Why is my tankless water heater not making hot water?

Read the display before anything else

A tankless water heater tells you what went wrong. Write down the code exactly as shown, including letters, before you press anything. Then use our brand tables: Rinnai, Navien, Rheem.

If you smell gas, skip all of this. Leave on foot, avoid anything that can spark, and call the gas utility and 911 from a safe distance^[6].

The codes behind most no-hot-water calls

Problem	Rinnai	Navien NPE-2	Rheem	Safe homeowner check
Burner did not light	11	E003	11	Gas valve to the unit open; other gas appliances work; propane tank not empty
Flame lit, then went out	12	E012	12	Note whether it fails when the furnace or dryer runs (a gas sizing clue)
Vent or air blocked	10	E110	10, 90, 99	Clear snow, leaves or nests from the outside terminals
Condensate backup (condensing units)	25	n/a	29, 93	Check the condensate line for kinks, clogs or ice
Overheat	14	E016, E030	14, 15	Do not keep resetting; call a pro
Scale or service due	LC, SS	E760	1L	Schedule a flush

Sources: Rinnai^[1], Navien^[2], Rheem^[3]. Code meanings are our paraphrase, not manufacturer text.

No code, no hot water

Rheem's service manual lists the causes when the display shows a normal setpoint but no hot water arrives^[3]:

• The unit needs a reset. Power it off and on once.
• The cold water inlet filter is clogged. Cleaning it is an owner's manual task on most units.
• Hot and cold lines were swapped at install. Common on new installs. A plumber fixes it.
• A plumbing cross-over. A failed single-handle faucet cartridge or a mixing valve can let cold water bleed into the hot line, which can keep the heater from firing or cool the output. A plumber can isolate fixtures to find it.

Also check the obvious: the cold supply valve fully open, and the unit plugged in. Tankless units need electricity to run the fan and controls, even gas models.

Hard water is the slow cause

Minerals in hard water form scale inside the heat exchanger. DOE's Building America guide notes that scale in a tankless unit restricts flow through the heat exchanger and can shorten the unit's life, unlike a tank where sediment mostly settles at the bottom^[4]. The makers build reminders for it: Rinnai's LC code is the only one that lets the unit keep running, as a warning that scale is building up^[1]. Rheem's 1L and Navien's E760 serve the same purpose^[2][3]. If you get one, book a flush and consider water treatment.

Cold weather and freezing

Outdoor and exterior-wall units can freeze if power is lost in cold weather. Rinnai code 02 means freeze protection was running but the burner could not fire, usually because of a gas supply problem^[1]. If you lose power during a freeze, follow the draining steps in your owner's manual, or call a plumber.

The cold water sandwich is not a fault

If hot water turns briefly cold and then hot again when you shut the tap and reopen it, that is the "cold water sandwich" effect. DOE describes it as a short temperature dip during frequent on/off use, and notes it is relatively rare because most units stay ready to fire^[4]. See our cold water sandwich page.

What it costs

HomeGuide's national figures (August 2025): tankless repairs run $300 to $1,300, a heat exchanger replacement $600 to $1,300, and a flush and descale $75 to $300^[5]. Rheem's guidance on repair or replace still applies: if a repair costs more than half the price of a new unit, replacement usually wins.

When to call a pro

• Ignition or flame codes (11, 12, E003, E012) that return after one reset with gas confirmed on.
• Any overheat code, or a code that names a sensor, fan, gas valve or circuit board.
• Vent codes that return after the outside terminals are clear.
• Hot and cold lines swapped, or a suspected plumbing cross-over.