Know The Difference: Boilers VS. Water Heaters

You’d be forgiven for thinking that boilers and water heaters are the same things. After all, you can’t exactly boil water without heating it*. Boilers heat water. Water heaters heat water. So what’s the difference?


That’s what we’re going to explore in this article. In short, the difference is in how the heated water is used. With a boiler system, hot water is used to heat a building, while water heaters are used to supply hot water to a building. There’s a notable exception to this rule (the combi-boiler), which we’ll discuss later.


The difference between water heaters and boilers is quite broad, and there’s a lot of ground to cover. Here’s how we’re going to break things down. First, we’ll discuss what boilers and water heaters are and how they work. Then, we’ll get into why there’s some confusion between the two terms. Finally, we’ll talk about how you can tell the difference between boilers and water heaters.


By the end of this article, you’ll never mistake a boiler for a water heater again. Let’s dip our toes into the world of water heaters and boilers – carefully. The water is hot!


*Yes, physics nerds, we know that you can boil water without heating it at very low atmospheric pressure. We’re trying to keep it simple.

What Are Boilers?

Boilers are closed tanks in which water is heated in order to provide heat to a building. There are many different types of boilers, including electric, natural gas, steam, hot water, condensing (high efficiency), and combi-boilers. One boiler can have many different attributes – for example, you could have a condensing combi-steam boiler fuelled by natural gas.

How do boilers work?

The answer to this question is actually pretty complicated because it depends on the type of boiler you’re using. In general terms, a boiler heats water, then a fluid (either steam or hot water) is transported through pipes to radiators throughout your building. As the radiators take heat from the fluid and transfer it to rooms throughout your building, the fluid cools down. Eventually, the fluid returns to the boiler, where the cycle begins again.


Each type of boiler has attributes that change part of this cycle. While both natural gas boilers and electric boilers use heat exchangers to heat cold water, they use different sources of fuel to conduct the heating process. Condensing boilers also capture heat from flue gases (waste gas) in order to heat water.


An interesting fact about boilers – the term is a bit of a misnomer! Modern boilers don’t heat water to its boiling point – most systems heat water to about 60 degrees Celsius. 


Of course, no boiler system is complete without a thermostat, which tells the boiler how much heat it needs to produce for a given room or building. 

What Are Water Heaters?

Water heaters are devices that provide hot water continuously to a building. Like boilers, there are several different types of water heaters. They can generate heat through the use of fossil fuels or electricity – that’s another way in which they’re similar to boilers. There are condensing models of water heaters as well.


You can probably already tell – water heaters and boilers share a lot of similarities. The main difference between the two is how the water is used – instead of heating rooms, water heaters provide potable hot water for washing, cooking, cleaning, and other activities. 


One big difference between water heaters and boilers is that not all water heaters use tanks. There are tankless water heaters that heat water on-demand. 

How do water heaters work?

How water heaters work will depend on whether you’re using a tank or a tankless system. 


Water heater tanks work in a way that’s similar to how boilers work. Cold water is fed into the water heater. There is a heating mechanism – in the case of electric heaters, a heating element is used, while gas heaters use a burner. Hot water rises to the top of the tank, while cold water settles to the bottom – the principle “heat rises” applies to water as well as air!


When you need hot water, cold water enters the system and displaces the hot water, which pours through your faucet, showerhead, or other outlets. This is why you can sometimes run out of hot water with tank systems – when you use hot water faster than cold water can be heated, cold water will start coming out of your water heater.


Condensing water heaters operate on the same principle as condensing boilers – they take energy from flue gases to provide additional heat for your water. There are also heat pump water heaters, which are well suited to warm climates but aren’t suitable in places like Winnipeg. They take heat from the surrounding air and use it to heat your water.


Tankless water heaters, on the other hand, heat water on-demand. When you need hot water, a sensor in the tankless heater activates an element or burner, which heats the water quickly as it passes through the heater. Once you turn the hot water off, the tankless heater also stops. This helps to avoid standby heat loss (heat leaving through the walls of the tank when hot water isn’t being used). It also means you’ll never run out of hot water.


The biggest problem with tankless heaters? They can only supply so much hot water at a given time – larger households and commercial buildings would need to install several tankless heaters. 


Traditional water heaters with tanks are often confused with boilers – there are some good reasons for this that we’ll discuss in the next section. Tankless water heaters, on the other hand, are basically never confused for boilers.


Thermostats are used in water heaters, but you’ll adjust them far less often than you would adjust the thermostat on your boiler. After all, most people are content with the maximum heat of their water being left at a consistent temperature. 

The Confusion Between Water Heaters & Boilers

At this point, you probably understand why it’s easy to confuse a tank water heater for a boiler. They look similar, and they heat water using similar mechanisms.


Things get even more confusing when you consider the fact that the term “boiler”, in a generic sense, is used to describe any vessel that heats fluids for almost any use. With this terminology, water heaters are, technically, boilers – as are boilers that are used for power generation and a whole slew of other types of boilers.


In the HVAC industry, however, the term “boiler” specifically refers to the types of boilers used for heating a building (aka central heating boilers). Water heaters are not boilers.


But here’s where things get really confusing: Boilers are sometimes water heaters!


Combi is short for combination – combi-boilers are combination water heaters and boilers. We love combi-boilers here at Howell Mechanical. They take up less space when compared to a separate boiler and water heater system, and they can be quite energy efficient if you get a condensing combi-boiler.


Of course, their existence makes it harder for people to understand the difference between water heaters and boilers. That’s okay – in the next section, we’re going to help you tell water heaters, boilers, and combi-boilers apart! Not the greatest party trick to impress your friends with, but helpful if you want to be a little handier.

How To Tell Water Heaters & Boilers Apart

If there’s a tankless water heater, this is a pretty easy job – the one with the tank is the boiler!


Another circumstance where it’s easy to tell water heaters and boilers apart: If a building has a furnace or no radiators, then a water heating tank is almost definitely a water heater and not a boiler.


But what if a space has a boiler and a water heater?


First, check the shape. Most modern boilers are square or rectangular, while water heaters tend to be cylindrical.


Next, check the labels on the tank. There should be clear information as to whether or not the tank you’re looking at is a boiler or a water heater.


Wondering if the unit you’re looking at is a combi-boiler? Check the number of pipes going into the boiler. Combi-boilers will generally have 6-7:


  • Hot water outlet
  • Cold water inlet
  • Central heating flow pipe
  • Central heating return pipe
  • Pressure relief pipe
  • Condensate pipe
  • Gas supply (for gas combi-boilers)

The More You Know

We hope this post has given you insight into the important differences between water heaters and boilers. We offer water heater and boiler repair in Winnipeg, which is why we’re so passionate about the topics! 


Boilers and water heaters serve very different purposes – even though they both heat water to accomplish their duties. Applied fluid dynamics is a fascinating topic, and we’re always finding new, more energy-efficient ways of supplying buildings with hot water and heat. We hope you enjoyed the read!