Tankless Water Heater Problems: Common Causes and When to Flush It

A tankless water heater is supposed to mean hot water that never runs out. So when the shower goes lukewarm, the water turns cold halfway through, or the unit flashes an error code and shuts down, it's frustrating. The good news is that most tankless problems we see in Carmel, Fishers, Westfield and North Indianapolis come down to a short list of causes, and several of them you can check yourself before calling anyone.

Common tankless water heater problems and what causes them

Lukewarm water or hot water that comes and goes

The most common cause in Central Indiana is scale. Our water is hard, and minerals build up inside the heat exchanger over time. Scale acts like insulation, so the burner works harder and the unit can't heat water as well as it should. Many units will show a warning or error code when scale gets bad, but plenty of homeowners just notice the water isn't as hot as it used to be. The fix is a descaling flush, covered below.

Hot water cuts out when the flow is low

A tankless heater only fires when enough water is moving through it. If you run a low-flow faucet at a trickle, the unit may not turn on at all, or it may shut off mid-use. Opening the tap a little more usually solves it. A clogged inlet filter screen can cause the same thing by cutting the flow before it reaches the burner.

Not enough hot water when two showers run at once

Every tankless unit has a limit on how many gallons per minute it can heat at once. In winter, the water coming into your home is much colder, so the unit has to raise the temperature further and can handle less flow. If hot water gets weak when the shower and dishwasher run together, the unit may be undersized for the house, it may be scaled up, or the gas line feeding it may be too small.

A burst of cold water in the middle of a shower

This is often called a "cold water sandwich." Someone turns the hot water off and back on, and the hot water still sitting in the pipe comes out first. Then a few seconds of cold water arrive while the burner fires back up, and then it's hot again. A short burst is normal for a tankless system. If it's long or getting worse, it's worth having us check the setup.

Error codes and the unit shutting down

Tankless heaters monitor themselves and shut down when something isn't right, like ignition trouble, a vent problem, overheating or a gas supply issue. Write down the code on the display and look it up in your owner's manual. Codes that keep coming back usually point to a problem that needs a plumber.

No hot water at all

Check the simple things first. A tankless unit needs electricity to run, so a tripped breaker, an unplugged cord or a power outage will stop it even on a gas model. Also make sure the gas valve to the unit is open and your other gas appliances are working.

What you can safely check before you call

• Write down any error code on the display.

• Check the breaker and make sure the unit has power.

• Confirm the gas shutoff to the heater is open. If you smell gas, leave the house and call your gas company or 911 first.

• Look at the vent and air intake outside the house. Leaves, snow, a bird's nest or a bush can block it.

• Run one hot tap at a time, opened most of the way, to see if the problem is flow-related.

• If your unit has an inlet filter screen and you're comfortable doing it, shut off the water to the heater and rinse the screen.

Anything involving the burner, gas line, venting or the inside of the unit is a job for a licensed plumber.

Flushing a tankless water heater: why it matters in hard water

Descaling is the most important maintenance a tankless heater needs. A pump circulates a descaling solution or food-grade white vinegar through the heat exchanger to dissolve mineral buildup, and then the unit is rinsed with fresh water. Most manufacturers recommend doing this about once a year. In hard-water areas without a water softener, it may need to be done more often.

A proper flush depends on isolation valves (service valves) on the hot and cold lines at the heater. Most newer installs have them. If yours doesn't, we can add them so future flushes are quick and clean.

A water softener helps a lot. Softer water means less scale in the heat exchanger and less frequent flushing. If you're seeing white crust on faucets and spotty dishes, read our guide to how hard water shortens water heater life or see our water softener and filtration service.

Repair or replace?

Many tankless problems can be fixed with a flush, a cleaned filter, a cleared vent or a part replacement. Replacement starts to make sense when the unit is older, the heat exchanger is badly scaled or leaking, or repair calls keep adding up. If you're on the fence about tankless in the first place, our post on whether tankless is worth it in Carmel and Indy walks through the pros and cons against a traditional tank.

How we handle tankless service

When you call ExpertPipe Indy about a tankless water heater, we check the unit, the gas supply, the venting and the water quality, and then explain what we found. You get options and an upfront price on site before any work starts. We install and service Navien tankless water heaters and Bradford White tank water heaters, and we pull the permit and haul away the old unit on replacements. If you have a written estimate from another licensed plumber, ask about our 10% price match.

Call or text (317) 731-3028, or visit our water heater repair and replacement page. We serve Carmel, Fishers, Westfield, Noblesville, Zionsville, Whitestown, Brownsburg, Cicero, McCordsville and North Indianapolis.

FAQ

How often should a tankless water heater be flushed?

About once a year for most homes. Homes with hard water and no softener may need it more often. Check your unit's manual for the manufacturer's schedule.

Why is my tankless water heater lukewarm in winter?

Incoming water is colder in winter, so the unit has to work harder and can heat less water at once. Scale buildup or an undersized unit makes it more noticeable.

Does a tankless water heater work during a power outage?

No. Even gas tankless units need electricity for the controls, fan and ignition.

Can I flush a tankless water heater myself?

If the unit has isolation valves and you follow the manufacturer's instructions, some homeowners do. If you're not sure, or the unit has no service valves, have a plumber do it so nothing gets damaged.