Tankless Water Heater Not Heating

Start Here: The Error Code and the Basics

Tankless units run diagnostics on themselves. When something is wrong, they usually show a number or blink a pattern. That code is the fastest clue, and it is different for each brand. Your owner's manual or the manufacturer's website has the list for your exact model. Write the code down and take a photo of the unit and its label before you call.

• Is the display on? No display means no power. Check the breaker and the plug if the unit has one. A gas tankless still needs electricity for its controls and fan.
• Is the gas on? Check the shut-off valve near the unit. On propane, check the tank gauge. Try another gas appliance to see if the supply is out.
• Is the water on to the unit? Make sure the isolation valves on the water lines are open.
• Is a code showing? Do not keep resetting the unit again and again. Reset once, note what happens, and then call.

If you smell gas at any time, leave the house first and call from outside. See smell gas in the house for the steps.

Common Reasons a Tankless Unit Will Not Heat

Low flow. A tankless heater starts heating when enough water moves through it. Every model has its own minimum, listed in the manual. A clogged aerator, a half-closed valve or a very weak tap can keep the flow below that trigger, so the unit never fires. Open a different, stronger tap and see if the water heats.

Cold water between hot bursts. Some units make a short blast of cold water when you shut a tap and reopen it soon after. This is sometimes called a cold water sandwich. It lasts a few seconds. If the cold stretch lasts a long time, the cause is more likely low flow, or a unit too small for the number of taps running.

Scale on the heat exchanger. Minerals in the water bake onto the inside of the heat exchanger. The unit runs hotter, flow gets restricted, temperature swings begin, and eventually it can shut down or throw a code. Manufacturers describe descaling with a pump and a descaling solution or vinegar, followed by a clean water flush. Follow your own manual, because the steps differ.

Venting trouble. A tankless gas unit needs a clear intake and exhaust. A blocked vent, a bird nest, a loose pipe joint or heavy snow can trip a safety shutdown. Do not run a gas unit that is showing a vent or exhaust error.

Too many taps at once. A shower, a dishwasher and a laundry load at the same time can ask for more hot water than the unit can make. The temperature drops or the unit seems to quit. Try one tap at a time.

What You Can Safely Try

• Reset the unit once, following the manual, and see if the code returns
• Clean the aerators on faucets that run weakly
• Check that the breaker, shut-off valves and propane supply are all on
• Try a different tap, and try only one tap at a time
• Look at the outside vent opening for snow, leaves or a nest, without opening the unit
• Photograph the label and the code

Stop and call

• You smell gas
• The same code returns after one reset
• You see water leaking from the unit or its pipes
• You hear loud rumbling, popping or banging from inside
• You are thinking about opening the cabinet or the combustion area

Wells and Mountain Homes

Many homes in the North Georgia mountains run on a private well. Well water can carry more minerals, so scale can form faster. It can also carry grit that clogs the small screen at the inlet of the unit. Whole-home filtration and softening reduce what reaches the unit. The page on water treatment explains what Ryan installs. Cold winters add the risk of frozen water lines to an unheated garage or crawlspace, so a frozen line can look like a unit that will not heat. If you suspect one, see frozen pipes.

A tankless unit's gas and vent lines are not something to adjust on your own. If the unit is on propane and the tank was just refilled, a purge of air from the line may be needed before it fires.

Repair or Replace, and What It Costs

Many tankless problems are a service call: descaling, a cleaned filter screen, a vent fix, or a part. Others point to a unit near the end of its life. Ryan installs and repairs tankless units and gives a free estimate. Water heater repair and replacement jobs typically run about $500 to $2,000 depending on the unit and the job, and you know the price before any work starts. There is no trip fee and no diagnostic fee. The tankless water heater page covers installation, and the Navien versus Rinnai guide compares two common brands.

"I had my water heater fail and fill my basement with water. Ryan replaced my water heater with a tankless and repaired my main water shut off valve the next working day. The work was done beautifully, quickly and the area cleaned up neatly when he was done. Super fair pricing as well."
Jean H., Mar 2025, Google review

A tank heater that runs out of hot water is a different issue. See hot water runs out fast or no hot water at all for tank problems.