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Witten by Bernard Morgan (Engineer) & Adam Apperley (Marketing). Published: November 13, 2023. Updated: March 11, 2024

How does a back boiler work?

A back boiler is a type of heating system that is typically installed behind a gas or solid fuel fireplace. The more popular gas version had a room heating fire at the front and a separate gas fueled boiler in the back. The system, which is essentially two heating systems in one shares a flue, often up a chimney.

It works by using the heat produced by the boiler to warm up water, which is then circulated through to the cylinder heat exchanger to warm the water in the cylinder. They also heat the radiators when switched to that water circuit.

A detailed breakdown of how a back boiler operates

image of a back boiler before removal

Combustion of Fuel

The process begins with the combustion of fuel (typically wood, coal, or gas) within the fireplace or stove. The heat generated from this combustion is used primarily to warm the room in which the stove or fireplace is located.

Heat Transfer

Gas burning back boilers generally have 2 burners, one at the front for the room it is in and one at the back which acts as a water heating boiler for hot water and central heating.

Solid fuel back burners tend to only have the one burning system to generate heat and heat water. The back boiler is situated directly behind the fire so that it can absorb this heat. The body of the boiler is a heat exchanger typically made from steel or cast iron, which is very effective at absorbing heat.

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Water Heating

Gas back burners use a boiler behind the room fire to heat water in a closed loop with the cylinder. This means that the hot water is transferred into a loop of pipe inside the hot water cylinder to transfer heat to the water you would use to wash up.

Solid fuel systems are the same, but there is no separate boiler behind the main coal or wood burner.

Circulation Through Radiators

The hot water is circulated through a network of pipes to radiators throughout the home. As the water flows through the radiators, it transfers its heat to them, warming up the rooms in which they are located.

Return for Reheating

After the water has passed through the radiators, it will have cooled down. It returns to the back boiler to be reheated by the ongoing fire, and the cycle repeats.

Hot Water Supply

In addition to heating the home, the hot water produced by the back boiler can also be used for domestic hot water needs. This usually involves a separate circuit that directs water to a hot water cylinder where it is stored for use in taps and showers.

A heat exchanger is used in the cylinder, for both fuel types, it is usually a coiled pipe inside the hot water cylinder that transfers the heat.

Exhaust Gases

The combustion process produces exhaust gases that contain carbon dioxide and water vapour, among other things. These gases are typically vented out through a flue that leads up the chimney.

A Quick Back Boiler History

Back boilers were a popular choice in the UK from the 1960s to the 1980s, especially because they could be installed without taking up additional space in the home. However, with advances in heating technology, they have been largely phased out due to their relatively low efficiency compared to modern condensing boilers. Additionally, since they are tied to the use of the fireplace for heat, they may not provide as consistent or controllable a heat source as other central heating systems.

It’s worth noting that since the 2005 Building Regulations were issued, back boilers have been made illegal in the UK. This was because back boilers do not meet government-set safety regulations alongside their carbon emissions being larger than other boilers, such as condensing gas boilers.

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The Efficiency of Back Boilers

Most working gas back boilers have a permanent pilot lite which can bring their efficiency to about 50%. This is why it can be so economic to replace them.

Early iterations of back boilers were notably lacking in efficiency, achieving less than a 70% efficiency rating. However, subsequent advancements in their design enhanced their performance significantly, allowing for efficiency levels to reach approximately 80%.

Despite these improvements, back boilers fall short when compared to the modern combi boilers available today, which can achieve efficiencies of 90% or more. We are a Viessmann Premium Patner and their Vitodens 200-W boiler model can reach 96% efficiency.

a back boiler being replaced

The Pros And Cons of Back Boilers

The back boiler, a heating system component, presents a mixed bag of benefits and drawbacks:


  • Space Efficiency

    Due to their compact design, back boilers are unobtrusive, making them suitable for smaller living spaces where conservation of space is a priority.

  • Cost-Effectiveness

    They are generally less costly compared to other heating systems, making them an economical choice.

  • Dual Functionality

    These systems adeptly provide central heating and domestic hot water, streamlining two critical functions into one unit.

  • Secondary Heat Source

    Back boilers can serve as an ancillary heat source, improving room warmth and reducing strain on primary heating systems.

  • Integrated Heating

    They can be connected directly to the central heating system, negating the need for additional equipment like immersion heaters for warming the house.

  • Reliability

    When maintained regularly and with care, back boilers demonstrate a high level of dependability.


  • Illegal to Install

    Can no longer be installed/used due to current building regulations.

  • Efficiency Limitations

    The primary limitation of back boilers is their efficiency. Modern boiler systems can reach efficiencies up to 98%, whereas back boilers peak around 80%, which can significantly influence energy consumption and environmental impact, particularly in cooler climates.

  • Delayed Hot Water Supply

    Unlike more contemporary systems, back boilers cannot provide immediate hot water on demand.

  • Ventilation Requirements

    The necessity for open-flued ventilation can introduce risks; inadequate ventilation may lead to the accumulation of harmful gases such as carbon monoxide, posing severe health risks.

  • Ventilation Drawbacks

    The same ventilation needed for safety can also be a source of unwanted drafts and noise within the home.

  • Restrictive for Remodelling

    The reliance on chimney flues for ventilation can considerably limit renovation possibilities.

  • Compliance Issues

    With the UK Government’s regulations stipulating that new boilers must achieve a minimum of 86% efficiency, the installation of new back boilers has become illegal.

Safety Concerns with Back Boilers

The main safety concern is that a back boiler needs ventilation into the room. They use air in the room for combustion rather than drawing it in from the outside like a modern boiler. This means it’s not a sealed unit and gases can flow both ways.
Back boilers are typically built to last between 20 to 30 years. However, as with any ageing appliance, the likelihood of safety issues rises over time.

Considering that back boiler installations predominantly ceased in the 1980s, most units in operation are now approaching or have surpassed their intended lifespan.

The halt in production and waning popularity of back boilers mean that obtaining replacement parts for maintenance can be challenging. This scarcity often results in less than optimal repair work, which can inadvertently heighten safety risks.

Furthermore, the natural process of expansion and contraction within back boilers is not inherently hazardous, but over time it can potentially lead to structural damage within the home, posing indirect safety issues.

What if my back boiler isn’t working?

Back boilers provide hot water to your home as well as heating, much like a modern condensing boiler. If your hot water or heating isn’t working there are two options;

  • find someone who is willing to try and fix it for you, and
  • replace it with a much more efficient and modern condensing gas boiler.

We would suggest replacing it. At best an old gas back boiler is probably only 50% efficient. This means that however much gas you use for the back boiler, half is being low through inefficiency. Replacing it could save 50% on your gas bill (not including any gas hobs or ovens).

Read through our back boiler information pages to see if it’s right for you. Here at Boilerhut we’ve replaced hundreds of back boilers over the years. It’s a relatively simple process, but it can be a bit time consuming. So grab an online quote and speak to one of our engineers today.

Need a new boiler?

If you need a new or replacement boiler or are looking at having a central heating system installed, get an online quote (it takes less than 1 minute), and you’ll get an instant price. But why choose Boilerhut:

  • Next day boiler installation.
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  • Finance options available for new boilers and central heating installation.
  • 10 to 12-year warranty on boilers.
  • ‘Excellent’ 5* customer satisfaction on Trustpilot.
  • Fixed prices. No hidden extras.
  • Get up to three quotes for different-tier boilers.

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