When the hot tap runs cold, the cause is almost always the water heater: no power, no flame, a thermostat problem or a failed part. A few safe checks can sort out the simple causes in 15 minutes. Anything involving gas lines, wiring or a leaking tank belongs with a licensed plumber.
This guide walks through the checks in the order we would run them, explains what changes after a power outage or with a tankless unit, and covers the pressure swings that often show up alongside the problem.
TL;DR
- Start at the source: check the breaker or fuse (electric heaters) or the pilot light and gas supply (gas heaters) before anything else.
- Keep the thermostat at or below 120 °F. Turning it up to “fix” the problem raises scald risk.
- After a power outage, a tank heater usually needs its breaker checked, and a tankless unit will not run until power returns.
- If the tank leaks, you smell gas, or the fix involves wiring or gas parts, stop and call a licensed plumber.
- Pressure that rises and falls is a separate problem. A $10 gauge tells you whether your home sits in the healthy range.
Table of Contents
- Why hot water stops: the short list
- Before you touch anything: safety first
- Step 1: Find out whether the heater has power
- Step 2: Check the pilot light on a gas heater
- Step 3: Look at the thermostat and safety reset
- Step 4: Think about sediment and age
- Step 5: Rule out a leak
- What if the cold water works but nothing hot comes out?
- No hot water after a power outage
- Tankless heaters: the cold water sandwich
- When water pressure is the problem too
- Quick reference: symptom, likely cause, next move
- Our perspective
- Need hot water restored? We can help
- Keep it from happening again
- FAQ
- Sources
Why hot water stops: the short list
A sudden loss of hot water usually comes down to one of five things:
- The heater has no power or no fuel. A tripped breaker, a blown fuse, an unlit pilot or a closed gas valve.
- The thermostat or a safety control has shut things down. Settings can be knocked, and a heater that overheats can trip its own safety switch.
- A heating element, burner or sensor has failed. These parts wear out over the years.
- Sediment has built up in the tank. Mineral deposits insulate the water from the heat source and cut the supply of hot water.
- The tank is leaking. A failed tank cannot be repaired and needs replacement.
The order below goes from the easiest and safest check to the ones that need a professional.
Before you touch anything: safety first
Two rules apply to every step that follows.
Do not raise the thermostat to compensate. To prevent scalding injuries, Ready.gov advises setting water heater thermostats to 120 °F or lower. If you are unsure what your setting should be, our guide to water heater temperature settings explains what to set and why.
Know where your limits are. Manufacturer guidance is clear that repairs involving gas lines, electrical components or a leaking water heater should be left to a professional. Resetting a breaker and relighting a pilot according to the label on your unit are homeowner tasks. Opening panels, touching wiring or working on gas valves is not.
If you smell gas (a rotten egg odor), do not try to relight anything. Leave the house and call your gas utility or 911 from outside.
Step 1: Find out whether the heater has power
Electric tank heaters. Go to the breaker panel and find the circuit for the water heater. If it has tripped, reset it once. A breaker that trips again right away points to a failed element or a wiring fault, and you should stop there and call a professional.
A related caution if you have drained the tank for any reason: an electric water heater must be completely full before power is applied, or the upper heating element will burn out. Whirlpool says to open a hot faucet and let it run full for at least three minutes before turning the power on. This matters after any plumbing work, and after a long outage or repair that involved shutting off the water.
Gas tank heaters. Confirm the gas valve to the heater is open and that other gas appliances, such as your range or furnace, are working. If they are not, the problem is with the gas supply, not the heater.
Tankless heaters. These have electronic controls, and many also need gas. Check the power outlet or breaker, the gas valve and any error code on the display. The error code is the most useful thing to tell a technician.
Step 2: Check the pilot light on a gas heater
A pilot that has gone out is one of the most common reasons a gas heater stops making hot water. Many units have a window or a viewing port and a lighting procedure printed on the front of the tank. Follow that label exactly.
If the pilot lights but will not stay lit, the cause is usually a worn safety sensor, a thermocouple or thermopile. Replacing or testing it means working around the gas valve, so this is a point to hand off to a licensed plumber rather than push through.
Step 3: Look at the thermostat and safety reset
Thermostats get bumped, and some units have a reset button that trips when the water overheats. Look at the setting first. If it is far below 120 °F, set it back to around that level. Do not go above it.
If your unit has a reset button and it has tripped, press it once. If hot water returns and then disappears again, something is making the unit overheat or fail, and repeated resets are not the answer. That is a diagnostic job for a plumber.
