Plumbing, Water Heaters

Under Sink Water Heater: How Instant Hot Water Works

Under Sink Water Heater Lg

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 tank under-sink water heaters work by heating water on demand. They are incredibly effective, and there is virtually no delay between turning on a faucet and receiving hot water.

Because hot water cools when sitting in pipes, it can take as long as three minutes for cold water to exit the pipe and hot water to arrive at the faucet. Under sink water heaters heat the cool water in the hot pipe, providing instant hot water.

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They allow you to enjoy hot water 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.

Our Favorite Under Sink Water Heaters

We have two favorites for under-sink water heaters; two tank models by Bosch and Thermoflow and one tankless model by EcoSmart.

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.

Under sink water heaters are good and can be a great addition to your holiday vehicle or home. Apart from being very powerful, they are energy efficient and compact in design. The heaters also provide on-demand hot water, thus saving you energy, water, and time.

However, you should take the time to select a water heater that is perfect for your needs. 

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

The Best Under Sink Water Heaters

There are so many under-sink water heaters that you can select from. Most of these heaters are eco-friendly, energy-efficient, and compact in design.

Let’s consider the top under-sink water heaters in the industry:

Top Pick - Tank Model
Bosch Electric Mini-Tank Water Heater Tronic 3000 T 2.5-Gallon

This 120/110-volt point-of-use tank water heater has a 2.5-gallon tank, fits in your cabinet or on the floor, and plugs into a standard electric outlet. We like that it can supply hot water to two sinks simultaneously.

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03/11/2024 01:29 pm GMT

Bosch Electric Mini-Tank Water Heater

The Bosch electric water heater is the best option if you have low hot water demand. 

We like the Bosch Electric Mini-Tank Water Heater Tronic 3000 T 2.5-Gallon.

The water heater is designed with premium glass-lined material. It’s also energy-efficient, thanks to the thick layer of CFC-free foam. 

It’s also designed in a compact size, making it fit under the sink perfectly. The heater also has a sleek design that makes it blend in with most decor. You’ll find a thermostat on the product where you can select your perfect temperature. 

Many users love this heater since it’s perfect for smaller uses. It comes with a 2.5-gallon, 4-gallon, or 7-gallon water tank to store enough water for smaller applications.

The heater also provides instant hot water as soon as you want it. For instance, after you open the faucet, you’ll get hot water in less than a minute. 

The heater works perfectly with a 120V power supply line. It’s designed with a recovery rate of 6.8 GPH. It functions well within a temperature range of 65 to 145 degrees Fahrenheit. The product weighs about 17.4 pounds. 

The Bosch Electric Mini-Tank Water Heater Tronic 3000 T 2.5-Gallon is a handy little water heater that can be installed under your sink and provides on-demand hot water whenever needed.

With a thermal efficiency of 98%, you’ll always have plenty of hot water on hand. Plus, the glass-lined material ensures a long service life.

This model has a 120v plug 36-inch power cord, so you’ll need to be able to connect it to a nearby standard outlet or have an outlet installed.

Top Pick - 240v Tankless Model
EcoSmart ECO 11 Electric Tankless Water Heater
$299.00 $203.25
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03/10/2024 03:54 pm GMT

EcoSmart ECO 11 Tankless Water Heater

EcoSmart ECO 11 Electric Tankless Water Heater is the best for the energy efficiency of an under-sink water heater. You should go for it if you intend to save energy

For instance, you can save money on energy and water bills by over 50%. According to EcoSmart, this heater has an energy factor of 0.98 and an energy efficiency of 99.8%. It’s also the best option for eco-conscious consumers. 

The water heater is also designed with self-modulating technology. This technology regulates the output energy, thus ensuring optimal performance in the long run. 

The heater is generally compact, meaning you can install it anywhere. It measures 11.5 by 8 inches. The heater is also lightweight, with a weight of 6.5 pounds.

You’ll also find a digital screen and thermostat where you can observe and set the temperature. The thermostat and screen are easier to use. 

The heater works with a 240V power supply and functions at a recovery rate of 2 GPM. It also operates within a temperature range of 80 to 140 degrees Fahrenheit. 

Thanks to its lifetime warranty, many consumers have fallen in love with this heater. However, the warranty won’t cover incidental damages. 

