If your water heater isn’t producing hot water, the most common culprits are a failed pilot light (gas), a tripped thermostat, or a burned-out heating element (electric). Tankless units add a flow sensor to that list. A few of these you can confirm yourself in minutes. Everything else is a job for a licensed plumber.
Start Here: Is It Just Your Heater?
Before you look at the unit, rule out a few upstream causes.
Check whether other hot water fixtures in the house are cold too. If only one faucet or shower is cold, the problem is probably a mixing valve or a local supply issue, not the water heater. Also verify your main water supply valve is fully open and that pressure is normal at a cold tap. A pressure drop across the house points to a supply problem.
If everything in the house is cold, the heater is the likely culprit. Go from there.
Gas Water Heaters: Pilot Light
On a conventional gas tank heater, the pilot light is the first thing to check. Open the access panel at the bottom of the unit. If you don’t see a small flame near the burner, the pilot is out.
Before you do anything: if you smell gas near the panel, stop. Leave the house, call your gas utility’s emergency line, and let them clear it before anyone approaches the unit.
If there’s no gas smell, most units have a relight procedure on the label. It usually involves turning the gas valve to “Pilot,” pressing and holding it while you ignite with a spark igniter or long lighter, then holding it for 30 to 60 seconds before releasing. If the pilot lights but goes out when you let go, the thermocouple is likely failing. That’s a sensor in the pilot flame that keeps the gas valve open while the pilot burns. When it fails, the valve shuts off gas as a safety measure.
Relighting the pilot is a reasonable homeowner check. Replacing the thermocouple, or anything else at the gas valve assembly, is work for a licensed plumber.
Gas Water Heaters: Thermostat and Burner
If the pilot is burning but you’re still not getting hot water, the burner isn’t firing. Before calling anyone, check the thermostat dial on the gas control valve (usually near the bottom front of the tank). Make sure it’s set to at least 120°F and wait 20 to 30 minutes.
If the burner still won’t kick on, the gas valve, thermostat, or burner itself needs a proper diagnosis. That means checking gas pressure, testing the control valve, and inspecting the burner for carbon buildup. All of that is plumber territory.
Electric Water Heaters: Reset and Breaker
Electric tank heaters have two heating elements (upper and lower), each with its own thermostat. Two quick checks before calling anyone:
Breaker. Check your electrical panel for a tripped circuit. Reset it and wait 30 minutes.
Reset button. If the breaker is fine, look for a red reset button behind the upper access panel, usually covered by insulation. Press it in firmly until it clicks. Wait 30 minutes.
If the water heats after one of those, good. But a reset button that keeps tripping means something is causing the system to overheat or fault. Common causes include a failing thermostat, a heating element that’s shorted, loose wiring, or sediment insulating the element. Don’t keep resetting it. That condition needs a plumber to diagnose.
If the reset holds but water is only lukewarm, or doesn’t heat at all, a heating element is likely burned out. Testing and replacing elements involves working near live electrical connections and draining the tank. Get a licensed plumber or electrician for that.
Tankless Water Heaters: Flow Sensor and Error Codes
Tankless units only heat water when they detect flow, so a dirty or failing flow sensor is a common reason for no hot water, especially if the unit makes no sound when you open a hot tap.
Two things you can check yourself:
Inlet filter. Pull and rinse the inlet screen (usually at the cold water inlet). Sediment buildup here is common and easy to clear.
Error code. Most tankless units display a fault code on a small screen. Look up your model’s manual (or search the model number plus “error code”) to see what it means. Don’t guess.
Beyond those two checks, diagnosing a tankless unit gets into the igniter, heat exchanger scale, flow sensor replacement, or control board faults. All of those need a plumber with the right tools.
What a Plumber Will Actually Do
When a plumber arrives for a no-hot-water call, the diagnostic is systematic. On gas: check gas pressure at the valve, test the thermocouple, inspect the burner for carbon buildup. On electric: measure element resistance, test both thermostats, check for loose wiring. On tankless: inspect the heat exchanger for scale, test the flow sensor, read stored fault codes from the control board.
The right tools make it fast. A plumber can read what’s actually happening instead of guessing through access panels.
When to Call a Licensed Plumber
Call a plumber if any of these apply:
- The pilot won’t stay lit after relighting
- You smell gas near the unit
- The reset button on an electric heater keeps tripping
- Water is pooling at the base of the tank (that usually means the tank itself is failing)
- The unit is old and losing performance (tank heaters typically last 8 to 12 years, depending on type and maintenance)
- You have an error code you can’t clear on a tankless unit
When you hire someone, verify their license at cslb.ca.gov. California law requires a licensed contractor for water heater work. Ask for the license number before work starts.
This site is informational only. The goal is to help you arrive at that service call with a clear picture of what’s happening, so you’re not starting from zero with the plumber.