Tankless water heater, no hot water? Check these first

Last verified: October 29, 2026

Disclosure: Some links on this site may be affiliate links; see our disclosure. Not affiliated with Rinnai. Safety first: if you smell gas, don't try to light any appliance, don't touch electrical switches or use a phone in the building, and call your gas supplier from a neighbor's phone (or the fire department if you can't reach them). [3] Rinnai says to call a service technician for any remedy that involves gas or electricity. [2]

Cold showers on the first chilly mornings are one of the most common complaints we see from tankless owners on X right now, along with "it's flashing a code." This guide uses Rinnai's own FAQ and manuals; other brands work on the same principle, but codes and settings differ, so check your model's manual. Tank-style heater instead? See electric water heater not heating or gas water heater pilot won't stay lit.

Free checks, in order

Gas, water and power. Rinnai's first answer to "no hot water when I open the tap" is to make sure the unit has gas, water and electricity: power on and gas turned on. [2] If error 11 (no ignition) shows, Rinnai says to check that gas is on at the water heater, gas meter, or cylinder. [2] Propane users: an empty cylinder counts.
Enough flow to fire. A tankless unit only lights when enough hot water is moving. Rinnai's gas units need at least 0.4 to 0.6 gallons per minute through the hot side to ignite and 0.26 gpm to keep running; below that, the unit shuts off because it senses hot water isn't needed. [1] Open the hot tap fully to test.
Went cold mid-shower after you turned the flow down? Rinnai says you may have dropped below the minimum flow. Lower the temperature setting on the controller or increase the flow. [2]
Lower the set temperature a couple of degrees. It sounds backward, but Rinnai says it changes the hot/cold mix so the unit activates for low-flow fixtures. Single-handle faucets can also have limit stops that need adjusting; check the faucet's manual. [1]
Clean aerators and low-flow heads. Rinnai lists cleaning or removing aerators from showerheads and fixtures, or replacing low-flow ones, as fixes for minimum-flow problems. [1]

Clean the inlet water filter

Sediment in the cold-water inlet filter cuts flow. Rinnai lists cleaning it as a fix for minimum-flow problems [1] and as the first remedy when "nothing happens when water flow is activated." [2] The in-line screen filter should be checked periodically for debris and cleaned. [1] Turn off power, gas and the water supply valves before you remove it, as Rinnai's maintenance section says. [2] Your manual shows where the filter is on your model.

Just installed, or plumbing recently changed?

For a unit that does nothing when water flows, Rinnai's manual says to make sure the hot and cold lines weren't reversed on a new installation and to check for "bleed over" (cold water crossing into the hot lines through the plumbing). [2] Diagnosing crossover means isolating the heater from the building's plumbing, so it's a job for your installer or a plumber.

Error codes

Rinnai's general rule: codes vary by product, so contact Rinnai or your installer to find the root cause. Rinnai Customer Care: 1-800-621-9419, Monday through Friday, 8 a.m. to 8 p.m. Eastern. [1]
Reset: Rinnai's code guide for technicians says to turn off all hot water fixtures and press the ON/OFF button on the controller twice to reset a code. [3] If the same code keeps coming back, stop and call for service.
LC (shown as "00" in code history): scale is building up in the heat exchanger and it must be flushed. Rinnai says failure to flush will damage the heat exchanger, and damage caused by lime buildup isn't covered by warranty. This is the only code that lets the unit keep running. [2] Rinnai's FAQ says periodic flushing should be done by a professional installer. [1]
10: air supply or exhaust blockage. Check that nothing is blocking the vent intake or exhaust outside, including snow and ice. [2] The rest of that list (vent parts, fan, settings) is for a technician.

Cold climate and freezing

Rinnai's freeze protection only works while the unit has both power and gas. Its FAQ gives freeze protection down to -22°F for indoor models and -4°F for outdoor models (protected from direct wind), and the pipes feeding the unit must be heat-traced and insulated. [1] The manual says damage from freezing isn't covered by the warranty, and if the heater won't be used during possible freezing weather, Rinnai recommends draining it. [2] Error 02 (no burner operation during freeze protection mode) is a service call. [2]

What's normal

Hot water takes a while to arrive: it has to travel through the pipes from the heater to the faucet. [2]
Fan keeps running after you shut the tap: designed behavior, to keep temperatures steady and clear exhaust. [2]
White "smoke" from the vent in cold weather: water vapor from the exhaust condensing. [1][2]

When to stop and call for service

You smell gas. Follow the safety steps at the top.
Any fix that involves gas or electricity, or anything inside the front panel. [2]
A code that returns after a reset, or an LC code that needs a flush you aren't set up for. [1][2]

Have the model number and the code ready when you call.

Other no-heat guides

Furnace: furnace won't turn on and gas furnace blowing cold air. Heat pump: heat pump blowing cold air.

We don't list repair costs or part prices. They vary by model and area.

Sources

General diagnostic guidance only. Error code meanings above are from the Rinnai VA series manual; your model's codes may differ. Last verified October 29, 2026.