The challenge to this heater is that it isn’t perfect for higher water demands. It won’t work well as a stand-alone water heater in colder regions but is the ideal complimentary water heater for a whole-house electric tankless water heater system. 

Thermoflow 2.5 Gallons 120 Volt Corded Electric Mini Tank Water Heater

The Thermoflow is a high-efficient 2.5-gallon tank water heater that can provide instant hot water to one or two sinks. Requires plugging into a standard 120 Volts electric outlet.

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03/10/2024 08:09 am GMT

Thermoflow UT10 Under Sink Water Heater

The Thermoflow UT10 Under Sink Water Heater is the best solution if you stay in a colder area. The model is designed with a frost protection mode, starting immediately if the temperature drops too low. Still, don’t place it where the freezing is likely to occur.

This heater is also designed with an energy-saving mode. The tank is lined with an EPS foaming heat system and stainless steel. Foam assists in heating water quicking eliminating long waiting periods. 

The heater also comes with a thermostat function that allows it to save even more energy. What the function does is that it automatically stops heating water when the unit reaches your set temperature.

It’s developed with a tank capacity of 2.6 gallons and a recovery rate of 2.6 GPM. The heater weighs 14 pounds and can operate at a power of 120 V. The unit is 18-inches tall by 12.2-inches wide and 11-inches deep.

However, this heater might be challenging to install (more complex than the Bosch), and its size may not work under some sink vanities.

Are Under Sink Water Heaters Worth it?

Under sink water heaters are worth it due to their compact design and energy efficiency. An under-sink heater works effectively as a single unit, or you can install one as a buffer in line with an existing tankless heater. Both system setups deliver hot water to the faucet or appliance quickly.

Water Heater Under Sink Lg

How Does an Under-Sink Water Heater Work?

There are two types of under-sink water heaters: tank and tankless heaters. The tank under-sink water heater features three parts that seamlessly deliver instant hot water.

Under sink water heaters are designed to provide hot water instantly from the faucet. These heaters are designed with three parts: the dispenser or valve, a heating tank, and an inline filter. 

The inline filter is linked to your cold water supply line to extract impurities for the water supply to remain fresh. The water moves to the heating unit from the filter. Here, there are two chambers, one for heating and holding hot water and another for storing the cold water supply. 

When you open your tap, the water heater will deliver instant hot water and at the same time draw filtered cold water from the cold water holding chamber. Water is therefore heated, and the process repeats itself continuously. 

The process takes a short time to complete, and you’ll get on-demand hot water

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

Buying Guide for Under Sink Water Heaters

Below are factors to consider while selecting an under-sink water heater:


This involves choosing between a tankless water heater or one with a tank. The tankless water heaters come at a higher cost, although they provide hot water instantly. Furthermore, they also cover a little space. 

A tank water heater is perfect in situations where you would like to access hot water in various corners of your home. The tank will store hot water, and you can access it anytime you need it. 

Flow Rate

If you’re seeking a constant flow of hot water, the under-sink water heater must come with a high GPH (GPM). The GPH (gallons per hour) or GPM (gallons per minute) represents the amount of hot water the product can supply. 

Energy Rating

Even though you might enjoy instant hot water, the cost of electricity might be a challenge. The heater’s energy rating represents its capacity to consume less energy. The lower the energy rating, the higher the energy used to heat a large amount of water. 


Since space is limited in most cases, you should use it well. For instance, an under-sink water heater containing a tank will take up more space. 

Tankless heaters are more compact and can be installed unnoticed in tight spaces. Before you buy a water heater, measure your available space. 

Heating Capacity

If your water heater effectively performs, you must balance demand with capacity. Here, it would help if you thought of the hot water you need at a time. Also, consider when the heater will heat water once the available stock is over. 

After you have used hot water, it’s immediately replaced with cold water in most cases. However, it might take just a few minutes to heat cold water. If you’re expecting a higher hot water demand, buy a heater that can supply the water quickly. 


When selecting a water heater, ensure that the electrical outlets in your unit can handle your water demand. Most water heaters function well with a 120V supply. If the heater requires 240V, you’ll need to hardwire a dedicated 30-amp breaker.

