Do Under-Sink Water Heaters Work? Let’s Find Out

Under-sink water heaters (also called point-of-use water heaters) are convenient if you constantly need to use hot water in your sinks. These compact devices are not only discreet but can be installed in any sink in your home.

Both tankless and under-sink water heaters with tanks work by heating water extremely fast. They are incredibly effective, and there is virtually no delay between turning on a faucet and receiving hot water. Under-sink water heaters also kill many germs in your home’s tap water. 

They allow you to enjoy hot water even while performing small tasks like washing your hands or face. Most importantly, they are time-saving and energy-efficient since they are on-demand heaters.

Under-sink water heaters limit the inconvenience of heating water from the central water heater. This guide will explore how under-sink water heaters work and what to look for when acquiring one.

Are Under-Sink Water Heaters Any Good?

Under-sink water heaters have many benefits and a few disadvantages that you should consider. They are typically regarded as being quite good at heating water efficiently, but whether or not they will work for you depends on your needs.

Below is a quick table of the main pros and cons of under-sink water heaters, and after that, I’ll take a more in-depth look at each point.

Benefits of Under-Sink Water HeatersDisadvantages to Under-Sink Water Heaters
Very fast at heating waterSometimes difficult to regulate the water temperature
Energy-efficientWater leaks are less noticeable
Use very little energy, which saves you moneyWater leaks can cause mold in your kitchen
Endless supply of hot water
Temperature adjustable
They save you time by not having to wait for hot water
Super easy to install and maintain
Stylish and discreet design
The pros and cons of under-sink water heaters

Our Favorite Under-Sink Water Heaters

Benefits to Under-Sink Water Heaters

There are many benefits to under-sink water heaters that make them an excellent investment for your home.

1. They are Very Fast

The under-sink water heater makes fast work of heating water because it heats small amounts of water compared to the central water heater.

That is one of the reasons that these heaters are excellent for the kitchen setup where you quickly need a little hot water.

2. The Units are Energy-Efficient

Tankless units are generally more energy-efficient than tank heaters. However, under-sink tank water heaters with an energy star label by the U.S. Environmental Protection Agency are also pretty energy-efficient.

3. They Save You Money

Both the tank and tankless under-sink heaters cut down on the energy consumption in your household, which means you save more money.

That is because you do not need to heat up or keep large amounts of water. Besides, the heating process is fast, which ends up using less electricity or gas.

4. They Supply Endless Hot Water

The under-sink water heater offers an endless hot water supply for the kitchen or bathroom sink. These are spaces where you may want to have water all the time instead of having hot water only in the shower.

5. The Units Are Adjustable

Both tank and tankless under-sink water heaters are adjustable. Many models come with thermostat control to adjust the temperatures.

That means you can even adjust the unit to heat the water to a specific temperature that is ideal for you. 

Adjusting the temperatures allows you to consume less energy and not overheat the water, yet you don’t want uncomfortably cold water either.

6. They Save You Time

Using an under-sink heater allows you to enjoy instant heated water. You do not have to wait for the water to heat before it is ready for use, as is the case with traditional central heaters.

The instant water can also be used to prepare a cup of coffee or to sterilize a baby’s bottle.

These tasks require heating water on a cooking appliance if you do not have an under-sink water heater. Boiling water on the stove is time-consuming and even tedious.

7. They Are Easy to Install and Maintain

The size and construction of these units make them easy to install and maintain. Of course, the tankless units feature a more complex installation process compared to the tank units.

However, once the tankless is in place, little to no maintenance is required because it is an electronic device.

The units that feature a storage tank need to be drained and the tank cleaned for efficiency. Because of their small size, the tank cleaning should not take long.

It is always best to have a professional installing both the tank and tankless units.

8. They Are Stylish and Discreet

One of the primary problems people have with regular water heaters is their size, making them an eyesore. But the under-sink water heaters are small and discreet. 

For example, the Bosch Electric Mini-Tank Water Heater Tronic 3000 T 2.5-Gallon can hold up to 2.5 gallons of water. It comes in discreet dimensions of ‎13.75 x 10.75 x 13.75 inches, which fit under the sink or mounted on the wall.

