Woodburning Stoves and Chimney Installations

Find out about the differences between woodburners and multifuel stoves and what you need to know for the installation process. With a glossary of terms...

There is a wide range of woodburners and multifuel stoves available and it is important to choose the correct one for the space and purpose.

Differences between Woodburner and Multifuel Stoves

1. A woodburner stove does not have a grate. The logs are burned on a bed of embers and ash, with the air for combustion arriving at the level of the wood. This is the most efficient way to burn wood.
2. A multifuel stove has a grate and ashpan. Coal and coke are burned and air for combustion comes from underneath. Logs can be burned on a multifuel stove but they will burn fast.

Choosing the correct size (kW) stove

It is important to make sure a stove provides the heat output required for the room size. The following formula can be used to work out how much heat is required:

• width x length x height of the room = its area in cubic metres
• then divide this by 14 to give the approximate kW
• (for example, 196 cubic metres of space requires a 14 kW stove)

Choosing the correct size boiler stove

Many factors affect boiler size. The advice of a central heating engineer is recommended to ensure that the right size boiler is chosen.

Where to Place a Stove

The distance a stove is placed from combustible and non-combustible materials is important to get right. The documentation of any stove must state the minimum distance it may be placed from materials surrounding it. These vary depending on the kW of the fire and are set by European standards.

In the case of the single skin pipe, the rule is that the pipe must be three times its diameter away from combustible material and five centimetres from non-combustible material. Walls, electric cables, plumbing pipes and furniture should all be taken in to account.

How to Light a Fire; What Fuel to Use

Manufacturer's guidelines must be followed in the installation and use of a stove. Many chimney fires are caused by the incorrect use of stoves, and the use of damp wood. A flue liner of a 150mm diameter can become completely blocked in 62 hours; 60 percent of house fires are caused by chimney fires.

First get the chimney up to temperature as fast as possible to stop smoke and condensation forming. Use small kindling to achieve this instead of large logs, and make sure the wood is well seasoned and dry.

Woods have different drying and storage times to become well seasoned. Soft wood such as pine takes a lot longer than hard wood such as oak. Its moisture content should ideally be below 20 percent, any wetter than that and condensation occurs, which causes tar build up over time.

An open fireplace takes from 4 to 12 hours to heat up correctly; a chimney with flexible liner takes 3 minutes.

Installing the Stove

Every heating appliance - a central heating boiler or a stove - needs its own individual liner. However, more than one liner can be put up a chimney provided there is a two centimetre area of free space around each.

Using an existing fireplace and chimney flue

The type and condition of the chimney is most important as it has a major effect on how a stove performs. Factors to take into account include the diameter, height, angle of run and whether it is structurally sound. For example if the flue is too wide then the up draught may not be sufficient to take the smoke/gases out quickly enough before they cool. Soot forms when the vapours cool to a temperature of 60°C or less.

Generally the use of flue liners is the quickest way to correct a chimney that will not function well. Precast concrete or terracotta liners can also be used although these entail more work to fit. The existing chimney must be swept before a new liner is fitted (residue from previous fires could catch fire and burn).

The flexible liner used must be correct for wood burning (those for gas appliances cannot resist the heat from solid fuel or wood); it must be installed the right way up. Fitting it is a two-person job, with one being needed on the roof with the other at the fireplace. The chimney should be big enough to accommodate the liner and leave 2cm all the way round. The distance through any material, including a brick or stone wall, needs to be 16cm from any combustible material such as a wooden floor. Once the liner is in place, secured by clamps top and bottom, the top of the chimney stack can be capped off, leaving ventilation top and bottom.

Creating a flue

When there is no existing chimney, a flue can be created with precast concrete or terracotta blocks, or the double skinned insulated metal system.

• The concrete block system must conform to the European standard EN 1858
• The terracotta system must conform to the European standard EN 1806

The thermal resistance (resistance thermique), measured in RU, must be taken in to account. This is the level of heat transfer lost from a particular type of pipe. A lower RU is better.

Different makes of flue pipe should never be mixed in a single installation. For example, Poujoulat is not compatible with Negarra.

A single skin solid pipe can only be used for leaving the actual stove. It should never be used to go through a wall, a ceiling, or to the outside.

Any chimney stack needs to finish at the right height for there to be nothing affecting the up draught. Numerous things can affect it such as roof elevation, position in relation to the ridge of the roof and also the environment surrounding the house such as trees or taller buildings.

Note: the rules are completely different for wood pellet stoves which reach higher temperatures in the flues and therefore must have a ceramic flue system.

The installation of a flue should not be considered as simple DIY work as there is a risk of fire from an incorrectly installed flue; if installed incorrectly house insurance becomes null and void.

Glossary of Terms

• Renewable Energy
• Septic Tanks & Waste
• Swimming Pool Laws
• Wastewater Treatment
• Woodburning Stoves
• Renovation
• Clearing Undergrowth
• Domestic Plumbing
• Gas Installation
• Natural Disasters