How a tankless unit is different
A tankless heater has no stored hot water. When a faucet opens, a flow sensor detects water moving, the control board starts the fan, lights the burner (or energizes elements on electric models) and modulates the flame to raise the water to the set temperature as it passes through a heat exchanger. When the faucet closes, everything shuts down.
That means far more electronics than a tank: a control board, a flow sensor, temperature thermistors, an igniter, a flame rod, a combustion fan and, on condensing models, a secondary heat exchanger and a condensate drain. Each one can throw a code. The upside is that the unit usually tells the plumber exactly where to look.
Common problems and typical costs
| Problem | Usual cause | Typical range |
|---|---|---|
| Scale or overheat error code | Mineral buildup in the heat exchanger | $150–$350 descale |
| Ignition failure code | Igniter, flame rod, gas pressure or venting | $200–$500 |
| Unit does not fire at low flow | Dirty inlet filter or failing flow sensor | $150–$400 |
| Hot, then cold, then hot water | Normal brief delay, or a failing thermistor | $150–$350 |
| Combustion fan noise or fan error | Worn fan motor | $300–$700 |
| Unit dead, no display | Control board or power supply fault | $400–$900 |
| Water dripping from the unit | Condensate line or a cracked heat exchanger | $150–$900+ |
Parts prices vary a lot by brand and model year. Some manufacturers sell boards for under $200; others charge several hundred. A good plumber will look up the part before quoting.
Descaling: the maintenance that prevents most repairs
The heat exchanger in a tankless unit is a narrow coil of copper or stainless tubing. Hard water leaves calcium inside it every time the water heats. As the scale builds, the unit has to work harder, temperatures swing, and eventually the overheat sensor shuts it down.
Descaling means isolating the unit with its service valves, connecting a small pump, and circulating a mild acid solution (often food-grade vinegar or a citric acid product) through the heat exchanger for about an hour, then flushing it clean. Most manufacturers recommend it once a year, more often in very hard water.
| Water hardness | Suggested descale interval |
|---|---|
| Soft, under 3.5 grains per gallon | Every 2 to 3 years |
| Moderate, 3.5 to 7 gpg | Every 1 to 2 years |
| Hard, 7 to 10.5 gpg | Every year |
| Very hard, over 10.5 gpg | Every 6 to 12 months, or add a softener or scale filter |
Our guide to hard water and your water heater shows how hardness varies across the country and why some cities go through tankless heat exchangers much faster than others.
The cold-water sandwich
Many tankless owners notice a burst of cold water in the middle of a shower when someone turns the water off and on again. The hot water left in the pipe comes out first, then a few seconds of cold while the unit relights, then hot again. It is a normal trait of tankless systems, not a defect.
If it is bothersome, the fixes are a small buffer tank, a recirculation system, or a newer unit with a built-in buffer. Do not let anyone sell you a new heater just for this.
Water that goes cold and stays cold at low flow is a different problem. It usually means the unit does not see enough flow to fire. A clogged inlet filter screen is the first thing to check.
Gas supply and venting
Gas tankless units need much more gas, at peak, than a tank heater. A 199,000 BTU unit can demand four times what a 40,000 BTU tank burns. When a tank is swapped for tankless without upsizing the gas line or meter, the unit starves under load: ignition faults, temperature drops when two showers run, and error codes during the morning rush.
Venting is the other common trouble spot. Condensing units vent through PVC; non-condensing units need stainless steel vent pipe. Wrong materials, too many elbows or a blocked termination all show up as ignition or combustion errors.
Electric tankless units
Electric tankless heaters skip the gas line and venting, but they make up for it with a huge electrical demand. A whole-house electric tankless unit can draw 100 to 150 amps on its own, which many older homes simply cannot supply without a service upgrade. When these units underperform, the cause is often not a fault at all but a home that cannot deliver enough current, or incoming water so cold in winter that the unit cannot raise it fast enough.
Real failures on electric models are usually a burned heating chamber element, a failed relay or triac on the control board, or a flow sensor. They do not need descaling quite as often as gas units, but scale still shortens element life in hard water.
Point-of-use electric tankless units under a single sink are a different story. They are small, cheap and simple. When one fails after many years, replacing it is usually more sensible than repairing it.
Simple checks before you call
Read the error code and note the model number from the label on the side. Check that the unit has power (most gas tankless units plug into an outlet) and that the gas valve on the supply line is open. Look at the vent termination outside for snow, leaves or a nest. Then try one faucet at full hot to make sure the problem is not a single fixture.
Some units let you clear a code by switching the power off and on. Do that once. If the code returns, the unit has a real fault and repeated resets can lock it out entirely.
Repair, or replace with another tankless?
A tankless heater that has been descaled on schedule can easily last 15 to 20 years, so most repairs are worth doing. The case for replacement is a failed heat exchanger on an older unit, or a brand whose parts are no longer stocked.
If you are thinking about moving from a tank to tankless, or back, our replacement page covers installed costs and our tank vs tankless guide runs the numbers over ten years.
Questions people ask
How much does tankless water heater repair cost?
Most repairs run $200 to $900. A descale is typically $150 to $350; control boards and heat exchangers are the expensive parts.
How often should a tankless water heater be flushed?
Once a year for most homes, every six months in very hard water, and every two to three years where the water is soft.
Why does my tankless water heater go cold in the middle of a shower?
Often it is the cold-water sandwich after someone else used water. If it goes cold and stays cold, the unit may not be seeing enough flow, or gas supply is too small.
Why is my tankless water heater showing an error code?
The code points to a specific fault such as scale, ignition, flow or venting. Note the code and model number before calling so the plumber can bring the right part.
Can a frozen tankless water heater be repaired?
Sometimes. If the heat exchanger cracked, it must be replaced, which on older units can cost close to a new heater.