One faucet or the whole house?
Run the kitchen tap on hot for a full minute, then a bathroom sink on the other side of the house. If both stay cold, go to the heater. If one runs hot and the other doesn't, the heater is doing its job and the cold fixture needs attention: shower valves are the usual culprit, and our guide to shower valve replacement explains why.
Gas heater: is the pilot lit?
Look through the small window near the base of the heater. A steady blue flame means the pilot is fine and the burner should fire when the thermostat calls. No flame means one of four things, and they are checked in this order.
- Gas supply. If the stove burners also won't light, the problem is the gas service, not the heater. Call the utility.
- A pilot that went out. Drafts, a gas interruption or a dirty pilot orifice can put it out. Relight it by following the label on the heater.
- A pilot that won't stay lit. If the flame dies when you release the knob, the thermocouple is usually worn. Fixr's August 2025 data puts a thermocouple replacement at $150 to $325.
- A lockout. Many newer gas heaters carry a flammable-vapor sensor that shuts the burner down after detecting fumes. Once tripped, some need a part or a service reset, and that's a call for a plumber.
Power-vent and direct-vent heaters with electronic ignition need household power for the fan and the igniter. When the hot water disappears after a storm or a tripped circuit, check the outlet or breaker that feeds the heater before anything else.
Electric heater: breaker, reset button, elements
An electric tank has fewer parts, and a homeowner can check the first two safely.
- The breaker. A water heater usually has a double-pole breaker in the panel. If it tripped, reset it once. If it trips again, leave it off and call.
- The high-limit reset. With the breaker off, remove the upper access panel and the insulation behind it. A red button that is popped out means the high-limit switch cut power because the water got too hot. Pressing it restores heat, but a switch that trips twice points to a failed thermostat or a shorted element.
- The elements. Testing them means measuring resistance with the power off, and replacing one means draining the tank to below the element. Fixr puts element replacement at $200 to $750, depending on access and on whether the plumber finds one bad element or two.
Tankless heater: flow, error code and scale
A tankless unit fires only when enough water flows through it. A partly closed fixture, a clogged aerator or a failed flow sensor can keep it from lighting at all. Most units show an error code on the front panel, and the owner's manual lists what each one means. In hard water, scale in the heat exchanger triggers overheat codes; the U.S. Geological Survey classes water above 180 milligrams per liter of hardness as very hard, and units in such areas need descaling more often.
When the hot water runs out fast instead
A heater that makes some hot water but not enough has a different set of causes. Sediment on the tank floor takes up space and insulates the water from the burner. A cracked plastic dip tube lets incoming cold water mix at the upper outlet instead of entering at the bottom. On electric models, a failed lower element leaves only the upper part of the tank heating. And sometimes nothing is broken: the household simply outgrew the tank, which our water heater size worksheet will show.
Turning the temperature back up safely
After a repair, resist the urge to set the thermostat higher to make up for lost time. The Consumer Product Safety Commission recommends 120°F and gives the exposure times at which most adults suffer third-degree burns.
| Water temperature | Time to a third-degree burn |
|---|---|
| 150°F | 2 seconds |
| 140°F | 6 seconds |
| 130°F | 30 seconds |
| 120°F | 5 minutes |
Source: U.S. Consumer Product Safety Commission, Avoiding Tap Water Scalds, Publication 5098. Children under five and older adults are most at risk.
What the likely fixes cost
A loss of heat is one of the cheaper calls a plumber takes, because the cause is usually a control part rather than the tank. Fixr's national figures for the parts involved: thermostat $100 to $250, gas control valve $150 to $300, thermocouple $150 to $325, heating element $200 to $750. The water heater repair guide compares these with the cost of a new heater and covers the other failures.
When the hot water is gone
Why is there hot water in the kitchen but not the shower?
Because the heater is working and the shower valve isn’t. A pressure-balancing cartridge that sticks, or a limit stop turned too far, sends only cold water to the head. The fix is the cartridge or the stop, not the heater.
How long until the water is hot again after a relight?
A full tank of cold water takes time to heat, usually an hour or more for a standard tank. The recovery rate printed on the rating plate tells you how many gallons it can heat per hour.
Is it safe to relight a pilot myself?
Follow the lighting instructions on the label attached to the heater, step by step. If you smell gas at any point, stop, leave the house and call the gas utility from outside.
Should I switch the heater off until the plumber comes?
For an electric heater with a suspected element fault, switching off the breaker is sensible and prevents a dry element from burning out if the tank is drained. A gas heater with its pilot out is already off; leave the control where it is.
Sources
- Water Heater Repair Cost, Fixr, updated August 29, 2025.
- Avoiding Tap Water Scalds, Publication 5098, U.S. Consumer Product Safety Commission.
- International Plumbing Code 2021, Chapter 4: Fixtures, Faucets and Fixture Fittings (§412.3, shower valves), International Code Council.
- Hardness of Water, U.S. Geological Survey Water Science School.
Reviewed September 27, 2026. Found an error? Write to admin@plumbingrepairquotes.com.