Start with the type of heater you have
Before anything else, look at the heater and work out what it runs on. A gas tank has a gas line with a shutoff handle running into a control valve near the bottom, and a metal vent pipe rising from the top. An electric tank has a heavy cable entering a junction box on top and no vent. A tankless unit is a flat box on a wall, often outside, with an error display on the front.
That one observation decides almost everything that follows. Gas heaters lose hot water when the flame stops; electric heaters lose it when an element, a thermostat or the power supply fails; tankless units usually tell you what is wrong through a code on the display.
Quick sorting: what you see and what it usually means
These are common patterns, not a diagnosis. Anything involving gas, wiring or the burner compartment belongs to a licensed plumber.
- IfGas tank, no flame visible through the sight windowThe pilot has gone out or the burner is not lighting. Try the relighting steps on the label once; if it will not stay lit, see the pilot guide.
- IfGas tank, pilot lit but burner never firesOften a control valve or thermostat setting problem. Check the dial has not been turned to Vacation or Pilot, then call a plumber if it still does not fire.
- IfElectric tank, completely coldCheck the breaker first. If it is on, the high-limit switch may have tripped or both elements may have failed.
- IfElectric tank, lukewarm water that runs out fastUsually one failed element, most often the lower one. A plumber can test and replace it.
- IfTankless unit showing a codeWrite the code down and look it up in the manual. Many codes point to ignition, flow or scale problems.
- IfAny heater with water on the floorTreat it as a leak first. Close the cold supply, turn the heater off and read the leak guide.
Safe checks you can do yourself
None of these require opening a burner compartment or an electrical panel on the heater.
- Confirm it is every tap.Run hot water at the kitchen and a bathroom. If only one fixture is cold, the heater may be fine and the problem is a mixing valve or the fixture itself.
- Check the setting.Make sure the gas control dial or the thermostat has not been bumped to a low or vacation setting.
- Check the breaker.For electric tanks and some tankless units, look for a tripped breaker in the main panel. If it trips again after resetting, leave it off and call a plumber.
- Look at the gas supply.Check that other gas appliances like the stove work. If none do, the problem may be with the gas service, which is a call to SoCalGas.
- Note the rating plate.Write down the brand, model number and manufacture date. It speeds up any diagnosis and helps decide whether a repair makes sense.
Common causes by heater type
| Heater type | Frequent cause | Usually |
|---|---|---|
| Gas tank | Worn thermocouple or dirty pilot | A part repair |
| Gas tank | Failed gas control valve | A part repair; may be weighed against tank age |
| Electric tank | Burned-out element | A part repair |
| Electric tank | Tripped high-limit or failed thermostat | A part repair after finding why it tripped |
| Tankless | Scale, ignition fault or low flow | Service, descaling or a part |
| Any | Leaking tank body | Replacement |
What to ask the plumber who comes out
- What exactly failed, and can you show me the part?
- Is this a repair on a sound tank, or is the tank itself near the end?
- Can I have the repair price and a replacement price in writing to compare?
- Is your CSLB license active, and will you or a subcontractor do the work?
Common questions
- Why do I have hot water in one bathroom but not another?
- If the heater is producing hot water elsewhere, the cold fixture is the likely problem, often a shower mixing valve or cartridge. That is still a plumbing repair, just not a water heater one.
- Can a tripped breaker cause no hot water?
- Yes, on electric tanks and on tankless units that need power. If the breaker trips again after one reset, leave it off; repeated tripping points to a fault that needs a professional.
- Is it safe to relight my own pilot?
- Many gas tanks have relighting instructions on the label, and following them once is reasonable. Never relight if you smell gas, and stop if the pilot will not stay lit.
General information for Fullerton, not a quote or professional advice. In an emergency, call 911.