Tankless Water Heater Not Heating?

1. Five-Minute Triage: Read the Display and Narrow It Down

Start at the heater, not the faucet. Look at the controller or the display on the front of the unit. If it is blank, the heater has no power: check that it is plugged in, that the outlet works, and that the breaker or GFCI feeding it has not tripped. A gas tankless unit still needs 120 volts for its fan, igniter and control board, so a power cut or a tripped GFCI in a garage takes out the hot water just as surely as an empty gas line.

If the display shows a code, write it down before resetting anything. The code is the single most useful clue you have, and resetting the unit by switching the power off and on often clears it without fixing the cause. Look the code up in the owner's manual for your exact model, because the numbers are not standardized across brands. On Rinnai units, for example, code 10 points to an air supply or exhaust blockage, 11 to no ignition, 12 to flame failure, 14 to an overheat condition and LC to scale buildup. Navien uses its own E-codes, such as E003 for ignition failure and E012 for flame loss.

Next, establish the pattern, because the pattern points to the cause. Is it every hot tap in the house or only one? Does it fail all the time, or only when two fixtures run together, or only when the furnace is also running? Does hot water arrive and then turn cold mid-shower? Is it worse in winter? A single fixture means the problem is almost certainly at that fixture. A whole-house failure with a code points at the heater, its gas or its vent.

Finally, check the obvious settings. Make sure the temperature setpoint on the controller has not been turned down, that a second controller in a bathroom is not set lower and in charge, and that the isolation valves under the unit are fully open after any recent service. A surprising number of no-heat problems appear the morning after someone flushed the heater and left a valve half closed.

2. Minimum Flow Rate: When the Heater Never Switches On

A tankless heater does not fire until its flow sensor sees enough water moving through it. That threshold is the minimum activation flow rate, and once lit, the unit stays on down to a slightly lower minimum flow rate. Rinnai and Rheem condensing models are commonly specified at about 0.4 gallons per minute to activate and about 0.26 to keep running. Navien's NPE-A2 series lists 0.5 GPM to activate, with an optional setting that allows much lower flow. Below those numbers, water simply passes through cold.

That is why one tap can be cold while others are fine. A bathroom faucet with a 0.5 GPM aerator barely meets the threshold, and opened halfway it does not. A single-handle faucet turned only partly toward hot sends a trickle of hot through the heater and the rest of the flow from the cold line. A low-flow showerhead with the mixing valve set warm rather than hot can draw less hot water than the activation rate. Open the tap fully to hot and see if the heater lights. If it does, the heater is fine and the fixture is the issue.

Restrictions upstream lower flow too. Most tankless units have an inlet water filter screen on the cold connection, which catches sediment and scale flakes and slowly clogs. Fixture aerators and showerheads clog with the same debris, especially after plumbing work or a water main repair. Unscrew the aerator on the problem tap and run it; if flow improves and the heater fires, clean or replace the aerator. Then shut off the isolation valves, drain the unit and clean the inlet screen as described in the owner's manual.

If low-flow fixtures are important to you, two fixes exist. Some units allow the activation threshold to be lowered or offer a small buffer tank accessory, and many have recirculation options that keep water moving. The other fix is fixture choice: a 0.5 GPM lavatory aerator and a tankless heater are a poor match. Our tankless sizing guide covers flow rates by fixture, and the best tankless water heaters guide notes which models handle low flows well.

3. Gas Supply and Undersized Gas Lines

A whole-house gas tankless heater can draw up to about 199,000 BTU an hour at full fire, several times what a typical tank heater uses. If the gas line, meter or propane regulator cannot deliver that while the furnace, range and dryer are also running, the pressure at the heater sags, the flame becomes unstable, and the unit shuts down with an ignition or flame failure code. The giveaway is a heater that works on a single shower in summer and fails in winter, or whenever a second fixture opens.

Start with the simple checks. Confirm the gas valve on the line to the heater is fully open. Check that other gas appliances in the house are working; if the range will not light either, the problem is the supply, not the heater. With propane, check the tank gauge, because a tank under about 20 percent may not deliver enough pressure in cold weather, and a regulator that ices up can starve the heater on a cold morning. If the heater was recently installed and has never been reliable, suspect the gas line size first.

Gas pressure has to be right while everything is running, not just at rest. Rinnai, for instance, specifies a flowing inlet pressure of 4 to 10.5 inches of water column on natural gas and 8 to 13.5 inches on propane, measured with all gas appliances firing at their maximum rate. Measuring that requires a manometer on the heater's test port, and it is a job for a licensed gas fitter or the installer, not a homeowner. If the reading drops below the minimum under load, the line or meter is undersized.

The fix for an undersized line is a larger line, commonly 3/4 inch to the heater, and sometimes a meter upgrade from the gas utility. That is an installation defect, so call the original installer first. In the meantime, you can reduce symptoms by not running the dryer, furnace and two showers at once. Our tankless water heater cost guide covers what gas line upgrades typically run if you are paying for it.

4. Scale: The Slow Cause That Ends in No Heat

In hard water, calcium and magnesium carbonate precipitate out of the water as it heats and coat the inside of the heat exchanger. The coating insulates the water from the burner, so the heat exchanger runs hotter for the same output and the unit struggles to hold its setpoint. Early signs are temperature swings and slightly lower maximum flow. Later the unit trips on an overheat sensor or displays a scale warning, and eventually it stops heating reliably at all.

Many units detect this before you notice. Rinnai uses the LC code for scale buildup, and it is one of the few codes that lets the heater keep running while it alternates with the temperature display, which is the heater asking to be flushed. Other brands use their own codes and may lock the unit out until it is serviced. Do not keep resetting a scale warning; each reset buys a little time while the deposit thickens.

