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Solid Fuel Boiler Explained


The ever increasing cost of heating oil is forcing more and more off-grid properties to turn to solid fuel boilers, a renewable and more cost-efficient heating solution. As such, both traditional solid fuel stoves and the more modern biomass boiler are popping up in many rural homes. But what exactly is a solid fuel boiler, and how does it work?

What is a solid fuel boiler?

A solid fuel boiler is a heating system that heats water for central heating and domestic use by burning solid fuels such as wood and coal. The hot water from the boiler is stored in a hot water cylinder and is pumped to your radiators or hot taps whenever there is demand.

Types of solid fuel boilers

There are essentially two types of solid fuel boilers, the solid fuel stove and the biomass boiler.

Solid fuel stoves

Solid fuel stoves are standalone heaters that burn wood or coal to heat a room. They include open fire heating systems, closed fire heating systems, and back boilers. Solid fuel stoves that run on coal are known as coal burners, while those that run on wood logs, wood chips, or wood pellets are known as wood burners.

Open fires are the simplest form of solid fuel stoves and consist of a fire basket attached to a high output boiler. Wood, coal, or smokeless fuel is loaded onto the fire basket, and the exhaust fumes and smoke from the combustion process are vented into the chimney.

Closed fires, also known as room heaters, are solid fuel stoves usually located inside a closed firebox with one or two doors. The fact that the fire burns behind a door means you can easily control the fire’s burning rate.

Lastly, back boilers or BBUs, consist of a boiler fitted to the back of an open fireplace. Due to their low efficiencies, these boilers were banned from being installed in new properties.

Advantages of solid fuel stoves

  • They have low running costs.
  • They are flexible and can use both wood and coal fuels
  • They increase the ventilation of the property as they have a chimney

Disadvantages of solid fuel stoves

  • They are messy and produce soot and ashes
  • Wet wood and coal produce a lot of tar and smoke
  • They are space consuming and require additional space for fuel storage
  • They are unsuitable for heating several rooms unless they are connected to a heating system

Biomass boilers

Biomass boilers are an upgrade on solid fuel stoves and work like condensation boilers. They burn natural wood materials such as pellets, chips and logs to provide both space and water heating in your home.

These boilers are considered a carbon-neutral heating solution since the carbon dioxide that it releases into the atmosphere is the one that was previously absorbed by the wood fuel.

Biomass boilers are easy and convenient to run as they can be manually or automatically refuelled depending on the model.

Advantages of biomass boilers

  • They are carbon neutral
  • They can reach efficiencies of 90% and above
  • They are easy to run
  • Provides a free heating solution if you have access to wood

Disadvantages of biomass boilers

  • Expensive to purchase and install
  • They are large boiler units and consume a lot of space
  • Requires separate fuel storage
  • Manually-fed units produce ash which needs to be cleaned out regularly.

Types of solid fuels


Both dry and wet wood logs can fuel a biomass boiler or a solid fuel stove. It is a cheap and readily available fuel but has a low heat output.


Although more efficient than wood, coal is becoming out-phased due to its high environmental impact. It is, therefore, not a recommended solid fuel for heating your home as the government is planning to phase it out.

Wood pellets

Wood pellets are small pieces of wood made from compressed wood shavings and sawdust. They are the primary fuel for the biomass boiler and are usually stored in a storage tank and automatically fed into the boiler.

Wood chips

Wood chips are small pieces of wood formed by chipping a larger part of a tree, such as stumps and Roundwood. They usually have more energy content than wood and are cheaper than wood pellets. However, their quality is not standardized, making the fuel less efficient.

Smokeless fuel

As the name suggests, Smokeless fuels are solid fossil fuels that don’t emit smoke when burned. They can either be natural or artificial and include anthracite, coke, and charcoal. These fuels usually have a higher energy level than bituminous coal and burn hotter and longer.

Cost of solid fuels

Cost is another factor besides availability and government regulations that significantly impact the choice of fuel for your solid fuel boiler. Here is a table of the varying costs of the different fuel types.

Fuel type Average price (pence per kilowatt-hour)
Wood 6.93 or free
Coal 4.13
Wood pellets 5.99
Wood chips 2.99
Smokeless fuel 9.03

Automatic and manual solid fuel boilers

An automatic solid fuel boiler, also known as the gravity-fed solid fuel boiler, has a large feeder box usually positioned above the firebox. The feeder box usually stores several days’ worth of solid fuel (either wood pellets, coal, or anthracite) and gradually releases it to the firebox.

On the other hand, a manual solid fuel boiler does not have a feeder box and require frequent refuelling like a conventional fireplace. It has to be hand-fed every time the existing batch of wood fuel runs out.

What is the cost of a solid fuel boiler?

A new solid fuel stove will cost you anywhere between £500 and £5,000 without factoring in the installation cost. Biomass boilers are a bit more expensive and cost between £4,000 and £21,000, depending on whether it is manually or automatically fed.

Heating controls in a solid fuel heating system

The simplistic nature of the solid fuel stove makes it incompatible with most of the existing heating controls associated with oil, gas, and electric heating systems. However, modern biomass boilers are compatible with simple heating controls such as thermostatic radiator valves and programmers.

The high limit thermostat is most compatible with the solid fuel heating system. This gadget is a safety device that regulates the hot water temperature in the storage cylinder and prevents the water from overheating.

Solid fuel boiler tips

Follow the following tips to get the best out of your solid fuel boiler.

  1. Sweep the chimney regularly

You should sweep your boiler’s chimney at least three times a year. This is because the soot that builds up in the chimney lowers the boiler’s efficiency and increases the risk of chimney fires.

  1. De-ash the boiler every time you use it

Make an effort to remove the ash in your appliance either in the morning or the evening since the build-up of ash would reduce heat transfer efficiency through the boiler.

  1. Service the boiler at least once every 12 months

Modern biomass boilers require annual maintenance to fix minor issues before they escalate.

  1. Always buy quality fuel.

When shopping for solid fuels, go for high-quality fuel rather than cheaper, low-quality ones. This is because the quality of the fuel will determine the quality of heat and hot water produced by the boiler. Cheap fuels are associated with high ash and soot content. They also have a low heat output, meaning it would generate lower water temperatures in the boiler.

  1. Check your boiler settings.

Ensure the appliance operates at the highest setting in the winter period and a lower thermostat setting in summer. Doing this will minimize your fuel usage and result in lower heating costs.

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