Start by naming the symptom precisely, because two very different faults get described the same way on the phone.
No hot water at all means the heater has stopped producing. Cold at every tap, no recovery after an hour, and on a gas tank no burner sound at all.
Hot water that runs out fast means the heater is producing but the tank is not delivering. You get five or ten good minutes, then lukewarm, then cold, and it comes back an hour later.
The first points at ignition, gas supply or a dead element. The second points at a dip tube, a lower element or sediment. Working out which one you have takes thirty seconds and saves a wasted service call.
Gas: is the pilot out, or is the thermocouple dead?
Look through the small sight glass at the bottom of the tank. On a standing-pilot heater you should see a steady blue flame. Nothing there means no pilot, and no pilot means no burner.
Relighting is a homeowner job on almost every tank, and the instructions are printed on the label. Set the control to PILOT, hold the knob down, light with the piezo igniter, and keep holding for a full thirty seconds after the flame appears before releasing. The wait matters: you are heating the thermocouple until it generates enough current to hold the gas valve open on its own.
What happens next is the diagnosis:
- It stays lit and the burner fires. The pilot was blown out. Drafts, a door slam, a spider web in the tube. Nothing more to do.
- It lights, then dies the moment you release the knob. That is the thermocouple, every time. It is a $150 to $350 job installed and the single most common gas water heater repair.
- It will not light at all. Check that the gas is on and another gas appliance works, then look at cause three.
Newer heaters use an electronic igniter and a status LED instead of a standing pilot. On those, count the LED flashes and read the legend on the label rather than trying to light anything.
Electric: has the high-limit reset tripped?
Kill power at the breaker. Take the upper access panel off the side of the tank, fold back the insulation, and look for a red button between the two terminals of the upper thermostat. If it has popped out, the energy cut-off has tripped.
Press it once. You should feel a click. Restore power and give the heater an hour.
If it holds, note the date and watch it. If it trips again, stop pressing it. That switch exists because a thermostat stuck closed or an element shorted to the tank will heat water past 180°F, and repeatedly resetting it puts a scalding hazard back in service. A tech will test both thermostats and both elements with a meter in about fifteen minutes.
While the panel is off, also check the breaker itself. A water heater on a 240V double-pole breaker can lose one leg and still look energized, which produces exactly the symptoms of a failed element.
Is gas actually reaching the heater?
The manual shutoff valve on the line into the heater should have its handle parallel to the pipe. Perpendicular is closed, and these get closed during other appliance work and forgotten.
Then test another gas appliance. If the range will not light either, the problem is upstream: a utility outage, an unpaid account, or a tripped excess-flow valve at the meter. Call the utility rather than a plumber.
On propane, check the tank gauge. Below about 15% you can be out of usable vapor pressure on a cold morning even though the needle is not on zero.
Electric: has a heating element burned out?
A standard electric tank has two elements, and they do not share the work. The upper one heats the top of the tank and hands over to the lower one, which does most of the day-to-day heating.
That division is what makes the symptom readable:
- The lower element is dead. You get a short burst of hot water from the top of the tank, then cold. This is the most common version.
- The upper element is dead. You get no hot water at all, because the sequence never hands over.
Elements fail for two reasons: they burn out at end of life, typically around eight to twelve years, or they are destroyed by being energized while not fully submerged, which is why a tank drained for maintenance must be refilled completely before the breaker goes back on.
Replacement runs $200 to $400 installed. It is a cheap part in a job that involves draining the tank, 240V terminals and a socket most homeowners do not own, so it is usually not the place to learn.
Is the thermostat set too low, or has it failed?
Water that is warm but never hot, on a tank that is otherwise behaving, is worth checking at the dial before anything else. Someone turns it down for a holiday, or for a child, and nobody turns it back.
Set it to 120°F. That is the usual recommendation: hot enough for household use, low enough to limit scalding, and it cuts standby losses at the same time.
If the dial is set correctly and the water is still lukewarm, the thermostat itself may be out of calibration or failed. On an electric tank there are two, and both are $200 to $350 installed. Measure the temperature at the nearest tap with a kitchen thermometer before you call, because “not hot enough” and “138°F at the tap” get a very different response from a technician.
Is the dip tube broken?
The dip tube is a plastic pipe that carries incoming cold water from the inlet down to the bottom of the tank, so cold water enters below and hot water leaves from the top. When it cracks or breaks off near the top, cold water is delivered straight to the outlet.
The symptom is distinctive: hot water that is never quite hot, and that fades within a few minutes at any tap, on a heater that is otherwise running normally and shows no leak. Sometimes you also find small white plastic flakes in aerators and shower heads, which is the tube itself disintegrating.
The part costs a few dollars. The labor is in draining the tank and pulling the inlet nipple, so budget $150 to $300 installed. On a tank past twelve years old, put that money toward a replacement instead.
Is sediment stealing the capacity?
On hard water, dissolved minerals drop out and settle as a layer at the bottom of the tank. On a gas heater that layer sits directly between the burner and the water, insulating one from the other. You lose recovery speed first and capacity second, and you usually hear it before you measure it: rumbling, popping or a kettle-like boiling sound as steam bubbles force their way up through the deposit.
Flushing is straightforward maintenance. Shut off the gas or the breaker, close the cold inlet, run a hose from the drain valve to somewhere safe, and let the tank empty while briefly opening the inlet to stir the bottom. Do this once a year and a tank lasts noticeably longer.
Two cautions. On a heater that has never been flushed in ten years, the drain valve may not reseat afterwards, so know where the shutoff is before you start. And if the water coming out is rusty rather than merely cloudy, you are not looking at a sediment problem, you are looking at a tank that is corroding from the inside, which is covered in our guide to a water heater leaking from the bottom.
What a healthy tank actually does
The numbers that tell you whether you have a fault or an expectation problem:
- A 40 or 50 gallon tank delivers roughly 70% of its volume as genuinely hot water before the temperature drops noticeably.
- A gas tank recovers about 40 gallons an hour at a 90°F rise. An electric tank recovers roughly half that, which is why electric heaters feel slower after a run of showers.
- Water at the tap should read within a few degrees of the dial setting, so about 120°F.
- A gas burner cycling on for a few minutes after a large draw is normal. A burner running continuously with no draw is not.
If the tank meets those numbers and the house still runs out of hot water on a Sunday morning, the heater is not faulty. It is undersized for the way the household uses it, and the answer is a bigger tank, a tankless unit, or spacing the showers out.
Repair or replace, honestly
Under eight years old, repair. A thermocouple, an element or a thermostat on a tank that age is a straightforward fix against a full replacement, and none of these faults says anything about the condition of the steel.
Between eight and twelve years, repair the cheap faults and price a replacement for anything involving the gas control valve or a second element failure in the same season.
Past twelve years, spend nothing significant. The failure that ends a tank is not the burner or the element, it is the tank itself, and no repair addresses that.
If you are replacing, check the space before you decide on the type. ENERGY STAR specifies that a heat pump water heater needs an interior location that stays between 40 and 90°F year round with roughly 1,000 cubic feet of air around it, so a tight utility closet rules it out even though the running cost is about a third of a conventional electric tank.