If you have no hot water in the house, the fault is almost always at the water heater, and the first check depends on which kind you own. On an electric heater, check the breaker first — a tripped breaker or a tripped high-limit reset button on the heater itself is the most common, cheapest cause. On a gas heater, check the pilot light first — if it is out, nothing else on the heater can work. Identify the type first: a gas heater has a vent pipe out of the top and a gas line into the bottom; an electric heater has a power cable into the top or side and no vent. Then confirm the thermostat is set near 120°F (49°C), and only after those supply checks should you consider a failed heating element, thermostat or dip tube.

Safety — read this before any check. If you smell gas, leave the house and call your gas company or emergency service from outside. Do not relight anything, do not operate electrical switches, and do not go back in to investigate. Never cap, plug or tamper with the temperature-and-pressure (T&P) relief valve on the tank. Turn the power off at the breaker before removing any access panel on an electric heater — its supply is typically 240 volts and can kill. Keep the thermostat at or near 120°F: the U.S. Department of Energy (DOE) recommends turning the thermostat down as its core energy-saving strategy, and hotter stored water scalds in seconds, especially children and older adults. If any step below feels beyond you, stop and call a licensed plumber — that is a correct outcome, not a failed one.

Step 1: Rule out the house, then name the heater

Before opening anything, run two checks. First, confirm cold water flows normally at several taps: if neither hot nor cold runs, the problem is the mains supply, not the heater. Second, check whether the loss is total. No hot water anywhere points at the heater’s supply or its main heating part; hot water that starts warm and fades within minutes points at capacity faults covered later — a failed lower element, sediment or a broken dip tube.

One pattern is worth naming before you touch the heater: hot water everywhere except at one tap or one shower. That is not a heater fault. A single fixture that runs lukewarm while the rest of the house is fine points at that fixture — usually its cartridge or a thermostatic mixing valve in the shower control. A. O. Smith’s manuals make the same split: check the other faucets first, and only blame the heater when the whole house agrees.

Then name the heater and its age. Look for the rating label on the tank: fuel type, model, serial number and often the manufacture year. A tank heater commonly lasts about 8 to 12 years; past that range, a heater that has lost heating may need replacement rather than another part. Any plumber or support line will ask for the label details first. Note also whether yours is a tank heater, which stores heated water, or a tankless (on-demand) heater, a wall-mounted box with no storage — their faults overlap less than their names suggest, and the tankless differences have their own section below.

Step 2 (electric): breaker, then high-limit reset

  1. Check the breaker. Find the double-pole breaker labelled for the water heater. If it has tripped to the middle or off position, reset it once: fully off, then on. Wait at least 30 minutes and test a hot tap. If it trips again, stop. A repeatedly tripping breaker signals a shorted heating element or a wiring fault — resetting it repeatedly does not fix that and risks worse damage.
  2. Press the high-limit reset. With the power off at the breaker, remove the upper access panel on the tank and the insulation behind it. On the upper thermostat sits a red reset button — the high-limit or ECO switch, which cuts all power if the water overheats. If it has popped, press it firmly. Refit the insulation and panel, restore power and wait for the tank to heat. A reset that trips again, or repeatedly, means the cause — commonly a failing thermostat or a grounded element — is still present, and testing it involves live electrical diagnosis that belongs to a professional.
  3. Check the thermostat settings. Most electric tanks have two thermostats, upper and lower, behind two access panels. Both should be set the same, near 120°F. A thermostat knocked down, or a single thermostat set far below the other, produces weak or short-lived hot water rather than none. Adjust only with the power off, and never set the tank above 120°F without a thermostatic mixing valve to temper the water at the taps — see the safety note at the top.

If the breaker holds, the reset holds and both thermostats sit at 120°F, yet there is still no hot water after an hour, the supply side is proven and the fault is in a heating part.

Step 3 (electric): heating elements and thermostats

An electric tank heats with two elements. The upper element heats the top of the tank first; only then does the lower element take over. That sequence explains the two classic symptoms:

  • No hot water at all usually means the upper element or upper thermostat has failed — the lower element never gets its turn, so nothing heats.
  • Some hot water, then cold quickly usually means the lower element has failed — the top of the tank heats, giving a short supply, and the rest stays cold.

Testing an element means isolating the power, disconnecting its wires and checking continuity and resistance with a multimeter, and checking whether it has shorted to the tank. Manufacturers treat element failure as the central electric diagnosis — Rheem’s guidance for its electric and hybrid models starts from power supply and thermostat setting before condemning parts, in its heat pump water heater care and troubleshooting guide. If you are confident with a multimeter and the power is verifiably off, the test itself is straightforward; if you are not, this is the natural handover point. Element replacement requires draining the tank below the element or working quickly against a full tank, plus a new gasket — and a leaking element port afterwards is a worse problem than a cold tank. Sediment matters too: a tank that rumbles or pops has mineral build-up coating the lower element and the tank floor, which shortens element life and slows recovery; flushing is maintenance a professional can combine with the repair.

Step 4 (gas): pilot light and gas supply

For a gas tank heater, work in this order — and re-read the gas warning at the top before starting. Any smell of gas at any point ends the job immediately.

