified service personnel. For how these vents are built, see water heater venting.
Tankless: read the code, then check flow, gas and power
A gas tankless heater shows a code on its controller when it shuts down. Write it down before you do anything else; codes differ by brand, and Rinnai’s FAQ says to contact Rinnai or your installer to find the root cause. Our tankless problems and error codes guide groups the common ones by symptom. Without a code, check these:
- Flow too low. Rinnai says its gas units need 0.4–0.6 gallons per minute to ignite and 0.26 to keep running; Rheem gives about 0.3 for its electric models. Open the tap further and clean clogged aerators. Rinnai also suggests a clean inlet filter and a lower set temperature, so less cold water is mixed in at the tap.
- No gas. Rinnai’s codes for no ignition (11) and no flame (12) start with checking that gas is on at the heater and meter, and that a propane tank isn’t empty.
- No power. Rinnai says its units won’t operate in a power outage. Electric tankless models can use several breakers; Rheem says make sure all are on.
- A blocked vent or full condensate trap: Rinnai’s code 10.
- Freezing. Rinnai’s freeze protection needs power and gas; if either is lost below freezing, it says to drain the unit. The pipes to the heater aren’t protected.
- Scale from hard water. Rinnai links its overheat code (14) partly to scale. See descaling a tankless heater.
Heat pump: check the mode, the display and the room
A heat pump water heater is an electric tank with a compressor on top, so the electric-tank checks apply: breaker, then error codes. A.O. Smith’s hybrid manual adds causes of its own:
- A blank display usually means no power. Check the breaker.
- Vacation mode holds the tank at 50°F on A.O. Smith’s models. Switch back to the default Hybrid mode.
- A room that is too cold or hot. The compressor runs only between 37°F and 120°F; outside that range A.O. Smith’s control shows HPO and heats with the slower elements.
- Error codes such as 001 (dry fire: powered before the tank was full) or element and sensor failures. Many put the heater in a “limp mode” that keeps some heat going.
- A dirty air filter, on A.O. Smith’s list for water that isn’t hot enough.
- A tripped ECO next to the upper element. Same rule: reset once, call if it trips again.
Troubleshooting table: symptom, cause, safe check, when to call
| What you see | Likely cause | Safe check | Call a pro when |
|---|---|---|---|
| Only one shower or faucet is cold | Fixture valve: supply stop, temperature limit stop or cartridge | Run hot water at other taps | Other taps are hot but you can’t fix the valve |
| Electric tank, everything cold | Breaker tripped or disconnect off | Breaker fully off, then on; disconnect on | The breaker trips again |
| Electric tank, power is on, still cold | ECO tripped, upper element or upper thermostat failed | Reset the ECO once, power off | No click, or it trips again |
| Gas tank, stove won’t light either | Gas outage, meter off, propane empty | Call the utility or propane supplier | Always; they restore gas and check for leaks |
| Gas tank, pilot out | Draft, gas interruption or thermocouple | Relight by the label | It won’t light or won’t stay lit |
| Gas tank won’t relight; arrestor discolored or vapor lockout | Flammable vapor shutoff | Remove solvents, ventilate; don’t relight | Right away |
| Gas tank, status light flashes 4 times | High-temperature shutoff (A.O. Smith) | Knob to OFF | Right away; the gas valve is replaced |
| Power vent, no lights or blower | Unplugged, switch off, power outage | Plug, switch and outlet | Power is fine and it’s still dead |
| Power vent, flashing code | Lockout, blocked vent, pressure switch | Check the outside vent for snow; one switch reset | The code comes back |
| Tankless, code on display | Varies by brand | Note the code; check gas, power, flow | It returns after a reset |
| Heat pump, cold, display shows a mode or HPO | Vacation mode, or room too cold or hot | Set Hybrid mode; check the room | An error code persists |
Sources: A.O. Smith electric (2026), atmospheric gas (2021), power-vent (2023) and hybrid heat pump manuals; Rheem thermostat bulletin and power-vent manual; Bradford White service manual; Rinnai manual and FAQ; Moen. Codes and procedures vary by brand and model; your heater’s manual and label come first.
When to stop and call
Rheem’s advice to homeowners is that checking the thermostat or using the reset button is fine, and anything involving gas or electrical risk belongs with a professional. In practice, stop and call when:
- You smell gas, or a carbon monoxide alarm sounds (leave first).
