How do tankless under sink water heaters work?

Tankless under sink water heaters, also known as instantaneous or on-demand water heaters, provide hot water instantly without the need for a storage tank. Unlike conventional tank water heaters that constantly heat and reheat water in a storage tank, tankless water heaters only heat water as it flows through the unit. This results in endless hot water on demand and increased energy efficiency.

How Does a Tankless Under Sink Water Heater Work?

Tankless under sink water heaters work by using powerful heating elements to rapidly heat water as it passes through the unit. Cold water enters the unit and flows past the heating elements which quickly raise the temperature of the water to the desired output. The now hot water exits the unit and flows from your faucet. When the hot water tap is turned off, the unit shuts off as no water is flowing through it. This on demand operation allows tankless heaters to save energy compared to traditional tank heaters.

There are two main types of tankless under sink water heaters:

Electric Tankless Water Heaters – Use electric heating elements to heat the water.
Gas Tankless Water Heaters – Use a gas burner to heat the water.

The main components and operation of each type is explained in more detail below:

Electric Under Sink Tankless Water Heaters

Electric under sink tankless water heaters consist of:

Cold Water Inlet – Brings in the cold water supply.
Flow Sensor – Detects when water is flowing and signals the heating elements to turn on.

Heating Elements – Electric resistive heating elements that rapidly heat the water.
Thermostat – Controls the temperature by turning the heating elements on and off.
Hot Water Outlet – The heated water exits the unit.

When the hot water tap is turned on, cold water flows into the electric tankless heater. The flow sensor detects the water flow and signals the heating elements to turn on. As the water passes the heating elements, it is rapidly heated to the temperature set on the thermostat. Once it reaches the desired temp, the water flows from the hot water outlet to the faucet. When the hot water tap is turned off, the flow sensor tells the heating elements to turn off as well since no water is flowing through the unit. This instant heating operation eliminates the standby energy losses associated with tank style heaters.

Gas Under Sink Tankless Water Heaters

Gas under sink tankless water heaters consist of:

Cold Water Inlet – Brings in the cold water supply.

Flow Sensor – Detects water flow and signals the gas burner to ignite.
Heat Exchanger – A copper tube that water flows through while the gas burner heats the exterior of the tube.
Gas Burner – Heats the water by burning natural gas or propane.

Thermostat – Turns the gas burner on and off to control the water temperature.
Flue – Exhaust gases are expelled through the flue.
Hot Water Outlet – The heated water exits the unit.

When the hot water tap is opened, cold water enters the gas powered tankless heater. The flow sensor detects the water flow and signals the gas burner to ignite. As the water flows through the heat exchanger tube, the burning gas heats the exterior of the tube, transferring heat to the water inside. The thermostat controls the burner to achieve the desired set temperature. Once heated, the hot water exits the tankless heater and flows from the tap. When the hot water tap is closed, the flow sensor tells the burner to shut off since no water is flowing. The instant on-demand operation of gas tankless heaters makes them highly energy efficient.

Benefits of Tankless Under Sink Water Heaters

There are several important benefits to using a tankless under sink water heater:

Endless Hot Water – Tankless heaters provide continuous hot water on demand without running out.

Energy Efficiency – Only heat water when needed so standby energy losses are eliminated.
Compact Size – Tankless heaters are very small and mount under the sink to save space.
Long Lasting – Tankless models last approximately 20 years compared to 10 years for tank heaters.

Lower Operating Costs – Save on monthly energy bills by using less energy.
Simple Installation – Can be installed under most sinks with easy hookups.

The combination of endless hot water, energy efficiency, and long lasting performance make under sink tankless water heaters a great choice for providing hot water in kitchens, bathrooms, utility sinks, and more.

How to Choose an Under Sink Tankless Water Heater

When selecting an under sink tankless water heater, there are several factors to consider:

Power Source

Choose between an electric or gas powered model based on your available power connections:

Electric – Requires a 120-240V electrical connection. Hardwired or plug-in options available. Best for urban settings.

Gas – Operates on natural gas or propane. Requires gas line connection. Best for rural settings. Venting is required.

