"No hot water" is the most common water heater call there is. It sounds like one problem, but the fault behind it depends heavily on what kind of heater you have. The same cold tap can mean a tired thermocouple on a gas tank, a burned-out element on an electric one, or a scaled-up heat exchanger on a tankless unit.
No hot water? Start here
- Run hot water at two or three different taps. All cold, or just one?
- If it's gas, check whether the stove or other gas appliances are working.
- If it's electric, check whether the rest of the house has power as normal.
- If it's tankless, write down any code shown on the display.
These are observations only. Don't open panels on the heater or operate its controls.
What "no hot water" usually means
The first split is between no heat at all and some heat that runs out. A heater producing nothing has stopped making heat: the burner isn't firing, the elements aren't heating, or the tankless unit isn't igniting. A heater producing some hot water that runs out quickly is working but losing capacity, often through sediment, a failed element on one half of an electric tank, or plain undersizing. This page covers the first case. The second has its own page.
Gas and propane tanks
On a gas or propane tank the chain is: fuel supply, ignition, flame, flame sensing, and the gas control that ties them together. A break anywhere stops the heat. The common failures are a worn thermocouple or flame sensor that no longer tells the control a flame is present, a pilot or igniter assembly that has fouled, a gas control that has failed, and on sealed-combustion tanks, a flame arrestor clogged with dust and lint so the burner can't get enough air.
That last one is common in California garages, where heaters sit near dryers, workbenches and stored boxes. On propane units, an empty or low supply tank is worth ruling out too; if other propane appliances have stopped as well, the supplier may be the first call.
Electric tanks
Most residential electric tanks have an upper and a lower heating element, each switched by its own thermostat, with a high-limit cutoff above them that shuts everything off if the water gets too hot. With no heat at all, the usual suspects are a tripped cutoff, a failed upper element or thermostat, or both elements gone.
In hard-water parts of the state, elements fail more often because scale crusts onto them. A cutoff that has tripped did so for a reason, such as a stuck thermostat or a shorted element, so a good technician finds the cause rather than just restoring power.
Tankless units
Tankless heaters only fire when they detect flow, so a very low-flow fixture or a clogged inlet filter can stop them heating at all. Beyond that, ignition and flame-sensing faults, fan problems and, most often in hard-water areas, scale in the heat exchanger all trigger shutdowns and error codes. A unit that heats briefly then goes cold is often telling you it needs descaling.
A good description on the phone gets the right parts on the truck.
California patterns worth knowing
Hard water is behind a large share of no-heat calls on electric and tankless units across the inland south and the Central Valley. Hardness varies by water district; your provider can give you figures.
Cold winter mornings in the valley, the Bay Area and the foothills expose weak components. A heater that limped through autumn may stop on the first cold week, when it works hardest.
Garage and closet locations matter: lint and dust near sealed-combustion gas tanks, sun-baked connections in exterior closets, and tight access in older interior closets all show up in the diagnosis.
Seismic strapping rarely has anything to do with a no-heat fault, but if the heater has shifted, the technician will check its gas, water and vent connections as part of the visit.
What to leave alone
For the technician only
- Relighting, the burner compartment and anything on the gas control
- Electrical access covers, elements, thermostats and the high-limit reset
- The relief valve and the drain valve
A cold tap is an inconvenience; a gas smell is not. If you smell gas, leave the building and call your gas utility and 911 from outside.
How the technician finds the fault
Diagnosis follows the energy. The technician confirms fuel or power is reaching the heater, then works through ignition, flame or element operation, the safety devices and the controls, testing rather than guessing. On tankless units they read the fault history and check flow, filters and signs of scale. They'll show you what failed, explain whether it's a repair or a sign of a bigger problem, and price the fix before starting.
What affects the cost
| Factor | Why |
|---|---|
| The failed part | Sensors and elements are modest; gas controls and circuit boards are not. |
| Heater type | Tankless and heat pump diagnosis usually takes longer than a standard tank. |
| Underlying cause | Scale or a clogged intake may need cleaning as well as a part. |
| When you call | After-hours visits can be priced differently; you'll hear it upfront. |
How long it takes
Most no-hot-water faults on standard tanks are found and fixed in a single visit when the part is in stock. If something must be ordered, the technician will say so and tell you the likely wait before you agree to anything.
Same-day visits are often available, subject to local demand.
No hot water questions
On gas and propane tanks, the most common causes are an ignition or flame-sensing fault, a gas control problem or a clogged air intake on sealed-combustion models. On electric tanks it's usually a failed element, thermostat or tripped high-limit cutoff. Tankless units often stop because of scale, an ignition fault or low flow. A supply problem, such as gas or power to the unit, is also possible.
It usually means the problem is in the heater rather than the gas supply to the house: its ignition, flame sensing, control or combustion air. That's useful information for the technician. Please don't try to open the burner compartment or operate the gas control; those are for the technician on site.
Electric tanks have two elements, each with its own thermostat, plus a high-limit safety cutoff. If the cutoff has tripped, or both elements or the upper thermostat have failed, you'll get no heat at all. Something caused the cutoff to trip, so the technician will look for the reason rather than just resetting it.
Codes differ by manufacturer and model, so we'd rather not guess. Write the code down, along with the make and model, and give them to us when you call. It helps the technician arrive prepared. On hard-water homes, many tankless codes relate to scale or flow, which descaling often resolves.
Probably not. A heater that has failed affects every hot tap. If only one shower or faucet runs cold, a failed cartridge or mixing valve at that fixture is more likely. The technician can check the fixture too, but it's worth saying so on the phone so the right person comes.
If you have some hot water that runs out, yes, capacity may be the problem, especially in winter when inlet water is colder. If you have no hot water at all, capacity isn't the explanation; something has stopped working. Our page on heaters not heating enough covers the running-out problem in more detail.
On a standard gas or electric tank, a methodical technician can usually pin down the fault within the visit. Tankless and heat pump units can take longer because there are more components and the fault history needs reading. You'll get the diagnosis and the price of the fix before any repair begins.
Generally yes. Cold water still reaches your taps, and a heater that has simply stopped heating isn't a hazard on its own. The exceptions are a leak, a relief valve discharging or a gas smell. If you smell gas, leave the building and call your gas utility and 911 from outside.