The Stiebel Eltron Tankless Water Heater only measures 4.6 by 16.6 by 14.5 inches, so it can be mounted onto the sink, taking up no room at all. Some people prefer to mount these water heaters in visible spots to give their spaces a modern-tech look. 

For more, see our list of best electric tankless water heaters.

Disadvantages to Under-Sink Water Heaters

As with nearly anything, there are a few disadvantages to under-sink water heaters. There aren’t any major deal-breakers, but below I’ll go over the few cons that you might want to consider.

1. It Can Be Hard to Regulate Temperature

This challenge is most often apparent in tankless units, which heat all the water uniformly as it passes through the system.

2. Leaks Can Cause Water Damage

Leaky tank heaters placed under the sink can cause humidity damage to the kitchen if left unattended. Their discreet placement makes it harder to notice leaks early.

How Does an Under-Sink Water Heater Work?

There are 2 types of under-sink water heaters: tank and tankless heaters.

How a Tank Under-Sink Water Heater Works

The tank under-sink water heater features three parts that work seamlessly to deliver instant hot water.

The parts include:

  1. An inline water filter 
  2. The heating tank
  3. The dispenser

Cold water from the pipe goes through the inline filter removing any impurities present in the water. That ensures that the hot water directly coming out of the faucet is fresh.

After the filtration process, the water moves into a tank that has two chambers. One chamber holds cold water while the other has the heating element, and it also retains the hot water at a steady temperature, ready for use.

And finally, the water goes through a valve or dispenser and out into the faucet. So, when you open the faucet, the water from the heating chamber flows out, making room for the water in the cold chamber to flow in and get heated.

How a Tankless Under-Sink Water Heater Works

This process does not feature any holding tank because the unit is tankless. When you turn on the faucet, the cold water comes out of the water pipe and runs through a heating element that turns on when the water begins to flow

The water heats up instantly as it passes through the system

Which Type of Under-Sink Water Heater is Best?

Storage/Tank Under-Sink Water Heater

This unit is a miniature version of the traditional water heater. It features a small tank to hold the water before and after heating. 

The tank can range between 2.5 to 7 gallons which also makes it portable. That allows you to go camping or on a road trip with your under-sink water heater.

While this is not an on-demand water heater like the tankless unit, the time between the flow of cold water out of the pipe, the heating, and the flow into the faucet is pretty fast so that you do not have to wait for hot water. 

Advantages of Tank/Storage Under-Sink Water Heaters

  • It is easy to install
  • The unit doesn’t require a high water flow
  • It is cheaper to acquire

Disadvantages of Tank/Storage Under-Sink Water Heaters

  • You need space to house the tank
  • The unit can lose energy by radiant heat loss
  • It limits you to only the hot water in the heating chamber at any given time

Tankless Under-Sink Water Heater

The tankless unit is a small compact device that can mount onto the sink. This unit is an on-demand heater, meaning an endless supply of instant hot water anytime you open the faucet. 

You do not have to wait for the water to heat in the tank before it flows out onto the sink.

You may notice that with the tankless under-sink water heater, the water is the same temperature every time you open the faucet. The first spray of water with a tank unit may be lukewarm or cold when you open the faucet. 

The tankless unit is also more energy-efficient compared to the tank option because it doesn’t have to store the water and maintain the heat. The tankless under-sink heater only begins to work when it detects water flow. 

Advantages of Tankless Under-Sink Water Heaters

  • It is energy-efficient
  • It offers instant hot water at anytime
  • It has a longer life span
  • The unit takes up less space
  • Has fewer maintenance requirements

Disadvantages of Tankless Under-Sink Water Heater

  • High initial installation cost
  • There can be inconsistent water temperatures if multiple faucets are opened simultaneously
  • No hot water during a power blackout
  • It is harder to achieve lukewarm water

What To Consider When Buying an Under-Sink Water Heater

When choosing an under-sink water heater to buy, the many options, features, and designs can quickly become overwhelming. Below are a few of the biggest considerations you should think about when buying one for your home:


This consideration only applies if you are buying a mini-tank under-sink water heater. The unit’s capacity will depend on your needs. 

