The shower goes cold and nothing you turn brings it back. When you have no hot water, the most useful move is deciding which of two unrelated problems you have, because nearly every troubleshooting list blends them. A tank that delivers nothing hot is failing at ignition or at power. A tank that gives you eight good minutes and then quits is failing at capacity or at one element. The fixes barely overlap.
This post splits the symptom three ways: nothing hot at all, hot water that runs out fast, and water that only reaches lukewarm. Gas and electric are handled separately, because their failure sets have little in common. Every temperature and limit figure comes from a named primary source: the manufacturer’s installation manual, CPSC, and DOE’s Energy Saver pages.
Key takeaways
- On a gas tank, nothing hot usually means the pilot or the gas control, and the A.O. Smith ProLine gas manual requires a 10 minute wait between lighting attempts.
- On an electric tank, a burned out upper element, a broken upper thermostat or a tripped energy cut off each give you nothing, while a failed lower element gives a short supply.
- A.O. Smith says not to restore power after the red reset button trips until the cause of the overheating has been repaired.
- Running out fast is a capacity problem first: the Department of Energy counts one shower as 20 gallons.
- CPSC recommends 120F, where a serious burn takes more than 5 minutes, against roughly 2 seconds at 150F.
Nothing hot at all and running out fast are different faults
Stand at the kitchen tap and time it. If the water never warms, not after two minutes and not at any fixture, the heater is not making heat. If it starts hot and turns cold after a few minutes, the heater is making heat fine and losing capacity. Those are two halves of the same machine, and the split is what most guides leave out.
The reason it matters is how an electric tank is built. The upper element heats the top of the stored water and the lower element heats the rest, in sequence rather than together. A.O. Smith’s ProLine Standard Electric manual states that the lower element does most of the heating, usually wears out first, and produces less hot water rather than none. That manual lists a burned out upper element under no hot water and a failed lower element under insufficient hot water.
A gas tank has one burner and one control, so it does not fail by halves. When a gas tank runs short rather than cold, the suspects are its volume against your demand, the dip tube, and sediment between the burner and the water above it.
The five minute check before you open any panel
Most of this narrows down with no tools. Work the sequence in order.
- Open the hot tap at the kitchen sink and run it two full minutes, because long pipe runs hold a lot of cold water. Then try a second fixture. One cold shower with hot water elsewhere is a failed mixing cartridge, not a heater fault.
- Decide whether you are getting nothing or a short supply. Time the hot water at a running shower. Under about five minutes on a full size tank is a capacity fault.
- Identify the fuel. A gas tank has a round control knob and a flue leaving the top. An electric tank has conduit entering the top and bolted access panels.
- On a gas tank, count the flashes on the status light. One flash is normal operation. No flashing means the control is off or the pilot is out.
- On an electric tank, check the breaker panel. The heater runs on a double pole breaker, so look for a handle out of line with its neighbors. Reset it once only.
- Before removing any electrical cover, switch the breaker off and confirm the circuit is dead with a non-contact tester, which the A.O. Smith manual requires.
- If you smell gas at any point, stop and leave the building.
Nothing hot at all on a gas water heater
On a gas tank the answer is almost always that the pilot is out or the gas control has shut itself down, and the status light tells you which. Bradford White is blunt about a dead pilot: under normal operation it should only have to be lit once, during installation and initial setup. A pilot that went out on a heater running for years did not go out at random, and relighting it without finding out why buys a few days.
The lighting procedure in the A.O. Smith ProLine Ultra-Low NOx gas manual goes like this. Turn the knob to Pilot, press it fully in and hold it, then click the igniter about once a second for up to 90 seconds until the status light blinks. If it does not blink inside 90 seconds, wait 10 minutes and try again, up to three attempts, because the circuitry in that valve requires the gap. After three failures, turn the knob to OFF and call the gas supplier.
When a pilot lights and then dies, the manual’s causes are short and physical: loose thermopile connections, air in the gas line, low gas pressure, dirt in the lines, cold drafts across the burner, or a dead igniter. The thermopile proves a flame is there, and when it weakens the valve shuts gas to the burner and the pilot on purpose. The same flame proving logic explains why a furnace that will not turn on often comes down to a similar circuit.
Combustion air is the cause people miss. The manual says inadequate combustion air can shut the heater off, and defines a confined space as under 50 cubic feet per 1,000 BTU per hour of input. For a 40,000 BTU heater that is 2,000 cubic feet, which a finished closet does not have. The flame trap at the base clogs too.
Nothing hot at all on an electric water heater
An electric tank that makes nothing is a power problem, an element problem, or a limit that has opened. The A.O. Smith troubleshooting table names them in that order: no electric power, a burned out upper element from dry firing, a tripped energy cut off, reversed inlet and outlet connections, a broken upper thermostat, and a hot side leak larger than the heater’s output. That last one matters, because a hidden leak empties the tank as fast as it heats, so if anything is damp near the unit, start with a water heater leaking.
