The Main Parts of a Tankless Water Heater

Main parts of a tankless water heater include the flow sensor, heat exchanger, burner or electric heating elements, control board, temperature sensors, water connections, and safety components. Each part plays a role in heating water only when you need it, allowing the system to provide hot water without storing it in a large tank.

Understanding these components can help you recognize warning signs, communicate clearly with a plumber, and make informed decisions about maintenance or repairs. The exact parts inside your unit will depend on whether it is powered by natural gas, propane, or electricity.

How Does a Tankless Water Heater Work?

A tankless water heater begins operating when someone opens a hot water fixture. Water movement activates the flow sensor, which signals the control board that hot water is needed.

The control board then activates the burner or electric heating elements. As water passes through the heat exchanger, it absorbs heat before traveling to the open faucet, shower, or appliance.

The system continually monitors water flow and temperature while the fixture is running. Once the hot water is turned off, the heater stops producing heat and returns to standby mode.

This basic process depends on several components working together:

• A water flow sensor detects demand.
• The control board starts and regulates the heating process.
• The heat exchanger transfers heat into the water.
• Temperature sensors help maintain the selected output temperature.

When one of these components becomes dirty, worn, or damaged, the unit may produce inconsistent temperatures, display an error code, or stop heating water entirely.

Water Connections and Flow Components

The water side of the unit controls how cold water enters, moves through, and exits the tankless water heater. Restrictions in this part of the system can reduce hot water output even when the heating components are functioning properly.

Cold-Water Inlet

The cold-water inlet is the connection through which the home’s water supply enters the heater. An inlet shutoff valve allows a plumber or homeowner to stop water flow when the unit requires maintenance.

Many tankless systems also have a small inlet filter or screen. This component catches debris before it can enter the heat exchanger and restrict its narrow water passages.

Flow Sensor

The flow sensor detects when water begins moving through the unit. Most tankless heaters require a minimum flow rate before they will activate.

A dirty or defective flow sensor may prevent the system from recognizing that a faucet is open. This can cause delayed heating, intermittent hot water, or a complete failure to ignite.

Hot-Water Outlet

After passing through the heat exchanger, the heated water leaves through the hot-water outlet. The outlet connects the unit to the home’s hot water piping.

The main water-related components commonly include:

• Cold-water inlet connection
• Water shutoff valve
• Inlet filter or screen
• Flow sensor or flow switch
• Heat exchanger water passages
• Hot-water outlet connection

Minerals, sediment, and debris can gradually restrict these components. Homes with hard water may need more frequent tankless water heater flushing to protect water flow and heat transfer.

The Heat Exchanger

The heat exchanger is one of the most important parts of a tankless water heater. It transfers heat from a burner or electric element into the water flowing through the unit.

Depending on the model, the heat exchanger may be made from copper, stainless steel, or a combination of materials. Some high-efficiency condensing models contain both a primary and secondary heat exchanger.

Because the heat exchanger is exposed to high temperatures and constant water flow, it can develop several problems:

• Mineral scale that reduces heat transfer
• Restricted internal water passages
• Corrosion caused by water chemistry or improper installation
• Leaks resulting from damaged joints or metal fatigue

Regular descaling helps protect the heat exchanger. Ignoring scale buildup can force the heater to run longer, reduce available hot water, and increase stress on other internal parts.

A leaking heat exchanger typically requires professional diagnosis. Depending on the unit’s condition, age, and warranty coverage, replacing the heater may be more practical than replacing the exchanger alone.

Gas Burner and Combustion Components

Gas-powered tankless water heaters use a burner assembly to produce heat. The burner must receive the correct mixture of fuel and air for safe, reliable combustion.

Gas Control Valve

The gas control valve regulates the amount of natural gas or propane sent to the burner. Modern systems can adjust, or modulate, the gas supply based on the amount of water flowing through the unit and the desired temperature rise.

A malfunctioning valve may cause ignition failures, low water temperatures, or abnormal burner operation. Gas valve problems should always be handled by a qualified professional.

Ignition System

The ignition system lights the burner when hot water is requested. Many units use an electronic igniter rather than a continuously burning pilot light.

A worn igniter, poor electrical connection, gas supply problem, or dirty burner can prevent the flame from starting correctly. The control board will usually shut down the unit and display an error code when ignition is unsuccessful.

Flame Sensor

The flame sensor, sometimes called a flame rod, confirms that the burner has ignited. If the sensor does not detect a stable flame, the system closes the gas valve to prevent unburned fuel from entering the unit.

Important gas heating components include:

• Gas inlet connection and shutoff valve
• Gas control or modulating valve
• Electronic ignition assembly
• Burner assembly
• Flame sensor
• Combustion fan or blower

Changes in gas pressure, blocked combustion air, venting problems, and dirty components can all interfere with burner performance. These concerns require proper testing rather than trial-and-error adjustments.

Electric Heating Elements

An electric tankless water heater does not have a gas valve, burner, flame sensor, or combustion vent. Instead, it uses one or more electric heating elements to warm the water.

The control board energizes the elements when the flow sensor detects demand. Larger electric units may activate heating stages based on flow rate and the temperature of the incoming water.

The main electrical heating components can include:

• Electric resistance heating elements
• Power relays or contactors
• Temperature-limiting switches
• High-amperage electrical connections

Electric tankless heaters require substantial electrical capacity. Breaker trips, damaged wiring, loose terminals, or a failed heating element can reduce output or prevent the unit from operating.

Electrical panels should remain closed unless service is being performed by a qualified technician. Tankless units can contain high-voltage components even when they are compact in size.

Combustion Fan, Venting, and Air Supply

Gas tankless water heaters need a controlled supply of combustion air and a way to remove exhaust gases. The combustion fan pulls in air, supports combustion, and helps move exhaust through the vent system.