Safety Features

It’s great to have additional features on your under-sink water heater. For instance, the features include a pressure relief valve, modulation, or thermostat. 

A thermostat makes it easier for you to control the water temperature. Modulation enables the adjustment of the heat output in response to the water’s flow rate. A pressure valve prevents any damage resulting from excessive water pressure. 

Installation and Maintenance

Whether you would like to call a professional plumber to help you out or you’re a handyman who can do everything by themselves, check the installation requirements keenly to understand the whole process. Some products only require you to plug them into the power outlet to function. 

However, others might require plumbing or electrical work to operate effectively. When it comes to maintenance, ensure you can use your heater correctly and repair it when necessary to prolong its life. 


There are so many risks that may arise after installing an under-sink water heater. A reliable warranty is likely to put your mind at ease. 

However, warranty terms vary from one manufacturer to the other. A longer warranty period indicates durability. Some models are sold with a lifelong warranty. 

How to Install an Under Sink Water Heater

Benefits to Under-Sink Water Heaters

Under-sink water heaters offer many benefits, making 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

There is no argument that under-sink water heaters are designed to be more energy efficient than traditional water heaters. These heaters can help reduce your home’s energy consumption since they require little power to heat the water. 

They only work when you need instant hot water meaning that you can save a lot of energy. 

3. They Save You Money

The tank and tankless under-sink heaters reduce your household’s energy consumption by lowering cold water waste, 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, 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 adjust the unit to heat the water to a specific temperature 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, tankless units feature a more complex installation process than 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 install the tank and tankless units.

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

8. Compact Size

Most under-sink water heaters are designed with a compact size, thus not requiring much space. Their compactness means you can install them anywhere—including in the kitchen, under your sink, and on the wall. 

9. Easy Installation

You can easily install any under-sink water heater if you have any background knowledge in plumbing or electrical setup. Most heaters come with instruction manuals to guide you through the installation process

The installation process will also take you less time than you expected.

Disadvantages to Under-Sink Water Heaters

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

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.

Which Type of Under Sink Water Heater is Best?

There are two types of these water heaters available in the industry. 

Tank Under Sink Water Heaters

The storage or tank water heaters heat a certain amount of water and then store it for use later. In most cases, the heater is designed with 2.5 to 7 gallons of tank capacity. They are the best solution, especially if you don’t need instant water. 

The benefit of these heaters is that they are easy to install. You can also plug them into any standard electrical power supply. 

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 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 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 Heaters

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. 

There is no water storage for tankless or instant water heaters. The heaters will heat water instantly as it flows through the system. You’ll access instant hot water if there is a water flow. 

The heaters are also energy efficient since they aren’t always working to keep the water hot. Your water heater will only function when it detects water flowing through it. 

However, the tankless water heater requires more setup than the storage tank models. For the water heater to function correctly, you should connect and hook up the wiring directly to a dedicated circuit in your electrical panel

After installation, you’ll always have access to hot water when you want it. 

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 than the tank 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

How Do You Get Instant Hot Water Under The Sink?

Watts Premier Instant Hot Water Recirculating Pump System with Built-In Timer 6" X 6"
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03/11/2024 04:34 am GMT

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 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. 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, hot water comes directly from the heater when you open a tap.

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

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

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


As we have seen, an under-sink water heater is designed to supply hot water in a condo, cottage, RV, garage, home, office, and at the point of service. These heaters are reputable for their energy efficiency, eco-friendliness, and compact design. 

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.

They also come with a friendly warranty to help you operate them risk-free. However, before buying an Under Sink water heater, consider different factors such as flow rate, heating capacity, safety features, dimension, energy rating, etc. 

You can install the water heater to save cost since the installation process is relatively straightforward. You’ll also get an installation manual when you buy the heater. When you find the installation complex, involve a professional electrician or plumber. 

Do You Need a Licensed Plumber?

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Hubert Miles | Licensed Home Inspector, CMI, CPI

Hubert Miles is a licensed home inspector (RBI# 2556) with more than two decades of experience in inspection and construction. Since 2008, he has been serving South Carolina through his company, Patriot Home Inspections LLC. As a Certified Master Inspector, Hubert is dedicated to providing his expertise in home inspections, repairs, maintenance, and DIY projects.