- A reset or relight fails twice, or the same code comes back.
- A breaker keeps tripping, or there are scorch marks or water near the panels or junction box.
- There are signs of a flammable vapor shutoff or a high-temperature shutoff.
- The vent is blocked or disconnected inside, or water drips from the draft hood.
- The pilot keeps going out. Bradford White lists poor combustion air, a clogged vent and an unsteady gas supply among the causes.
Tell the technician
- Fuel and type, plus the model and serial number from the rating plate (the serial also gives the age; see how old is my water heater)
- Any code or status light pattern, exactly as shown
- What you already tried: breaker, reset, relight, mode change
- Whether the water got very hot or the relief valve discharged before it went cold
- Whether it’s every tap or some
Common questions
Where is the reset button on an electric water heater?
Behind the upper access panel on the side of the tank, under the insulation, on top of the upper thermostat. It is usually red and may be labeled RESET or ECO. Turn the breaker off before you remove the panel.
Why does my water heater reset button keep tripping?
The button is a high-limit switch that trips when the water gets too hot. A.O. Smith says that in most cases an element or thermostat problem is the cause, and Rheem says frequent trips mean additional problems. Leave the power off and have a technician test the elements and thermostats.
My gas water heater’s pilot is lit. Why is there no hot water?
The thermostat may be set too low, the gas supply may be weak, or the gas control may be faulty. Bradford White lists these causes for a pilot that lights but a main burner that won’t come on. Check the dial setting; beyond that, call a technician.
Why is there hot water in the kitchen but not the shower?
Then the heater is working and the shower valve is the problem. Moen’s troubleshooting checks that the supply valves are fully open, then the temperature limit stop on the handle, and then replaces the cartridge and flushes the valve body if that doesn’t fix it.
How long does a water heater take to heat after a reset or relight?
A.O. Smith says a full tank of cold water may take several hours to heat, and its electric manual says to troubleshoot further only if there is still no hot water after two hours. A gas tank of the same size usually recovers faster than an electric one.
Does a tankless water heater work in a power outage?
Not without backup power. Rinnai says its tankless heaters won’t operate in a power outage, and their freeze protection needs both power and gas, so in freezing weather the manual says to drain the unit if either is lost.
My heat pump water heater shows HPO. Is it broken?
On A.O. Smith models, HPO means the heat pump is outside its operating range: the room is below 37°F or above 120°F, or the tank water is below 55°F. The heater switches to its elements and returns to the heat pump when conditions allow.
Sources
Sources checked October 4, 2026. Prices, codes and rebates change. Check current details with your installer and local permit office.
- Residential electric water heater installation instructions and use & care guide 100404292 (March 2026) ↗ · A.O. Smith, hosted by Lowe’s
- Technical Service Bulletin: How an electric thermostat works ↗ · Rheem
- What to Do Before Calling a Plumber or Replacing Your Water Heater (December 2025) ↗ · Rheem
- Signature 100 gas water heater installation instructions and use & care guide 100341468 (2021) ↗ · A.O. Smith
- Service manual 238-51542-00C, Defender FVIR gas water heaters (2019) ↗ · Bradford White, hosted by G.T. Sanders
- Installation instructions and use & care guide, residential power-vent gas water heaters, 100366722 (2023) ↗ · A.O. Smith, hosted by Lowe’s
- Low NOx FVIR gas water heater use & care manual AP16412-1 (2017) ↗ · Rheem, hosted by InspectAPedia
- Hybrid electric heat pump water heater installation instructions and use & care guide ↗ · A.O. Smith, hosted by Lowe’s
- Residential FAQ (minimum flow, power outages, freeze protection, error codes) ↗ · Rinnai
- Tankless water heater installation and operation manual 100000773 (SENSEI RSC) ↗ · Rinnai
- Electric tankless water heater installation instructions and homeowner’s manual ↗ · Rheem
- Water temperature is not hot enough on tub/shower units ↗ · Moen
- Emergency gas shut-off ↗ · San Diego Gas & Electric
- Propane safety information brochure (Propane Education & Research Council, 2005) ↗ · Mississippi State Fire Marshal
- Winter safety tips: utility equipment and ventilation in snowy, icy conditions ↗ · Baltimore Gas and Electric (BGE)
- Electrical fire safety brochure (2023) ↗ · Oregon State Fire Marshal