Tankless Water Heater Problems: The Common Faults

By Ray Bennett · Updated 2 October 2026 · 7 min read · Checked against the sources at the end

Tankless Water Heater Problems: The Common Faults

Short answer

A tankless heater only lights once enough water is moving, so a tap barely open or a clogged aerator gives you cold water with nothing broken. After that the list is short: scale in the heat exchanger, a gas line that cannot keep up under load, a blocked intake or exhaust, and whatever its error code is telling you.

Tankless water heater problems look alarming because the unit shows you a code and then does nothing. In practice the list of things that go wrong is short, and the first one on it is not a fault at all.

A tankless heater only fires when water is actually moving through it. Below that, it sits there and you get cold water from a perfectly healthy machine.

Before you start

What to carry: a torch, a bucket and a hose. Ten minutes for the checks, about an hour if it needs flushing.

Gas or electric? A gas unit has a gas pipe and a vent pipe through the wall, and it is what most houses have. An electric one has no vent and a bank of very large breakers, often three or four of them, and a tripping breaker is a common complaint. Most of this page is about the gas kind.

Find the model number and the manual. Every maker uses its own error codes, and a code list from another brand will send you to the wrong part. The number is on a plate on the side.

Three things to leave alone. The vent. A tankless unit vents through a sealed pipe, and it cannot be lengthened, shortened or joined into anything else without the maker’s figures. Bad venting puts combustion gas in the house.

The gas line. A tankless unit burns several times what a tank does while it runs. Resizing a gas line is a licensed job, not a weekend one.

An electric unit’s wiring. Those breakers carry a great deal of current. Switching them off is fine. Opening the unit up is not.

No hot water, or hot at one tap and cold at another

Below the minimum flow the burner never lights. That is the design, not a fault, and it explains the tap that always runs cold.

Check 1Free, one minute

Open the tap properly and see if it fires

Most units need somewhere between 0.4 and 0.75 gallons a minute before the burner will light. A tap cracked open a quarter turn does not get there.

Open it fully and listen. If the unit fires, nothing is broken and you have found the pattern behind the cloakroom basin that never runs hot while the shower is fine.

If it still will not fire, work back along the water. Unscrew the aerator on the tap and look for grit. Check the isolating valve under the basin is fully open. Then check the small filter screen on the cold inlet at the heater itself, which clogs with the same grit that settles in a tank: where that grit comes from.

It runs hot, then cold, then hot again

That burst of cold part way through a shower has a name, the cold water sandwich, and it happens when somebody turns the hot tap off and straight back on.

The water already sitting in the pipe is hot. Behind it is the water that went through while the unit was shutting down, which is not. So you get hot, then a slug of cold, then hot again as the burner catches up.

It is normal behaviour rather than a fault, and it is worse on older units. What fixes it properly is a small buffer tank or a recirculation loop, and that is a fitting job rather than a repair.

What is not normal is water that goes cold and stays cold under a heavy draw, and that is two sections further down.

The temperature has dropped over the years

The heat exchanger scales up the same way a kettle does. Flushing it through the service valves is the maintenance a tankless unit actually needs.

Check 2

Flush the scale out through the service valves

Hard water leaves scale on the inside of the heat exchanger, and as it builds the unit gets less heat into the water. Output slowly drops, the unit runs louder, and eventually it throws a code.

If there are two valves with hose ports either side of the unit, this is an hour at home. Gas and power off, both service valves closed to the house, a hose from one port into a bucket of descaler, a small pump sending it back into the other, and forty five minutes to an hour round the loop, depending on the maker. Then clean water through to rinse.

Once a year in hard water, A.O. Smith says, and less often in soft. A.O. Smith calls hard water the biggest threat to a tankless unit’s life.

If there are no service valves fitted, adding a pair is the first thing worth paying for, because without them this job means cutting pipe.

It lights and then quits when you use a lot of hot water

Check 3Free to diagnose

Watch what happens with two taps running

A tankless unit at full output burns several times what a tank does. If the gas line, the meter or the propane regulator cannot deliver that, the unit lights, runs for a minute and then drops out or throws a flame loss code.

The pattern is the giveaway. One tap is fine, two together kills it. Or it works all summer and fails on the coldest morning, when the gas pressure at the street is already under load.

Check whether anything else gas fired misbehaves at the same moment, and look at the gas pipe size at the unit against what the manual asks for. This one ends with a licensed fitter, but it is worth arriving at that conversation knowing what to say, because an undersized gas line gets misdiagnosed as a failing heater every winter.

There is a code on the display

Check 4

Read it against your own manual, not a list online

Write the code down, then find it in the manual for your model. The same two digits mean different things on different makes, which is how people end up replacing a flame rod when the problem was the vent.

Most codes fall into four families. Ignition failure, which is gas supply, the igniter or the flame rod. Flame loss, usually the same list under load. Overheat, which is scale or restricted flow. And air supply, which means the intake or the exhaust outside is blocked.

That last one is worth a look before anything else, because it is free. Go outside and check the vent terminal for a nest, a wasp, lint from a dryer vent nearby, or snow banked against it.

If your unit is fine and the problem is actually a tank heater somewhere else in the house, the order for one of those is different: how to work a tank heater through in order.

Cold weather, and units that live outside

An outdoor or garage unit protects itself from freezing with a small heater inside it, and that heater needs power. A power cut in a cold snap is how tankless units burst.

If the power goes out and the temperature is below freezing, drain the unit. There are two drain plugs for exactly that, and the manual shows where.

The other winter pattern is a unit that simply cannot lift the water far enough. Incoming water at 40 degrees instead of 60 means the same unit delivers noticeably less hot water at once, and that is physics rather than a fault.

Questions people ask

How long should a tankless unit last?

A.O. Smith says 15 to 20 years with proper care, and calls hard water the biggest threat to that. That is the whole argument for the flush.

Can I use vinegar instead of a proper descaler?

Rinnai’s own flushing steps use undiluted food grade white vinegar, and Takagi’s use food grade citric acid. What matters more is that it goes round the loop for long enough and gets rinsed out properly afterwards.

Why does it take longer to get hot than my old tank did?

Because there is no tank of hot water waiting. The unit has to sense the flow, light and heat, and then that water still has to travel the pipe. A recirculation loop is what removes the wait.

The unit is fine but the water is never quite hot enough.

Check the set temperature on the panel first, then think about how many taps are running. Output falls as demand rises, which is not a fault but is a size limit.

Do I really need to descale if my water is soft?

Less often, but the flush also tells you the service valves still work and gives you a look at the unit once a year. It is worth keeping the habit.

Is a tankless worth it over a tank?

It saves standing losses and never runs out, and it costs more to fit and needs yearly attention. For a house that regularly runs out of hot water, yes. For a small household that does not, the sums are much closer than the marketing suggests.

Sources: U.S. Department of Energy Building America "Strategy Guideline: Proper Water Heater Selection" (revised April 2015) (minimum flow, cold water sandwich, start up delay), Takagi troubleshooting guide 321923-000 (inlet filter, descaling), Rinnai VB Series indoor operation and installation manual (gas line sizing, flushing, freeze protection), A.O. Smith "How Long Do Tankless Water Heaters Last?" (lifespan, hard water), and EcoSmart ECOS electric tankless use and care manual (breakers)