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What is a heat-only boiler?

Are you confused by all your options with a boiler replacement? With so many different makes and models of boiler out there, starting the search for a new one can seem like a daunting task. The first thing to think about is what type of boiler you need. In this handy guide, we explain what a heat-only or conventional boiler is; how it differs from the other two types of boilers; and discuss their pros and cons. So, if that’s something you’re interested in, read on!

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Confused about what a heat-only boiler is? If so, we understand!

Looking for a boiler replacement can be very confusing. There is an overwhelming range of makes and models of boiler to choose from. So, starting the search for a new one can seem like a daunting task.

 But don’t worry, The Heating People are here to help!

The first thing to think about is what type of boiler you need.

Boilers fall into three basic types: heat-only or ‘conventional’ boilers; combi boilers; and system boilers.

In this handy guide, we explain what a heat-only or conventional boiler is; how it differs from the other two types of boilers; and discuss their pros and cons.

You might also like our related blog: Your options with a boiler replacement.

How do heat-only boilers compare to the other types of boiler system?

Before we look at heat-only boilers, let’s see how they compare to the two other types of boiler system:

1. Heat-only or ‘regular’ or ‘conventional’ boilers


Heat only boilers are also called ‘regular’ or ‘conventional’ boilers.

Many people think of them as being the traditional, or ‘conventional’ way of heating a home. 

They provide both heating and hot-water with the use of an additional cold-water tank and hot-water cylinder.

They’re a good choice for large families with a high demand for hot water. This is because they store large quantities of hot water in a cylinder.

Heat-only boilers need a large feed tank in your attic or loft. This tank receives cold water from the mains and feeds it down to the boiler.

When the fuel ignites, the heat exchanger warms the water. A pump sends the water to the hot water cylinder until its needed.

Heat-only boilers are either sealed or open vented.

Open vented systems are open to atmospheric pressure. They have a feed and expansion tank at the highest point in the property (usually a loft or attic). This feed and expansion tank maintains water pressure whilst a pump circulates the hot water to your home’s radiators.

Sealed systems are closed to the atmosphere and don’t have a feed and expansion tank. They maintain water pressure using an expansion vessel (which can be internal or external to the boiler).

Combis and system boilers can also work on a sealed system.

Read the pros and cons of heat-only boilers here.

2. System boilers verus heat-only boilers

System boilers are sometimes confused with heat-only boilers, because they also use a cylinder to store hot water.

This means that they are also a good choice for larger homes with a high demand for hot water.

But unlike heat-only boilers, system boilers don’t need a cold-water tank in the attic. They take their water supply directly from the mains – and in this way – system boilers are similar to combi boilers.

Since system boilers take their water directly from the mains, they don’t need as much space ass a heat-only boiler. It also means that they offer better water pressure. So you’ll get a more consistent flow rate throughout your home.

You can read about the pros and cons of system boilers here.

3. Combi boilers versus heat-only boilers

‘Combi’ is short for ‘combination’ boiler because it provides both your heating and hot water from one all-in-one (combined) unit.

Unlike heat-only or system boilers, combis don’t need any additional tanks or cylinders – everything it needs is inside the unit.

As soon as you turn on a hot water tap, a combi heats water taken directly from the mains. It doesn’t store hot water for later use, it heats it on demand. This eliminates the need for tanks and cylinders.

Combi boilers are a popular choice for homes that are limited on space.

However, combis are best suited to homes with relatively small hot water demands at any one time.

If you’re a household where everyone wants showers at the same time, then a combi won’t be suitable. And this is where a heat-only or system boiler would be more appropriate.

Check out our related blog: Pros and cons of combi boilers.

How does a heat-only boiler work?

With an unvented heat-only boiler, there will usually be:

  • two water tanks in the loft or attic;
  • a cold water tank and a small feed and expansion tank; and
  • a hot water cylinder in an airing cupboard.

The cold-water tank fills with water from the mains supply. It is then fed to the hot water cylinder, with the help of gravity.

The boiler fires up by igniting its fuel (usually gas); and the heat exchanger warms up the water in the cylinder. It is stored here until you need it.

When you set your thermostat, a pump circulates hot water around your radiators. The radiators emit the heat to warm up your rooms.

The hot water is also sent to your taps and showers, when you turn them on.

The feed and expansion tank brings in cold water to regulate the water levels. It helps to deal with the expansion that occurs when the water is heated. It also replaces any water that is lost because of evaporation or leaking.

Are heat-only boilers condensing boilers?

The terms ‘condensing boilers’ and ‘non-condensing boilers’ are misleading. They actually describe the technology within the boiler itself, rather than what sort of boiler it is.

In the past, we had non-condensing boilers which were very inefficient. But today, all modern boilers that you can buy are condensing boilers, which are much more efficient.