Dry firing is the most common way a brand new electric heater arrives with nothing hot in it. The manual is explicit that the tank must be empty of air and full of water before power is connected, and that running a hot tap wide open for at least three minutes is what gets the air out. Energize a partly empty tank and the upper element, sitting in air, burns out in seconds. A.O. Smith calls that an installation error its warranty does not cover.
Reversed connections confuse people badly. Cold water arriving at the hot port enters the top of the tank beside the outlet and goes straight to the fixtures, so the tank fills with hot water you never get. Check it on any heater just replaced, before blaming low water pressure in the house.
A reset button that keeps tripping is a warning
The red button on the upper thermostat is the energy cut off, usually written ECO, and it is not a nuisance switch. It opens when the water got hot enough to be dangerous. Pressing it with the power off and listening for a click is a legitimate test, and a click means it had tripped. What you do next is the part that matters.
A.O. Smith’s instruction is one sentence worth reading twice: do not turn the power back on until the cause of the overheating has been identified and repaired. An ECO only opens after something else failed toward more heat. A thermostat with welded contacts, or an element shorted to the tank, keeps pouring energy into the water with nothing calling for heat. A large hot discharge from the relief valve points at the same two parts.
The numbers show why this is not small. Bradford White states an ECO limit of 188F on residential models, and the A.O. Smith gas control shuts down above 189F. CPSC puts a serious burn at about 2 seconds at 150F, so water approaching 190F at a tap is not theoretical. A heater that trips its ECO twice has told you its temperature control no longer works, and the fix is a new thermostat and element, not pressing the button again. The manual cites ASSE 1070: a thermostat should never be the only thing between the tank and a scald.
Gas controls carry the same limit inside the valve. The A.O. Smith gas manual resets it by turning the knob to OFF, unplugging the thermopile, waiting about three minutes, reconnecting and restarting at PILOT, then says that if the limit actually tripped, a qualified person should replace the gas control valve.
Hot water that runs out faster than it used to
Start with arithmetic before parts, because a tank that was always marginal feels like a tank that just broke. DOE’s sizing worksheet puts one shower at 20 gallons, shaving at 2, hand dishwashing at 3, an automatic dishwasher at 7, and a clothes washer at 25 for a top loader or 15 for a front loader. Its worked example reaches 66 gallons of peak hour demand, and DOE’s rule is that the first hour rating on the label must at least match it.
If the tank is big enough and still runs short, work the A.O. Smith list for insufficient hot water: a faulty mixing valve at a faucet, thermostats set too low, a tank too small for demand, a melted dip tube, a plumbing leak, a failed lower element or thermostat, and low supply voltage. The lower element is the usual answer on an electric tank, and the signature is consistent: the first few minutes are properly hot, then it falls away rather than stopping.
The dip tube is the sleeper. It carries incoming cold water down to the bottom so the burner or lower element heats it and the hot stays at the top. Crack or melt it near the top and cold mixes straight into the outlet, halving usable capacity without breaking anything electrical. Bradford White’s service manual tells technicians to check the inlet nipple for cracks and replace rather than repair it.
Sediment is the other long term cause. The A.O. Smith gas manual says to drain and flush every 6 months and warns that sediment causes noise and premature tank failure. Bradford White’s FAQ is plainer: water heaters are generally not self cleaning, and its Hydrojet system does not replace draining the tank. A tank never flushed in 10 years carries scale that insulates the burner from the water above. If flushing brings heavy debris and recovery is still poor, read about when to replace a water heater before buying more parts.
Water that is only lukewarm and never hot
Lukewarm is its own category with a short suspect list. On an electric tank it points at the upper element or thermostat doing part of a job, or both thermostats set too low. A.O. Smith says to set both the same and never the upper above the lower, because that makes the tank run short. On a gas tank, look at the knob: it adjusts from 55F to 155F, Hot is roughly 120F, and the vacation setting sits near 55F. A knob left on VAC produces exactly this complaint.
A crossed connection survives every parts replacement. If a single handle faucet has a failed cartridge, or a thermostatic mixing valve has given up, cold water feeds backwards into the hot side and dilutes everything downstream. Shut the cold supply valve at the heater’s inlet and open a hot tap. If water keeps running steadily after the tank’s pressure drops, something is pushing cold into the hot line.
The 120F setting and what these repairs cost
CPSC recommends setting a residential water heater to 120F and gives the reason in seconds. Time to a third degree burn for an adult is about 5 minutes at 120F, about 30 seconds at 130F, about 6 seconds at 140F, and about 2 seconds at 150F. A.O. Smith ships electric tanks set to approximately 120F, and its table adds about 10 seconds at 135F.