The fix is descaling: isolating the heater with its service valves and circulating a descaling solution or white vinegar through it with a small pump for about an hour, then flushing with fresh water. Our step-by-step guide on how to flush a tankless water heater covers the pump, the hoses and how often your water hardness calls for it. If the unit heats properly afterward, scale was the cause, and the question becomes how to keep it from coming back.

Prevention depends on hardness. Rinnai's published water-quality limit puts total hardness at up to about 200 mg/L, roughly 12 grains per gallon, and manufacturers generally exclude scale damage from heat exchanger warranty coverage. If you are flushing more than once a year, a whole-house softener or a scale-reduction system upstream of the heater is cheaper than an early heat exchanger replacement. Our best water softener systems guide covers the options.

5. Venting and Combustion Air

A gas tankless heater needs to pull in combustion air and push out exhaust through its vent, and its fan and pressure sensors watch both. If the intake or exhaust is blocked, the heater will refuse to light or shut down mid-run with an air supply or exhaust code. On Rinnai units that is code 10; other brands have their own equivalents. Because the vent is outside, these failures often line up with weather: a snowdrift over a sidewall termination, ice in a vent cap, leaves blown into an intake, or a wasp nest in a summer-idle outdoor unit.

Check the terminations outside first. Look for snow, ice, leaves, insect nests, plastic bags or anything stacked against the wall. Make sure nothing new has been built in front of the vent, such as a deck, a shed or a privacy screen, and that a dryer vent or another appliance exhaust is not blowing into the heater's intake. Clear what you can reach safely. If the vent is on the roof, or the code returns once the termination is clear, call a technician to inspect the vent run itself.

Condensing units add a second airflow-related cause: condensate that cannot drain backs up into the unit and trips a safety switch. Rinnai's diagnostic manual describes a dedicated code for condensate backing up into the trap. Check that the condensate line is not kinked, frozen, blocked with debris, or draining into a condensate pump that has failed, and that a neutralizer cartridge is not clogged. Our condensing vs non-condensing comparison explains why that drain exists and how to keep it from freezing.

Two less common causes are worth knowing. High-altitude installations need the correct altitude setting or the burner runs out of balance and can trip flame or air codes. And a unit installed in a cramped closet without enough makeup air, or in a space with chemical fumes from laundry products or pool supplies, can struggle for clean combustion air. Both are installer issues rather than owner fixes.

6. The Cold-Water Sandwich vs a Plumbing Crossover

Two problems are often confused with a heater that is not heating. The first is the cold-water sandwich: you shower, turn the water off briefly, turn it back on and get a few seconds of hot water, then a slug of cold, then hot again. It happens because the hot water left in the pipe comes out first, followed by water that passed through the heater during the few seconds it took to relight. It is normal tankless behavior, it lasts seconds, and a recirculation system or a unit with a small built-in buffer reduces it.

The second is a plumbing crossover, and it is a real fault. A crossover happens when cold water leaks into the hot line somewhere in the house, usually through a failed cartridge in a single-handle faucet, a worn pressure-balancing or thermostatic shower valve, a mixing valve at a utility sink, or a thermostatic bypass valve used for recirculation. The result is lukewarm water at every tap, or a heater that will not fire at a fixture because cold water from the crossover is filling the flow the heater should be seeing.

The crossover test is simple. Close the cold-water inlet valve on the heater, then open a hot tap somewhere in the house. With the heater's supply shut off, nothing should flow from a hot tap except a brief drain-down of the pipe. If water keeps running steadily, cold water is getting into the hot side somewhere. Then turn off fixtures or close their stop valves one at a time until the flow stops; the last one you closed is the culprit, and a new cartridge or valve usually fixes it.

Tankless systems are more sensitive to crossovers than tank systems for a simple reason. A tank heater will push hot water into the line regardless, and the crossover just dilutes it. A tankless heater depends on seeing enough flow through itself to fire, and a crossover can divert part of that flow around the heater entirely. If you have a crossover recirculation valve under a sink, as many tankless setups do, it is the first fixture to suspect.

7. Electric Tankless Units, and When to Call a Pro

Electric tankless heaters fail differently, because there is no gas, flame or vent. Whole-house electric units use several heating elements, each usually fed by its own double-pole breaker. A large 27 kW unit typically needs three 40-amp double-pole breakers. If one of them trips, the heater still runs but delivers lukewarm water, because it has lost a third of its power. Check every breaker serving the unit before assuming anything inside it has failed, and if one keeps tripping, stop resetting it and call an electrician.

Elements can burn out, and the most common reason is dry firing: the unit is powered while there is air rather than water around the elements, typically after plumbing work or a flush when the lines were not purged. Always run hot water through a fixture until it flows steadily, with no sputtering, before restoring power after any work. A burned-out element shows as one stage that never heats, and on many units elements are replaceable, but it is an electrical repair inside a high-amperage appliance.

The other electric limitation is physics, not failure. Heating water takes a fixed amount of energy, so a unit that comfortably heated 3 GPM of 65-degree summer water may only manage half that in January when the inlet is 40 degrees. If the water is lukewarm only in winter and only at higher flows, the heater is working at its limit. That is a sizing issue, covered in our tankless sizing guide, and the options are lower-flow fixtures, a second unit or a gas model.

When to Stop and Call a Professional

Call a licensed technician for anything involving gas pressure, gas line size, burner, igniter, flame rod, fan, circuit board or internal wiring; for any vent damage or carbon monoxide alarm; for a code that returns after you have cleared the obvious causes; for water leaking from the cabinet or the condensate line when the unit is not running, which can indicate a leaking heat exchanger; and for any repair on a unit still under warranty, because unauthorized repairs can void coverage. If the heater is old and these failures are starting to pile up, our guide to how long tankless water heaters last covers when repair stops making sense.