A tankless water heater that is not heating is usually held back by one of four things: water flow that is too low to switch the heater on, scale on the heat exchanger, a clogged inlet filter, or a fault in the gas, ignition, venting or power supply. Most units show an error code that names the system at fault, so read the display and write the code down before you reset anything.
Why is my tankless water heater not heating up?
A tankless unit heats water only while it flows, so a problem shows up as water that is cold, lukewarm or inconsistent rather than as an empty tank. The pattern you see points at the cause.
| What you notice | Usual cause | What a technician checks |
|---|---|---|
| Water stays cold and the display shows an error code | Ignition, flame sensing, venting, gas supply or a safety lockout | The code history, gas pressure, venting and the igniter or flame rod |
| Water is only lukewarm at every tap | Scale on the heat exchanger, a clogged inlet filter, or demand above the unit's capacity | Inlet filter, heat exchanger condition, flow and temperature rise |
| Heater does not turn on at a low-flow tap | Flow below the minimum needed to activate the unit | Flow sensor, filter, fixture aerators and valve position |
| Hot water turns cold for a few seconds, then returns | A short gap between the burner shutting down and restarting, often called a cold water sandwich | Recirculation setup, flow rate and fixture behavior |
| Works fine, then goes cold when a second fixture opens | The unit is sized for fewer simultaneous fixtures than the household is using | Sizing against the household's real peak demand |
| No display, no heat | No power to the unit, a tripped breaker or a failed control board | Power supply, breaker and board |
Safe checks to run first
- Test more than one tap. If only one fixture runs cold, the problem is likely that fixture's valve or cartridge, not the heater.
- Read the display. Note any error code and the model number from the rating label. Codes are specific to each brand, so the code means little without the model.
- Open the tap fully. Many units will not fire below a minimum flow, so a barely open tap can read as a failed heater.
- Check the basics. Confirm the heater has power and the gas shutoff valve on a gas model is open, with the handle in line with the pipe.
- Check whether the household is asking for too much. Running a shower, a dishwasher and a washing machine at once can exceed the unit's capacity, especially in cold weather when the incoming water is colder.
- Look at the temperature setting. A setting that was bumped or reset after a power outage can look like a failure.
Do not open the unit's case or work on gas, venting or electrical connections yourself. Cleaning the inlet filter is a task many manuals allow, but only with water and power or gas shut off as the manual specifies.
Gas tankless water heater: ignition, gas supply and venting
On a gas tankless unit, a no-heat condition is most often a failure to ignite or a safety lockout. The unit needs a clean supply of gas, a working igniter and flame sensor, and clear intake and exhaust venting. If any of those is out of range, the control board stops the burner to protect the home. Causes include a flame sensor coated with residue, a partly blocked vent or air intake, a gas supply that cannot keep up when the whole house is running, and a condensate drain that has backed up on a condensing model. Repeated lockouts and resets are a reason to stop and call, because a venting fault is a carbon monoxide risk. Our tankless water heater repair page explains what a technician tests on a gas call.
Electric tankless water heater not heating
Electric tankless units draw a very large amount of current, so they run on dedicated high-amperage circuits, often with more than one breaker. If one breaker has tripped, the unit may still power up but heat only partway, giving lukewarm water. Other causes include a failed heating element, a tripped high-limit switch, a flow sensor that is not registering, and scale on the elements. If a breaker trips again right after you reset it once, stop and call. Repeated resets can damage the unit and are not a safe fix.
Tankless water heater not hot enough or only lukewarm
Lukewarm water from a tankless unit is a capacity or flow problem more often than a failed part. The heater can raise water temperature only by a fixed number of degrees at a given flow rate, so colder incoming water or more simultaneous fixtures can leave the output short of what you want. Scale makes this worse by reducing how well the heat exchanger transfers heat. A fixture with a worn cartridge, or a cross-connection that lets cold water into the hot line, can also mimic a weak heater. If the heater sat at the right temperature for years and then slipped, scale or a clogged filter is the first thing to rule out.
Hot water goes cold for a few seconds, then returns
This pattern, often called the cold water sandwich, happens when you shut the tap briefly and reopen it. Hot water already in the pipe cools while the unit shuts down and restarts, so you get a slug of cold water between two stretches of hot. It is a characteristic of how many tankless systems work rather than a failure, and a technician can often reduce it with settings or a recirculation arrangement. If it started suddenly, or it comes with an error code, treat it as a fault and have it checked.
Scale: the quiet cause of tankless problems
Minerals in the water gradually coat the inside of the heat exchanger, the part that transfers heat to the water. The coating insulates the metal, so the unit runs hotter, heats less effectively and eventually trips its safety limits. Descaling means circulating a descaling solution through the unit, following the manufacturer's procedure. How often depends on the water hardness and the manual, and skipping it is the most common reason a tankless unit loses performance and years of life. The full list of tankless upkeep tasks is in our annual water heater maintenance checklist, and the effect on service life is covered in our guide to how long tankless water heaters last.
Tankless water heater error codes: what to do
Error codes are brand and model specific, so a code from one maker means something different on another. The useful steps are the same for all of them: write down the exact code and the model number, note what the unit was doing when it appeared, and check whether it clears after a single power reset. A code that returns, or a unit that locks out repeatedly, needs a technician. Resetting again and again does not fix the cause and can hide the pattern a technician needs to see.
When to stop and call a technician
- You smell gas, or a carbon monoxide alarm sounds.
- The unit locks out or shows the same code more than once after a single reset.
- A breaker trips again after one reset.
- You see water leaking from the unit or its connections.
- The unit has not been descaled in years and performance has dropped.
- You are unsure what the code means, or cannot find the model number.
Frequently asked questions
Why is my tankless water heater not heating but has power?
If the display is on and the unit is powered, the usual causes are low flow, a clogged inlet filter, scale on the heat exchanger, a gas supply or ignition fault, or a safety lockout. The error code narrows it down.
How often should a tankless water heater be descaled?
Follow the interval in your manual, which many manufacturers set at about once a year. Hard water shortens it. A unit that has never been descaled is overdue regardless of age.
Can I fix a tankless water heater myself?
Reading the code, checking the power and gas shutoff, and opening the taps fully are safe. Cleaning the inlet filter is allowed by many manuals when water and power or gas are shut off. Gas, venting, electrical and heat exchanger work belongs to a licensed technician.
How long should a tankless water heater last?
Commonly 15 to 20 years or more with regular descaling. Our guide to tankless water heater lifespan covers what changes that number.
Is a tankless water heater not heating an emergency?
It is disruptive but not dangerous by itself. Treat it as urgent if it comes with a gas smell, a carbon monoxide alarm, a breaker that keeps tripping or a leak. See our emergency water heater repair page.
We repair tankless water heaters across Lake Stevens, from older homes with retrofitted units to newer builds in North Lake Stevens and Glenwood. If the problem is wider than the tankless unit, start with our guide to why a water heater is not heating. Call any time, 24/7.