Colton Deck

By: Colton Deck on May 4th, 2023

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What Is a Combi Boiler?

Tankless Water Heaters

If you’ve been researching tankless water heaters, you’ve likely come across combination boilers, or combi boilers for short. While tankless water heaters and combi boilers share similarities, they are still worlds apart in many ways. 

For over 15 years, Monkey Wrench Plumbing, Heating & Air has served the Greater Los Angeles area by installing and servicing tankless water heaters and combi boilers. In that time, we’ve learned this hardware inside and out. Now, we’re passing some of that knowledge to you. 

In this article, we’ll discuss what a combi boiler is and how it works. We’ll also define a few key differences between tankless water heaters and combi boilers so you can carry out your research with a leg up. 

Now, let’s get the ball rolling by addressing the elephant in the room: what is a combi boiler? 


What Is a Combi Boiler?



Put simply, a combi boiler provides hot water in the same way a tankless water heater does, but it also handles heating your home if you have hydronic heating. While combi boilers resemble tankless water heaters, this dual design makes them fundamentally different. 

Let’s pause here to address a key factor in whether a combi boiler is for you before we get too far ahead of ourselves. Combi boilers are only an option for you if you have hydronic space heating like in-floor radiant heating or individual radiators throughout your home. Now, let’s pick things back up.

Aside from “What is a combi boiler?” another common question we get is, “How does a combi boiler heat water and the house?” This is where combi boilers get more complex than a standard tankless water heater and can even be divisive in some cases. 


How Does a Combi Boiler Work?

To understand how a combi boiler works, we’ll first have to take stock of some key components. Combi boilers feature a dual heat exchanger design. The dual heat exchangers are responsible for heating water for both domestic hot water and space heating, while the stainless steel flat plate mixes the domestic hot water for home use

Let’s break it down step by step, starting with how a combi boiler handles water heating. A combi boiler works to heat water with the following process.

  • The boiler will turn on once hot water is needed at a fixture, meaning the fuel source will enter the combustion chamber and ignite.
    • If you are running your hydronic heating and call for hot water, the combi boiler halts heating your home and prioritizes providing hot water.
  • The heat generated will warm the heat exchangers.
  • Then, the heat exchangers heat the cold water. 
  • The water then passes through the stainless steel flat plate, which mixes the water for the home, ensuring the temperature is safe.
  • Hot water travels through your plumbing to the fixture. 

In this way, combi boilers are similar to tankless water heaters, providing you with hot water on demand. Some combi boilers also have a pre-heat capability, a small chamber where water is kept at a set temperature to provide more immediate hot water. 

Now, let’s check out how a combi boiler handles general heating

  • If your thermostat detects a drop in temperature, it will signal the combi boiler to turn on.
  • The boiler will turn on just like it did in our breakdown of how combi boilers heat water. 
  • The dual heat exchangers heat the water.
  • Hot water is pumped throughout your hydronic heating system.
  • Once the water cools, it returns to the combi boiler through a separate line, where it is heated by the boiler once again.
  • This process continues until your thermostat detects your home has reached the set temperature. 

Sounds pretty great, right? But there are a few things to consider before you go all in on a combi boiler. Let’s dive deeper into who a combi boiler is ideal for and for who it might not be so practical. 


Is a Combi Boiler Right for My Home?

As it is with any investment, you might be asking yourself, “Is a combi boiler right for me?” The only person who can answer that question is you and your family. But there are a couple of key reasons why a combi boiler may or may not be right for you. Let’s take a look. 


Combi Boilers Don’t Heat Water and Space Simultaneously

Because of their core design, combi boilers can’t heat water and your home simultaneously. If you’re using your combi boiler to heat your home and need access to hot water, it will stop heating air but keep its fan running while switching gears into heating water. This type of design may not work for every home type. 


Ideal for Smaller Homes

Combi boilers are not typically recommended for larger homes in our experience because the loss of heat when the space heating is cut is greater than that of a smaller home with less square footage to heat. Simply put, it takes a long time to heat a large space compared to a smaller one. 

Expanding your system with a separate tankless water heater can supplement your combi boiler and ensure hot water runs endlessly while your home maintains its temperature. This workaround adds complexity to your system and ultimately adds to your overall investment. 


Ideal for Space-Saving with Two-in-One Design

Going hand in hand with being ideal for a smaller home is the space-saving factor of a combi boiler. Because combi boilers handle both space heating and water heating, it eliminates the need for a standalone boiler or separate water heater. 


Higher Upfront Cost than a Tankless Water Heater

It’s probably no surprise that a combi boiler is more expensive than a standard tankless water heater. The two-in-one nature of these machines informs the higher upfront investment since you’re effectively buying two major heating elements in one. 

While the higher upfront investment won’t be worth it for those who don’t need space heating on top of water heating, it can be more cost-effective than upgrading a boiler and tankless water heater separately. 

As we’ve mentioned before, this decision is one specific to you and your home. Whether investing in a combi boiler for your home is worth it or not will rest solely on what you need and see fit. 


Combi Boilers in a Nutshell

After touching on what a combi boiler is, how it works, and who it might be best for, you should have a firm grasp on the basics of combi boilers. While combi boilers may not be for everyone, staying informed will help as you search for the best water and space heating options for your home.

With over 15 years in the field, we can confidently tell you that not every home is the same and that different situations require different solutions. If you’re not sure of where to start, use one of the buttons below to get in touch or book an appointment through our online scheduler