How to Install an Under-the-Sink Water Heater

Installing an under-the-sink water heater can be an efficient solution if you need hot water at a specific location, such as your kitchen sink or a bathroom tap. You’re only heating the water you need to use, unlike a boiler system featuring a water tank that’s designed to keep large amounts of water hot, whether you’re using it or not.

Here, we break down what an under-the-sink water heater is, what the benefits are, and a step-by-step process on how to fit an under-sink water heater so you can feel confident doing it yourself.

What is an under-the-sink water heater?

An under-the-sink water heater is a compact and specialised appliance designed to provide hot water directly at the point of use, typically placed under sinks or near specific taps. Unlike traditional water heaters that supply hot water to the entire house from a central location, under-the-sink water heaters are installed near your desired hot water outlet, eliminating the need to wait for hot water to travel from a distant water heater.

These under-sink water heaters are small in size and can be easily mounted on a wall or placed in a cabinet under the sink. They are designed to heat a relatively small volume of water quickly and efficiently, ensuring that hot water is readily available whenever you need it, without wasting time or energy.


How to install an under-the-sink water heater in 11 steps

Before you get started installing your under-the-sink water heater, you’ll need the following equipment: 

  1. Under-the-sink water heater kit (including the water heater unit, connectors, and mounting hardware)
  2. Adjustable wrench
  3. Pipe cutter or hacksaw
  4. Pipe joint compound or plumber's tape
  5. Screwdriver
  6. Measuring tape
  7. Pencil
  8. Safety goggles
  9. Gloves

Note: You'll also have to make sure you're following all local building codes and regulations before you begin. If you're not confident in your abilities, we recommend you hire a licensed plumber for the installation.

  1. Choose the location

Determine the ideal location for your under-the-sink water heater. The heater  should be close to the tap you want to supply with hot water.

You will need to ensure there is enough space to accommodate the unit and allow for installation and maintenance.

  1. Shut off the water supply

Next, you will need to locate the main water stop tap in your home and turn it off - to stop the water flow during the installation process.

Your inside stop valve is normally located just after the water pipe enters the house. This is often under the kitchen sink, but can also be:

  • In an airing cupboard
  • Under the stairs
  • Under the floorboards
  • Near the front door
  • In the garage, utility room, bathroom or cellar
  1. Prepare the area

Before you can start fitting the water heater, you will need to clear out the space under the sink and remove any obstacles that may hinder installation. Place a cloth or towel on the floor to catch any water drips during the installation.

  1. Measure and mark

Next, use a measuring tape to determine the distance between the hot water supply pipe and the intended location for your under-sink water heater - marking the spot where the unit will be mounted on the wall.

  1. Install the mounting bracket

Use the mounting hardware provided with the water heater kit to install the mounting bracket on the wall. You must align the bracket with the marked spot and secure it firmly to ensure the water heater can be secured in place.

  1. Cut the hot water supply pipe

Once you’ve done that, it’s time to measure and mark the hot water supply pipe where you plan to install the water heater unit. Use a pipe cutter or hacksaw to carefully cut the pipe.

Take precautions and wear safety goggles and gloves during this step.

  1. Connect the under-sink water heater unit

Attach the connectors provided with the water heater kit to the water inlet and outlet ports on the unit.

To do this, you’ll need to use an adjustable wrench to tighten the connections securely. Apply pipe joint compound or plumber's tape to the threaded connections for additional sealing.

  1. Connect the supply pipes

Connect one end of the hot water supply pipe to the water inlet port of the water heater unit. Use an adjustable wrench to tighten the connection securely.

Once you’ve done that, you should attach the other end of the supply pipe to the previously cut hot water supply pipe. Again, make sure you use an adjustable wrench to securely tighten the fastening.

  1. Connect the tap

Connect the tap to the water outlet port of the water heater unit using the provided connectors. Follow the manufacturer's instructions to ensure a secure and watertight coupling.

  1. Turn on the water supply

Open the main water supply valve to restore water flow to your home. Ensure everything is tightly sealed to avoide any leaks.

If leaks are present, shut off the water supply and recheck the connections.

  1. Test the system

Turn on the hot water tap connected to the water heater and check if hot water flows.

You may need to give the water heater time to heat up fully according to the manufacturer's instructions before regular use.

Remember: It's important to refer to the specific installation instructions provided by the manufacturer of your under-the-sink water heater. Following these steps will give you a general idea of the installation process, but specific details may vary depending on the model you've chosen.

If you encounter any difficulties or are unsure about any step, it's best to consult a professional plumber for assistance.

What are the benefits of an under-sink water heater?

Instant hot water

An under-sink hot water solution eliminates the waiting time for hot water to flow from a distant water heater. This can be particularly useful if you have a kitchen or bathroom where quick access to hot water is frequently required.

Energy efficient

By heating water directly at the point of use, under-sink water heaters eliminate the energy losses associated with long pipe runs and standby heat loss (the heat that gets lost from a tank water heating system that's not being used) that can occur in traditional water heating systems.

This targeted approach will help you to reduce energy consumption and, subsequently, lower your energy bills.


Under-the-sink water heaters can be convenient in situations where installing a traditional water heater may not be feasible or practical.

For example, if you live in a small flat or home with limited space, or find adding additional plumbing connections may be challenging, these compact water heaters provide a viable solution.


Does an under-sink water heater need an expansion vessel?

Under-sink water heaters are typically designed to heat a limited amount of water on demand, without significant storage. Since the water is heated directly at the point of use and used immediately, the chances of significant thermal expansion are minimal. This means most under-sink water heater models do not require an expansion vessel.

However, it's important to consult the manufacturer's specifications and installation guidelines for your specific under-sink water heater model. The manufacturer's instructions will provide accurate information on whether an expansion vessel is necessary for that particular unit.

How much electricity does an under-sink water heater use?

The electricity consumption of an under-sink water heater can vary depending on several factors, including your particular unit's power rating, usage patterns, and efficiency.Under-sink water heaters' power ratings usually range from 1,000 watts to 3,000 watts. The higher the wattage, the more electricity the unit will consume. However, since under-sink water heaters are designed for localised use and heating smaller volumes of water compared to whole-house water heaters, their energy consumption is generally lower.