Power Output

Select a wattage or BTU rating adequate to heat water to desired temperature:

Under Sink Use – Around 5-6 kW (electric) or 11-13 kW (gas) output is typical.

Light Commercial Use – 7-9 kW (electric) or up to 199,000 BTU (gas).

Flow Rate

Make sure the tankless heater’s flow rate meets your water demands:

Residential – At least 1.5 GPM flow rate.

Commercial – 3-8 GPM flow rates or more.

Temperature Settings

Look for adjustable temperature settings up to at least 140°F (60°C).

Safety Features

Seek units with:

Overheat protection
Leakage protection
Freeze protection

Warranty

Choose a unit with at least a 5-year warranty, longer is better.

Efficiency Rating

Higher uniform energy factor (UEF) ratings indicate greater efficiency.

Dimensions

Make sure the tankless water heater will fit in the under sink space.

Tankless Under Sink Water Heater Maintenance

To keep your under sink tankless water heater running efficiently:

Flush the Heat Exchanger – Annually flush built-up sediment out of the heater.
Check Water Filters – Replace any inlet water filters as needed.

Inspect Burners – For gas models, inspect burners yearly and clean as needed.
Test Temperature – Verify unit is heating to the desired temp.
Check Valves and Connectors – Replace any leaky valves or connectors.

Clear Air Intakes – Clean dust or debris from venting intakes.
Check Electrical Connections – Confirm wires are tight and connections intact.
Inspect for Leaks – Check unit and water lines for any leaks.

Proper maintenance will keep your under sink tankless water heater working properly for years.

Under Sink Tankless Water Heater Repairs

Some common repairs for under sink tankless water heaters include:

Heating Element Replacement

For electric models, the heating element may need replacement after years of use. Turn off power, disconnect wiring, and remove screws to access element.

Thermostat Repair

Defective thermostats that no longer properly control water temperature will need to be replaced. This involves opening the unit and swapping in a new thermostat.

Flow Sensor Repair

If the flow sensor malfunctions, the unit may not heat water properly when the tap is opened. The sensor will need to be replaced.

Leak Repair

Leaking tanks, valves, or water lines will need to be repaired. This may involve replacing o-rings, pipe fittings, hoses, or other components.

Circuit Board Replacement

If the circuit board malfunctions, it will need to be replaced. Circuit boards control the functioning of tankless heaters.

Gas Valve or Burner Replacement

For gas models, a faulty gas valve or burner will need to be replaced if they are no longer functioning properly.

Knowing when to make repairs versus replacing your under sink tankless water heater depends on the age and extent of the issues. Routine maintenance helps minimize repairs.

Replacing an Under Sink Tankless Water Heater

When it comes time to replace your under sink tankless water heater, follow these guidelines:

Select a New Unit – Choose a replacement unit that meets your hot water needs.
Turn Off Water – Shut off the cold water line to the old unit.

Drain Unit – Open hot water tap to drain out residual water.
Disconnect Wires – Unhook the electrical connection wires.
Disconnect Gas Line – For gas models, detach the gas supply line.

Remove Old Unit – Unscrew mounting screws/brackets and remove old heater.
Install New Unit – Mount and connect the new tankless water heater following instructions.
Connect Water – Reconnect cold water in and hot water out lines.

Connect Power – Re-establish electrical or gas connections.
Flush Unit – Flush the system per manufacturer instructions.
Test Operation – Open taps to test that the new unit works properly.

Follow all local codes and permit requirements when replacing your under sink water heater. With proper installation, your new tankless water heater will provide endless hot water.

Conclusion

Tankless under sink water heaters provide an efficient and convenient way to get instantaneous hot water for faucets, sinks, and light commercial uses. They work by rapidly heating water only on demand rather than constantly heating water in a storage tank. The compact size allows installation under sinks to save space while the endlessness hot water capacity enhances convenience. Choosing the right size unit along with proper maintenance will provide years of reliable service. Replacing older under sink tankless heaters is straightforward. Following manufacturer instructions for installation will ensure your new on-demand heater works flawlessly.