Step 4: Think about sediment and age
If the heater runs but the hot water runs out faster than it used to, or the water is only lukewarm, sediment is a likely suspect. EPA recommends flushing the tank annually to reduce sediment buildup, and also checking the temperature and pressure valve at least once a year. Our water heater maintenance tips cover what a yearly routine looks like.
Age matters too. When a heater is old, has needed repeated repairs or shows rust at the base, replacement often makes more sense than another repair. We list the warning signs in water heater replacement signs.
Step 5: Rule out a leak
Water around the base of the tank can mean the tank itself has failed, or that a fitting, valve or connection is leaking. These have very different fixes. A fitting can often be repaired. A tank that has corroded through cannot. Our five-minute paper towel test helps you work out where the water is coming from before you call.
Turn off the power or gas to the heater and close the cold water supply valve while you wait for help.
What if the cold water works but nothing hot comes out?
That pattern, normal cold water and nothing from the hot side, tells you the home’s main supply is fine and the problem is on the hot side. Work out how widespread it is:
- Only one faucet has no hot water. The water heater is probably fine. Look at that fixture: a clogged aerator, a failed cartridge or mixing valve, or a stuck shutoff. A fixture repair is a small job.
- Every hot tap in the house is cold. The heater, its power or fuel, or its controls are the cause. Work through the steps above.
- Hot water is there, but only briefly. Think sediment, a failing element or a heater that is too small for the household’s demand.
- Hot water comes and goes in short bursts. On a tankless unit, this can be a minimum flow issue or the cold water sandwich effect (see below).
If the checks do not reveal the cause, a plumber can test the heater, its controls and the lines methodically. Our plumbing diagnostics guide explains how that process works.
No hot water after a power outage
What you do depends on the type of heater you have.
Electric tank heaters. Once power returns, the heater normally heats again on its own. If it does not, check the breaker first, since a surge or restoration can trip it. Allow time for the tank to reheat. A full tank can take a while to come back to temperature, so do not assume it is broken after ten minutes.
Gas tank heaters. Many keep working through an outage because they do not need electricity to heat, though some models with electronic ignition do. A pilot that blew out during the outage will need to be relit using the label on the tank.
Tankless heaters. Tankless water heater electronic controls do not operate during a power outage, even if the unit is gas-fired. The same Department of Energy guide notes that a UPS battery backup would be needed to keep the heater running. For most households, the practical answer is to wait for power to return and then check for error codes.
If hot water does not come back after power is restored and the breaker is reset, move on to the other steps above.
Tankless heaters: the cold water sandwich
If your hot water runs hot, turns cold for a few seconds, then turns hot again, you may not have a failure at all. The Department of Energy’s guidance describes a cold water sandwich effect during frequent on and off operation, which causes a temperature drop for a few seconds. It is most noticeable when you turn a tap off and on in quick succession.
If the effect is long, frequent or paired with error codes, a technician can check flow, sensors and settings. Our guide to water heater types compares how tank and tankless systems behave.
When water pressure is the problem too
Many people who search for no hot water also notice pressure that rises and falls, drops suddenly or feels inconsistent. Pressure and temperature problems can overlap, but they usually have different causes.
Is it the whole house or one tap?
Run a few fixtures. If only one tap is weak or surging, the cause is probably local: a clogged aerator, a failing cartridge or a partly closed supply valve. If every tap, hot and cold, swings up and down, look at the supply to the house.
Measure it
A pressure gauge screwed onto an outdoor hose bibb gives you a real number. EPA’s guidance says to take the reading with all water-using fixtures and systems turned off. WaterSense recommends incoming service pressure between 45 and 60 psi for efficient water use and proper system function. Whirlpool’s guidance for water heater owners gives a slightly different range, 50 to 60 PSIG.
Check the gauge at different times of day. Pressure that is fine at noon and high late at night is a common pattern, and a single reading can miss it.
Why high pressure matters for your heater
High pressure is not just noisy. The same WaterSense technical sheet notes that it can damage piping, fixtures and appliances, and that most U.S. plumbing codes require a pressure reducing valve (PRV) where supplied pressure exceeds 80 psi. Our guide on pressure above 80 psi shows how to start with a simple gauge reading.
A PRV can also create a closed system. The Department of Energy points out that modern systems with PRVs may need thermal expansion management to protect the heater’s temperature and pressure valve and the tank. If your T&P valve drips after the heater cycles, tell your plumber. It can be a sign that an expansion tank is missing or has failed.
