Mark Brinkley examines chimney and flue options for new and old homes. Chimneys are expensive to build, and stoves or fires are quite unnecessary in a modern, centrally heated home, but some self-builders still long for a focal-point fireplace.
Look around any 21st century housing scheme and you will see lots of clean-cut roofs, with rarely a chimney in sight. That is because chimneys are expensive to build, and stoves or fires are quite unnecessary in a modern, centrally heated home. Cost-cutting developers, quite logically, omit them.
But self-builders sing to a different hymn sheet and most of them long to have a focalpoint fire. Top of the self-build pops is the wood burning stove, combining as it does elegance and efficiency. Others seeking more convenience will go for a gas fire or, less frequently, an electric fire. Very few people still choose to burn coal in an open grate — it’s gone right out of fashion.
Whereas a gas fire can often be fitted with a through-the-wall balanced flue, a wood burning stove or a solid-fuel open fireplace needs a flue and/or a chimney. But they are not quite the same thing. A chimney is a masonry structure designed to act as a flue, but you can have a flue without having to build a chimney. So, what are the choices?
A chimney is now built around a core of clay or concrete flue liners, whereas once they were simply constructed in brickwork as an open duct, terminating with a terracotta pot. For a chimney to work well, it requires a good flow of air and for the flue to maintain as high a temperature as possible, so there are exacting Building Regulations about chimney design. For instance, it’s important for chimneys to be insulated, as this keeps the smoke warm and lessens the chances of it condensing as tar deposits. This is particularly important with wood – burning appliances, as they burn cooler than coal. Material costs for a new chimney start at around £1,000 and can spiral upwards depending on the design. However, there is generally a considerable labour element.
The budget option with a wood burning stove is a prefabricated stainless-steel flue. Whereas a gas-burning appliance can use a single-skin flue, with solid fuel you have to use an insulated twin-wall flue section in order to stop the smoke condensing inside the flue. Typical material costs for a 7m twin-wall stainless-steel flue are around £1,300-£1,500. Main manufacturers are Rite-Vent, Poujoulat, SFL and Selkirk.
Ceramic twin-wall flues are a more upmarket option, and are 30-50% more expensive than stainless steel. Ceramic flues tend to come with a 30-year guarantee (rather than ten years for stainless-steel flues). They’re recommended for heavy use, especially with biomass boilers.
Pumice stone prefabricated flues provide a very simple way of building a long-lasting, insulated masonry chimney system, at a similar cost to a twin-walled stainless-steel flue. Instead of having to source components from several different suppliers, these chimney systems are supplied as kits for easy assembly. Names to look out for here are Isokern and Anki.
In short, chimneys require foundations while steel or ceramic flues don’t. If you want a traditional pot, then you need a chimney, as flues have terminals instead. While flues work with any construction type, chimneys work best with blockwork.
Renovating Existing Chimneys
Many people will want to keep an existing chimney and adapt it for use with a wood – burning appliance. You need to check that the chimney is functional and adequate — a chimney sweep should be able to help here. And it is now commonplace to install flexible flue liners, usually pushing them down the existing chimney. Whilst often not essential, a flexible flue liner will help with the free flow of smoke and make for a more efficient chimney. Expect to pay between £400 and £600 for flexible flue liners, plus the labour to fit. Bear in mind there may be a lot more work you need to undertake to bring an existing chimney back into use.
Most gas fires can be installed using a through-the-wall balanced flue, just like a gas boiler. You can also used powered flues, which allow you to duct the exhaust gas over longer routes incorporating bends. This enables you to place a gas fire away from a suitable external wall.
There are some very precise regulations dealing with chimneys and flues which have grown more complex over the years. Appliances need an adequate air supply, and the flue/chimney must be designed so that it can carry the resulting smoke, while the flue terminal must be placed somewhere where it is not going to inconvenience others. Plus, of course, there are stringent fire precautions that have to be met. In England and Wales, these regulations are laid out in Approved Document J, which is available for free download from the planning portal website (planningportal.co.uk).