Condensing boilers make the fuel you’re burning (usually gas) go further. They do this by recycling the wasted exhaust gas that is created during a boiler’s normal operation.

By re-using the exhausted gas, water vapour and steam back through the system, some of the heat is retained. This makes the boiler’s operation more efficient.

They can typically extract 90-92% of heat from fuel, compared to 50-80% that a non-condensing boiler can.

According to the Energy Saving Trust, you could save £340 a year by replacing an old non-condensing boiler. And given the current price of gas, the savings are likely to be even bigger!

But don’t worry, the Boiler Plus Regulations) require all new boilers to use condensing technology as standard. So you won’t have to choose between condensing and non-condensing technology.

How efficient are heat-only boilers?

As we’ve mentioned, all new boilers have condensing technology which makes them much more efficient than in the past.

New models are ErP A rated, achieving up to 90% efficiency in optimum conditions.

However, heat-only boilers are generally the least efficient type of boiler because they have the fewest control options.

Since hot water is stored in a cylinder, you may waste energy by heating more water than you use. There will also be some heat loss from the cylinder. It’s therefore important to ensure that your hot water cylinder is properly insulated to prevent heat loss.

Can a heat-only boiler run out of hot-water?

Unfortunately, yes. But only for as long as it takes the boiler to heat up more water.

This is where heat-only boilers (and system boilers) are different from combis.

 A combi boiler will never run out of hot water because it heats water instantaneously on demand. But this also means that a combi will struggle to supply multiple outlets at the same time.

Heat-only and system boilers work differently because they heat water and store it in a cylinder for use later. Once all the water has been used, you’ll have to wait for more water to be heated again. 

During your survey, your engineer will ask you about your hot water usage. This will help with the correct sizing of your cylinder. This will of course be limited by the space available in your home.

What are the advantages of a heat-only boiler?

A woman looking at a diagram of a heat-only boiler.Let’s look at some of the advantages of a heat-only boiler:

Advantage: Heat-only boilers work well in large households with multiple bathrooms

Heat-only boilers store hot water in a cylinder. This means that they can supply large amounts of hot water to multiple outlets at the same time.

They’re ideal for bigger families who want to be able to shower and run taps at the same time.

This is a particular advantage over combi boilers, which heat water instantaneously, rather than storing it.

Advantage: Heat-only boilers are effective in areas with low water pressure

Another advantage that heat-only boilers have over combis, is that they don’t need good pressure systems to work.

Heat-only systems are fed by gravity (hence the tank being in your loft or attic). This makes them perfect for regions where the water pressure is too low for a combi boiler to work.

Advantage: Heat-only boilers can use an electrical immersion heater

Electric immersion heaters can be fitted to the hot water cylinder of a heat-only boiler system. This can heat the water instead of the boiler.

This is handy if your boiler ever suffers a breakdown. You can switch to your immersion heater, and won’t be left with no hot water.

Advantage: Heat-only boilers are compatible with renewable heating systems such as solar thermal

Another advantage of heat-only boilers is that they can be integrated with renewable heating systems. This can save you money whilst helping the environment.

Solar thermal heating systems use free heat from the sun to heat your water. This means you won’t have to burn gas, which currently costs a lot!

What are the disadvantages of a heat-only boiler?

But as with all things, there are drawbacks too. Here are the cons with a heat-only boiler:

Disadvantage: Heat-only boilers require a lot of space

Heat-only systems require the most space out of all three types of boilers. This is because they need external tanks and cylinders in addition to the boiler itself.

Disadvantage: heat-only boilers don’t have instant hot water

Combi boilers produce hot water instantly, on demand. They never run out of hot water, although they struggle to supply multiple outlets at once.

This is different from heat-only boilers which heat water and store it in a cylinder, for use later.

This means that the amount of hot water a household can use is limited by the size of the tank. After the cylinder is emptied, you’ll need to wait for the boiler to heat up more water. 

Disadvantage: Heat-only boilers are generally the least efficient type of boiler

As we mentioned earlier, heat-only boilers are generally the least efficient type of boiler. Because they have the fewest control options, and they store hot water in a cylinder.

If you’ve ever noticed how warm an airing cupboard is, you’ve experienced heat loss from a hot water cylinder!

Your boiler will be working harder to keep the cylinder warm, and you’ll be spending more on your energy bill. Therefore, insulating your cylinder is a must.

Final thoughts…

What is a heat-only boiler?

Heat-only boilers are a good choice for larger homes with a high demand for hot water.

 But if you live on your own; or have a small household where people aren’t wanting to use multiple hot water outlets at the same time; a combi might be fine for you.

For tailored advice, call one of our friendly team today to book your free survey.

Useful Links:

Viesmann: What is a heat-only boiler

Vaillant: What is a heat-only boiler

Worcester Bosch: Boilers explained

Energy Saving Trust: Boilers

Which? Reviews – energy efficiency