There is a real tension here. Higher storage temperatures control bacteria better, and A.O. Smith cites 140F as the setting that does it, while CPSC points at 120F for burn risk. DOE puts water heating at about 18% of a home’s energy use, the second largest energy expense in most houses. The usual resolution is storing hotter and mixing down at the outlet, which is what Bradford White describes a mixing valve doing.
| Symptom | Most likely cause | Where to check |
|---|---|---|
| Nothing hot, gas tank | Pilot out or gas control shut down | Status light flash count |
| Nothing hot, electric tank | Breaker, upper element, or tripped ECO | Panel, then upper thermostat |
| Nothing hot on a new install | Dry fired upper element | Upper element resistance |
| Hot then cold within minutes | Lower element or melted dip tube | Lower access panel, inlet nipple |
| Gradual loss over years | Sediment or an undersized tank | Flush the tank, check first hour rating |
| Lukewarm only | Setting, mixing valve, or crossed line | Thermostat dial, then isolate the cold |
On cost, this post quotes HomeAdvisor’s published 2025 ranges and nothing else, because no federal or manufacturer source publishes repair pricing. HomeAdvisor lists $200 to $300 for a professional heating element replacement, $150 to $200 for a thermostat with the part around $20, about $150 for a thermocouple, about $350 for a gas control valve, and about $200 for a tank flush. Those are national figures, not a quote for your house.
| Status light | What it means | What to do |
|---|---|---|
| No flash | Control off or pilot out | Follow the lighting sequence |
| One flash | Normal operation | Look past the gas control |
| Two flashes | Low thermopile voltage | Thermopile needs replacing |
| Four flashes | Overheat failure | Stop and call a technician |
| Ten flashes | Contaminants in the chamber | Stop and call a technician |
When to stop and call a professional
If you smell gas, follow the instruction printed on the front of every gas water heater manual. Do not try to light any appliance. Do not touch any electrical switch. Do not use any phone inside the building. Leave, call the gas supplier from a neighbor’s phone, and call the fire department if you cannot reach them. Switches and phones are named because either can make a spark.
Stop on a repeat ECO trip. Once is a fault you can chase. Twice is a temperature control that has failed toward more heat, and the tank can run away toward that 188F to 189F limit while you stand next to it. Replacing a thermostat and element is within reach with the breaker locked off and a meter, but a second trip means the circuit is still wrong.
Stop on the gas control valve itself: the A.O. Smith manual is direct that a tripped limit calls for a qualified person to replace it. Stop if the relief valve is discharging hot water, because pressure or temperature is out of range. Stop if the tank is weeping from the shell rather than a fitting, because that is rust through.
Frequently asked questions
Why is there no hot water when the breaker has not tripped?
Power can be present and the heater still dead. A.O. Smith lists a burned out upper element, a broken upper thermostat and a tripped energy cut off as separate causes, and none trip the breaker. Switch the breaker off, confirm with a non-contact tester, then press the red button and listen for a click.
How long should you wait between attempts to light a gas pilot?
Ten minutes, according to the A.O. Smith ProLine gas manual, which states the circuitry in the gas valve requires that gap. It allows up to three attempts of 90 seconds each. If the status light has not blinked after the third, turn the knob to OFF and call the gas supplier.
Can sediment really stop a water heater from making hot water?
Rarely outright, but it cuts how much you get and how fast the tank recovers. A.O. Smith’s gas manual specifies draining and flushing every 6 months and warns that sediment causes noise and premature tank failure.
What size tank stops the hot water running out?
Match the first hour rating on the label to your peak hour demand. The Department of Energy counts a shower as 20 gallons, a top loading clothes washer as 25 and an automatic dishwasher as 7, reaching 66 gallons in its worked example.
Is 120F hot enough, or should the tank be hotter?
CPSC recommends 120F and A.O. Smith ships electric tanks set there. A serious burn takes more than 5 minutes at 120F and roughly 2 seconds at 150F. If you want 140F for bacteria control, the manufacturer’s position is that point of use mixing valves become essential.
Name the symptom precisely before buying anything. Nothing hot at all, a supply that quits after five minutes, and water that never gets past lukewarm send you to three different parts of the same appliance, and a flash count or a click from a reset button usually tells you which. Where the trail leads to a gas valve, a repeat limit trip, or a smell of gas, the answer stops being a part and becomes a phone call from outside.
Sources
- US Department of Energy, Energy Saver, Water Heating
- US Department of Energy, Energy Saver, Sizing a New Water Heater
- US Consumer Product Safety Commission, Tap Water Scalds, CPSC Publication 5098
- A.O. Smith, ProLine Standard Electric Water Heater Instruction Manual 100309797
- A.O. Smith, ProLine Ultra-Low NOx Gas Water Heater Instruction Manual 100263190
- Bradford White, Residential Water Heater Frequently Asked Questions
- Bradford White, RG2PDV40S10N Residential Gas Water Heater Service Manual
- HomeAdvisor, How Much Does a Water Heater Repair Cost in 2025