The venting configuration depends on the heater model and installation. Some units use separate intake and exhaust pipes, while others use a concentric vent that places both passages within one assembly.

Common venting and air-supply concerns include:

• Blocked intake or exhaust openings
• Loose or improperly sealed vent connections
• Incorrect vent material or pipe sizing
• Improper vent slope on a condensing system

A venting problem may cause repeated shutdowns, unusual noises, combustion errors, or unreliable heating. Never bypass a safety switch or continue resetting a heater that repeatedly reports an exhaust or airflow problem.

Condensate Components in High-Efficiency Units

High-efficiency condensing tankless water heaters capture additional heat from their exhaust. As the exhaust cools, it creates liquid condensate that must be collected and drained properly.

A condensing system may include:

• A secondary heat exchanger
• A condensate collection trap
• A condensate drain line
• An acid-neutralizing cartridge when required

A blocked drain or dry condensate trap can cause leaks, error codes, or improper operation. The condensate is mildly acidic, so the drain materials and installation must be suitable for the unit.

Homeowners should not assume that water beneath a condensing unit is normal. A small drain system should carry condensate away without allowing moisture to collect around the heater.

Control Board and User Interface

The electronic control board acts as the brain of the tankless water heater. It receives information from sensors, activates heating components, monitors safety conditions, and adjusts operation as water demand changes.

The user interface may be built into the heater or connected through a wall-mounted controller. It allows you to choose a temperature and may display operating information, maintenance reminders, or diagnostic codes.

Controls and sensors commonly found in a tankless system include:

• Main electronic control board
• Digital temperature controller
• Inlet water temperature sensor
• Outlet water temperature sensor
• Overheat sensor or high-limit switch
• Water flow sensor

An error code can provide a useful starting point, but it does not always identify the exact failed part. For example, an ignition code could result from a dirty flame sensor, inadequate gas pressure, blocked venting, or another condition.

A professional diagnosis combines the error history with physical inspection and operating tests.

Pressure Relief and Other Safety Components

Tankless water heaters contain several protections intended to stop operation when unsafe conditions are detected. The number and type of safety components vary by model and local installation requirements.

Common protections include:

• Overheat sensors and high-temperature limits
• Flame monitoring on gas-powered units
• Freeze-protection heaters on supported models
• A pressure relief valve when required by the unit or installation

Freeze protection is especially important for units installed outdoors or in unconditioned spaces. Built-in protection often depends on the unit having electricity, so it may not function during a power outage.

Never plug, cap, or modify a relief valve or discharge line. Safety devices should be inspected when there are signs of leakage, corrosion, overheating, or repeated shutdowns.

Optional Recirculation Components

Some tankless systems have a built-in recirculation pump, while others can be connected to an external pump. Recirculation moves hot water through the piping so it reaches certain fixtures faster.

A recirculation setup may use a dedicated return line or a crossover valve. Controls can operate the pump according to a schedule, temperature setting, motion sensor, or manual demand button.

Potential recirculation components include:

• An internal or external circulation pump
• A dedicated hot water return line
• A thermal crossover valve
• A timer, temperature sensor, or demand control

Recirculation can improve convenience, but improper settings may cause the heater to operate more often than necessary. Pump sizing, piping layout, and control settings should match the home and the manufacturer’s requirements.

Signs a Tankless Water Heater Part May Be Failing

Many tankless water heater problems begin with small changes in performance. Paying attention to these changes may help prevent a complete loss of hot water.

Warning signs can include:

• Water that alternates between hot and cold
• A noticeable drop in hot water pressure
• Repeated ignition or combustion error codes
• Unusual rattling, buzzing, or rumbling sounds
• Water collecting beneath or inside the unit
• The heater shutting off during normal use

Some symptoms have simple causes, such as a clogged inlet filter. Others may point to scale inside the heat exchanger, a failing sensor, an electrical problem, or improper fuel supply.

If your system is not operating correctly, Deer Valley Plumbing & Air Conditioning provides tankless water heater repair and installation services for homeowners who need professional evaluation.

How to Protect the Parts Inside Your Tankless Water Heater

Preventive care helps protect the heat exchanger, water passages, burner, sensors, and other internal components. Maintenance frequency should be based on the manufacturer’s guidance, water hardness, installation conditions, and household usage.

Helpful homeowner practices include:

• Keep the area around the heater clean and accessible.
• Check outdoor air intake and exhaust openings for visible blockage.
• Watch for leaks, corrosion, error codes, and unusual sounds.
• Arrange professional maintenance and descaling as appropriate.

A professional service visit may include:

• Inspecting water, gas, electrical, and vent connections
• Cleaning the inlet filter and flushing mineral deposits
• Testing sensors, burner operation, and safety controls
• Checking condensate and recirculation components when present

Tankless maintenance is not limited to flushing the heat exchanger. The entire system should be checked to confirm that it can safely produce the expected amount of hot water.

Homeowners with a traditional tank unit or those comparing replacement options can also explore Deer Valley’s complete water heater repair and installation services.

Schedule Tankless Water Heater Service

Knowing the main parts of a tankless water heater can make troubleshooting easier, but internal gas, electrical, combustion, and safety repairs should be left to a trained professional. Repeated error codes, leaks, temperature changes, or ignition failures deserve a proper diagnosis.

Call Deer Valley Plumbing & Air Conditioning at (623) 227-4322 to schedule tankless water heater service. You can also contact the team online or view Deer Valley Plumbing & Air Conditioning on Google Maps.

Frequently Asked Questions

Not around here? Well, then check out the 5-star tankless water heater professionals at Albuquerque Plumbing, Heating & Cooling!