  1. Check the pilot. Look through the sight glass or inspection opening at the base of the heater. A lit pilot is a small, steady blue flame. If there is no flame, the pilot is out and the burner cannot fire. Many modern gas valves also carry a status light: on A. O. Smith heaters, one flash every few seconds means the pilot is lit and the valve has detected no problem; a different flash pattern is a fault code, and the chart for your valve is in the heater’s manual.
  2. Confirm the gas supply. Check the shut-off valve on the gas line to the heater: its handle should lie parallel to the pipe (open). Check whether other gas appliances work. If nothing gas-powered works, the supply to the house is interrupted and the gas company, not a plumber, is the call.
  3. Relight only by the label. If gas is reaching the house and there is no smell of gas, relight strictly by the lighting instructions printed on the heater’s own label — typically: turn the control knob to Pilot, press and hold it, press the igniter repeatedly until the pilot catches, keep holding for 30 to 60 seconds, release, and turn the knob back to On. Rheem’s own troubleshooting support follows the same discipline: the model’s instructions first, and its error-code and service route when the fault persists, as set out in Rheem’s troubleshooting help. Then wait at least 30 minutes for recovery before judging.
  4. Read what the pilot is telling you. A pilot that will not light at all suggests no gas at the pilot, a clogged pilot tube or a failed igniter. A pilot that lights while you hold the knob but dies the moment you release it is the classic thermocouple or thermopile failure: that sensor sits in the pilot flame and proves it to the gas valve; when it fails, the valve closes for safety. A pilot that stays lit while the main burner never fires, with the thermostat calling for heat, points at the gas control valve or thermostat. All three are gas-side repairs for a licensed technician — do not bypass, tape down or defeat the sensor or the control to keep a flame alive.

Step 5: Hot water that runs out too fast, and the dip tube

If the heater fires but showers run cold early, think capacity rather than ignition. The dip tube is the plastic pipe inside the tank that delivers incoming cold water to the bottom, near the burner or lower element, so hot water is drawn from the top. When it cracks or disintegrates, cold water pours in at the top and mixes with the hot water leaving — the tank holds plenty of heat that you never receive. Clues are lukewarm water from the start of a draw, white or pale plastic fragments in aerators, and an older heater, from the era of deteriorating tubes. Dip-tube replacement means shutting off water and power or gas, draining and opening the cold inlet — feasible, but on an older tank it is often combined with the replacement conversation.

The other capacity thieves are a thermostat set too low (raise it only to 120°F), heavy sediment insulating the water from the heat source, and undersizing after household demand grows. A 40-gallon tank delivers its top portion as a full-temperature draw before mixing pulls the outlet temperature down; if a household has added occupants, a soaking tub or back-to-back showers, the heater may be healthy and merely outmatched — the DOE sizing guidance linked from its water-heating pages is the right starting point for that judgement.

Tankless heaters: what changes

A tankless heater has no stored water, no pilot (electronic ignition instead) and no dip tube — so the checks translate rather than transfer. No hot water from a tankless unit most often means: power or gas supply interrupted (check the breaker and that the unit’s display is alive), flow too low to trigger ignition (a single low-flow tap may not reach the activation rate — test with a tap fully open), an error code on the display (record the exact code; manufacturers diagnose by it, and Rheem’s support route begins with the code), or scale build-up on the heat exchanger in hard-water areas, which needs descaling with a pump kit or a service visit. A tankless unit that delivers hot water that turns cold mid-shower under several simultaneous draws is usually undersized for that combined flow, not broken. Do not open a tankless heater’s case beyond the maker-approved user checks: the gas, venting and electronics inside are technician territory.

When to call a professional

Call a licensed plumber — and for a gas-supply failure, the gas company first — if any of these apply: you smell gas; the breaker or the high-limit reset trips repeatedly; a pilot will not stay lit, or the burner will not fire with the pilot lit; an element or thermostat needs testing or replacement and you are not equipped for electrical work; the tank leaks (a leaking tank cannot be repaired, only replaced), the T&P valve discharges repeatedly, or there is rusty water with falling performance on an old tank; or a tankless heater shows a persistent error code. The same supply-first method runs through the neighbouring plumbing jobs: how to fix a leaky faucet begins with its shut-off valves just as this job begins with the breaker and pilot, and toilet keeps running is the other common “water doing the wrong thing” fault to rule out now. If the cold taps misbehave alongside electrical symptoms elsewhere, why lights flicker helps separate a house electrical fault from an appliance one.

Frequently asked questions

How long should I wait after resetting the breaker or relighting the pilot? At least 30 to 60 minutes for noticeably hot water, and longer to recover fully. Judging after five minutes leads to unnecessary part replacement.

Why did my breaker trip once and never again? A single trip can follow a power surge or a momentary fault. Reset once and monitor. Tripping again — immediately or repeatedly — is the signal that an element or wire is faulty, and that needs a professional, not another reset.

Is 120°F hot enough? Yes for household use, and it is the DOE-recommended setting: hot enough for showers, laundry and dishwashers with their own heaters, while reducing scalding risk and standby heat loss compared with the 140°F many heaters ship set to.

Should I drain my tank to replace an element myself? Element work requires the power off and the tank drained below the element level, with a new gasket on refitting. If that is not already familiar territory, treat it as the professional handover point above.

This article is general information based on the manufacturers’ and U.S. Department of Energy guidance linked above and checked in November 2026. Water-heater designs, controls and codes vary by maker and model — follow the instructions and warnings on your heater first, never bypass a safety sensor or relief valve, and use a licensed professional for gas components, internal electrical testing and any leaking tank.