Guide to the Most Common Tankless Water Heater Problems and Their Solutions

The most common tankless water heater problems are not enough hot water, temperature swings, mineral scale buildup, ignition failure on gas units, tripped breakers on electric units, unusual noises, and leaks. Most come down to three causes: hard water scale in the heat exchanger, a flow rate below the unit’s minimum, or gas, venting, or power supply issues. Flushing the unit once a year fixes or prevents many of them.

Tankless water heaters are efficient and supply hot water on demand, but they aren’t trouble-free. When yours stops heating, shows an error code, or starts making strange noises, the cause is usually one of a handful of common problems. This guide covers each one, what causes it, and what you can safely check yourself. If the fix involves gas, electrical work, or internal parts, Brock Mechanical’s water heater repair team in Howell, MI can help.

Before you start, check the unit’s display for an error code. Most tankless heaters from Rinnai, Navien, Noritz, Rheem, and Takagi show a code that points straight to the problem, and your owner’s manual explains what each one means.

Common Issues With Tankless Water Heater and Their Solutions

Tankless water heaters offer energy efficiency and endless hot water, but they can encounter problems too. Here are some solutions to common water heater issues to help ensure your water heater operates efficiently and lasts for a long time.

Insufficient Hot Water

Common causes

• The unit is too small for your home’s demand. Tankless heaters are sized by flow rate (gallons per minute, GPM) and how much they need to raise the water temperature. Michigan’s cold groundwater means the unit has to work harder in winter, so a heater that keeps up in July may fall short in January.
• Mineral scale on the heat exchanger reduces how well it transfers heat.
• Too many fixtures running at once go beyond the unit’s GPM capacity.
• A clogged inlet water filter restricts flow.

Solutions

• Run fewer hot water fixtures at the same time during peak use.
• Clean the inlet filter screen (see your manual for its location).
• Flush and descale the heat exchanger.
• If the problem never goes away, have a plumber check whether the unit is sized correctly. Some homes need a larger unit or two units linked together.

Fluctuating Water Temperature

Common causes

• “Cold water sandwich”: a short burst of cold water between hot ones when you turn the tap off and back on. The leftover hot water in the pipes comes out first, then cold water passes through while the burner relights.
• Flow below the minimum activation rate. Most units need about 0.5 GPM or more to stay lit. Low-flow showerheads or a barely open faucet can make the burner cycle on and off.
• A dirty air intake filter (on some gas models) or a clogged inlet water filter.
• A faulty temperature or flow sensor.

Solutions

• Open the hot tap a little more, or check that showerheads and aerators meet the unit’s minimum flow rate.
• Clean the air intake and inlet water filters.
• Ask about a recirculation system or a unit with a built-in buffer tank if cold water sandwiches bother you.
• Have a technician test and replace faulty sensors.

Mineral Buildup and Scaling

Common causes

Hard water minerals (calcium and magnesium) form scale inside the heat exchanger. Many homes in Howell and the rest of Livingston County are on hard well water, so scale builds up faster here than in areas with soft water. Many units show a scale warning code when this happens.

Solutions

• Flush the unit with a descaling solution or white vinegar, using the service valves and a small pump. Do this once a year, or every 6 months if you have hard water.
• Install a water softener to stop scale before it forms. See Brock’s water softener services, and make sure your water softener hardness setting matches your water.

Ignition Problems (Gas Heaters)

Common causes

• The gas valve is partly closed, or the gas supply is low.
• The gas line is too small. Tankless units often need a 3/4-inch line and much more gas than a tank heater, so a unit installed on an existing 1/2-inch line may struggle when other gas appliances are running.
• A dirty burner or igniter, or a failed flame sensor.
• Blocked venting or air intake (snow, ice, leaves, or a bird’s nest).

Solutions

• Make sure the gas valve is fully open and other gas appliances are working.
• Check that outdoor vent and intake openings are clear, especially after snow.
• Reset the unit by turning off its power for a minute, then restoring it.
• If the error code returns (for example, an ignition or flame failure code), call a technician. Don’t take apart gas components yourself.