But the typical gallon sizes range from 2.5 to 7 gallons. You can also opt for larger gallons.


The size of the unit depends on the size of your kitchen and the section under the sink.

If you have an apartment, it is best to have a tankless unit that is discreet and hideable. You can easily unmount the unit and go off with it into your new space when you move out.

Also, the tankless units allow you to enjoy an endless supply of water. The smaller the capacity of a tank unit, the smaller the size. And vice versa.

Safety Features

Ensure that your water heater has excellent safety features that keep you safe. Most have a thermostat feature to regulate the temperature. 

With the thermostat in place, the unit is not losing radiant heat and posing a fire hazard under your sink.

You should also ensure that your unit comes with modulation, which will change the amount of heat the heater produces in response to water flow.

That way, the unit doesn’t produce too much heat and ruin some of the elements inside. Instead, the unit maintains the ideal temperature depending on the flow rate in your pipes.

Also, test the temperature and pressure valves regularly. The temperature and pressure relief valve (T&R valve) on a tank unit helps keep the heater safe. It is at the top of the water heater, and it protects the unit from too much temperature and pressure by discharging the overheated water.

Ease of Installation

Because the units are smaller, there is less work when it comes to installation. Also, it is easier to handle a smaller unit. 

Of course, the tankless unit is more complex to install. So, it is always best to get a plumbing professional to do this because of the electrical or gas connections required. The process can also be complex due to the cramped space and intricate structural components in the kitchen.

A tank unit may be simpler to install. You need to connect the dispenser faucet to the sink mounting hole. And mount the hot water chamber to the main bathroom or kitchen faucet. Make sure the tank is mounted vertically.

But, it is also best to let the professionals handle the installation of tank heaters because of the plumbing connections under the sink.

Energy Consumption

The good news is that under-sink water heaters tend to be more energy-efficient than traditional water heaters.

However, you can ensure that the unit is energy efficient by looking for the Energy Star label on the unit. The more energy-efficient the unit, the more money you save on your electricity bill. 


Under-sink water heaters are available at different costs ranging from budget to high-end prices: the more the bells and whistles, the more expensive the unit.

When it comes to quality and reputable brands, you may have to pay a bit more. 

Ensure the unit comes with a long-lasting warranty. The more extended the warranty, the more confident the manufacturer is with their product. That means that you can also trust in the durability of the product.

How Do You Get Instant Hot Water Under The Sink?

If you rely on the central water heater and do not want to invest in an under-sink water heater, it may be best to consider a hot water recirculation pump.

This pump attaches to the water heater, and it returns unused hot water to the heater so that the water doesn’t become cold.

The plumbing system creates a loop directly from the water to the faucet and back to the heater again. Because the unused hot water is drawn back into the heater, you instantly get it anytime you need hot water.

Without the assistance of a recirculating pump, the hot water does not recirculate. Instead, it stays in the pipe where it cools down. That is why, when you open the faucet, you first encounter cold water before hot water comes gushing out.

The cold water in the pipes must flow out to make way for the heated water.

But with the pump, all the hot water goes back into the heater when the faucet is off, leaving the pipes empty. So, when you open a tap, hot water comes directly from the heater.

Unfortunately, it can be pricey to buy and install the pump, plus maneuvering the plumbing system to accommodate the loop in the system.

It is most likely cheaper to buy an under-sink water heater to get instant hot water under the sink.

One popular brand of under-sink water heaters is made by Marey. Read our brand review at Are Marey Tankless Water Heater Worth It: Review, Problems & Comparisons


Most under-sink water heaters are tankless. That is because the tankless units are more convenient. An under-sink water heater eliminates the need to relocate your main water heater closer to the kitchen or bathroom.

Also, it makes it easy to access hot water anytime you need it, even for mundane tasks. The best part is their energy efficiency and timeliness.


Photo of author

Hubert Miles

I've been conducting home inspections for 17 years. I'm a licensed Home Inspector, Certified Master Inspector (CMI), and FHA 203k Consultant. I started to help people better understand the home inspection process and answer questions about homeownership and home maintenance.
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