For more on causes and fixes, see our homeowner’s water pressure guide and our practical guide to improving water pressure.
Quick reference: symptom, likely cause, next move
| What you see | Likely cause | What to do |
|---|---|---|
| No hot water anywhere, electric heater | Tripped breaker, failed element or thermostat | Reset breaker once; call a plumber if it trips again |
| No hot water anywhere, gas heater | Pilot out, gas valve closed, failed sensor | Check the valve and relight per the label; call if the pilot will not stay lit |
| Hot water runs out fast | Sediment, failing element, undersized tank | Schedule a flush or inspection |
| Hot water only gone at one tap | Fixture problem | Repair or replace the cartridge or aerator |
| Water around the tank | Leak at a fitting or failed tank | Shut off power or gas and water, then call |
| Cold blast in the middle of a hot shower, tankless | Cold water sandwich effect | Wait a few seconds; call if it is frequent |
| No hot water after an outage, tankless | Controls need power | Wait for power, then check for error codes |
| Pressure rising and falling at every tap | Supply or PRV issue | Test with a gauge and call a plumber if it is outside range |
Our perspective
We get calls about hot water at every hour, and the pattern is consistent: most homeowners who call us have already checked the breaker or the pilot. What they need next is someone who can tell, quickly, whether the heater is worth repairing.
Our approach is to diagnose first and quote second. We look at power, fuel, controls, sediment, venting and the condition of the tank, and then explain what we found in plain terms. If a repair makes sense, we say so. If the tank is at the end of its life, we show you why.
We also look beyond the heater. A pressure reducing valve that is failing, a missing expansion tank or an old supply line can all shorten a heater’s life. Those are the checks that keep the problem from coming back, and they are part of how we work in Burbank homes and businesses.
Safety is not negotiable for us. Gas, electrical and leaking tanks are licensed work, and we treat them that way.
Serghei
Need hot water restored? We can help
PSV Plumbing is a licensed and insured plumbing company based in Burbank. We handle water heater repair, installation and maintenance for homeowners, property managers and commercial clients, and we offer free quotes.
If your checks did not bring the hot water back, see our water heater services or contact us through our website for a free quote. Property owners with a commercial building can also look at our commercial plumbing service.
Keep it from happening again
A few habits cut down on surprise cold showers:
- Flush the tank every year and check the T&P valve, as EPA recommends.
- Keep the thermostat at or below 120 °F for safety and to reduce strain on the unit.
- Test your water pressure with a gauge and fix anything outside the 45 to 60 psi range.
- Watch for early signs: rumbling noises, rusty hot water, a pool of water at the base, or hot water that runs out sooner than before.
- Do not ignore smells. If your hot water smells like rotten eggs, our guide to checking hot versus cold first helps you decide what to do.
For a broader sense of when a problem has moved past DIY, our homeowner’s triage checklist lists eight signs that call for a plumber now.
FAQ
Why would I suddenly have no hot water?
The most common causes are a tripped breaker, a pilot light that went out, a failed heating element or a closed gas valve. A power outage can also trigger it, especially with a tankless unit. A leaking tank is the cause you should never ignore, since it needs prompt professional attention.
What is the first thing to check when there is no hot water?
Check the power or fuel. For an electric heater, look at the breaker panel. For a gas heater, confirm the gas valve is open and look at the pilot. Those two checks fix a large share of cases and take only a few minutes.
How do I reset my hot water heater?
For electric tank models, reset the breaker and, if your unit has one, press the reset button on the thermostat once. Do this only after turning off power and following the label on your heater. If it trips again, stop and call a licensed plumber.
Why is there no hot water but the cold water works?
The main supply is fine, so the problem is on the hot side: the heater, its controls or a single fixture. If every hot tap is cold, start with the heater’s power or fuel. If only one tap is affected, the fixture itself is the likely cause.
Is it safe to turn the water heater up to get hotter water?
Not above 120 °F. Ready.gov recommends that setting or lower to prevent scalding. If the water is not hot enough at that setting, something is wrong with the heater or its supply, and it needs a diagnosis.
Sources
- Preventing and Treating Burns, Ready.gov
- Home Maintenance, US EPA WaterSense
- WaterSense Labeled Homes Technical Sheet: Service Water Pressure, US EPA
- Electric Water Heater Repair and Troubleshooting, Whirlpool
- Measure Guideline: Transitioning to a Tankless Water Heater, US Department of Energy
- Strategy Guideline: Proper Water Heater Selection, US Department of Energy
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