Electrical Glitches (Electric Heaters)

Common causes

• A tripped breaker. Electric tankless units draw a lot of power and often need two or more large double-pole breakers. Undersized wiring or an overloaded panel will trip them.
• Failed heating elements or a faulty control board.

Solutions

• Check and reset the breakers for the unit.
• If breakers keep tripping, have an electrician confirm the wiring and panel can handle the load.
• A technician can test elements and the control board.

Excessive Noise

Common causes

• Banging or popping: scale buildup in the heat exchanger.
• Whistling or humming: a partly clogged inlet filter or a valve that isn’t fully open.
• Rattling: loose mounting hardware or internal parts.
• Note that a quiet click or fan sound when the unit starts up is normal.

Solutions

• Flush and descale the unit.
• Clean the inlet filter and make sure the isolation valves are fully open.
• Tighten mounting brackets and accessible fittings.
• Call a professional if the noise continues or you see damage.

Water Leaks

Common causes

• Loose connections on the water lines or service valves.
• A dripping pressure relief valve, which can mean the water pressure is too high.
• A clogged or disconnected condensate line (on high-efficiency condensing units).
• An internal leak in the heat exchanger.

Solutions

• Tighten loose fittings and check the service valves.
• Make sure the condensate line drains freely.
• If water is coming from inside the unit, shut off the water and power (and the gas, on gas units) and call a professional right away.

Freezing and Winter Problems

Michigan winters are hard on tankless water heaters, especially units on exterior walls or in unheated garages.

• Most units have built-in freeze protection, but it only works when the unit has power. A winter power outage can leave the heat exchanger unprotected.
• Cold air coming in through the vent can freeze the unit even indoors. Wind blowing into the vent makes it worse.
• Signs of freezing: no hot water on a very cold morning, an error code, or water dripping from the unit after it thaws.

Prevention:

• Keep the unit plugged in and powered all winter.
• If you lose power during a long cold stretch, drain the unit according to the manual.
• Insulate exposed water lines, and ask a plumber about a vent damper or a freeze protection kit.

Keep Your Tankless Water Heater Running Smoothly

Most tankless water heater problems are preventable with a little routine care:

• Flush and descale the heat exchanger once a year (every 6 months with hard water).
• Clean the inlet water filter and air intake filter every few months.
• Keep outdoor vents clear of snow, ice, and debris.
• Address error codes and new noises early, before a small problem turns into a costly repair.
• Schedule a professional inspection once a year to check gas pressure, venting, and internal components.

For problems involving gas, electrical systems, or internal parts, call a licensed plumber. Brock Mechanical repairs, maintains, and installs tankless water heaters in Howell and throughout southeast Michigan, including Brighton, Hartland, Milford, South Lyon, Fenton, and Grand Blanc. Call 248-775-5746 to schedule tankless water heater repair. Contact us today!

Frequently Asked Questions

Why is my tankless water heater not getting hot?

The most common causes are scale buildup in the heat exchanger, a clogged inlet filter, too many fixtures running at once, or a unit that’s too small for your home. Start by checking for an error code and cleaning the inlet filter.

What is the cold water sandwich in a tankless water heater?

It’s a brief burst of cold water between bursts of hot when a tap is turned off and on again quickly. It happens while the burner relights. A recirculation system or a small buffer tank can reduce it.

How often should I flush a tankless water heater?

Most manufacturers recommend flushing once a year. If you have hard water, as many Livingston County homes do, flush every 6 months or install a water softener.

Why won’t my tankless water heater ignite?

Common causes are a partly closed gas valve, a gas line that’s too small, a blocked vent or air intake, or a dirty igniter or flame sensor. Check the gas valve and vents, then call a technician if the ignition error code returns.

Can a tankless water heater freeze?

Yes. Freeze protection only works when the unit has power, so outages and cold air coming through the vent can freeze it. Keep the unit powered, and drain it if you lose power during a long cold spell.

How long do tankless water heaters last?

With regular maintenance, most last about 20 years, which is longer than a typical tank heater. Skipping annual flushing in hard water areas can shorten that significantly.