Twin Wall Flue - Ideal if You Don't Have a Chimney for your Stove

Few people can resist the charm and appeal of a wood burning stove. Who doesn’t like the idea of snuggling up on a cold winter night with a hot chocolate and a DVD? But what if you don’t have a chimney? Can you still have your dream stove? In today’s post, we look at twin wall flue systems…the solution to that problem.

The Purpose of a Chimney

The purpose of the chimney is, rather obviously, to allow smoke and combustion gases to exit the building safely. Prior to modern central heating systems, open fires were the preferred method of heating a home. However, most modern and new build homes have no chimney system at all.

A wood or multi-fuel stove burns solid fuel and will require a chimney or flue system to operate correctly and safely. Householders with older properties usually choose to break out their fireplace and to install a chimney liner in their existing chimney. With liner kits typically costing £300-600, this is a relatively cheap means of ensuring safe and efficient stove operation. But what if you don’t have a chimney?

Twin Wall Flue Systems

In nearly every case where there is no existing chimney, a twin wall flue system is specified. As the name suggests, these consist of an internal diameter pipe (usually 150mm) through which flue gases pass and an external wall with insulation between the two layers.

The insulation in a twin wall flue serves two purposes. The first is to keep the heat within the flue system to ensure an adequate draw and to prevent condensation. The second is to reduce the exterior temperature of the flue system so that it can pass relatively close to combustible materials. A single skin flue pipe must be at least three times its diameter away from combustible materials. Accordingly, a 125mm (5”) single skin vitreous pipe would need to be at least 375mm from a floor joist or roof truss. The manufacturer sets the distance to combustibles for twin wall flue pipe and this usually ranges from 50-80mm. As such, you can install a 200mm (8”) external diameter twin wall flue between joists with narrow 400mm centres.

Twin Wall flues are sectional with each component clipping to one another with a strong locking band. There are many manufacturers to choose from, but our preferred system is the excellent Schiedel ECO ICID. A wide array of straight lengths, angled bends, terminals, and structural bands and supports are available. As such, you can add a twin wall flue to most homes or buildings.

Cost of a Twin Wall Flue System

In comparison to lining an existing chimney, twin wall flue systems are expensive. The price of a complete system can vary dramatically depending on its length and complexity. Costs typically range from around £1,000 to £2,000 for the flue system components. However, you must be view this in the context of the problem the flue solves. Without it, you would need to construct an entire chimney from scratch, the cost of which would far exceed a twin wall flue. In that regard, twin wall flues are exceptional value for money and are a flexible and versatile solution that allow stove placement in a variety of locations.

Things to consider with a twin wall flue

The manufacturers and Building Regulations set numerous design and installation rules for twin wall flues. A competent stove installer will advise you on what you can and cannot achieve in your home. In addition to the previously mentioned clearance from combustibles, there are three other key considerations:

Flue Height and Termination

To achieve a satisfactory draw, the flue must be a certain height. This is usually specified by the stove or chimney system manufacturer (or both). The terminal of the flue must also project a certain height above the roof slope or ridge, depending on where it exits the building. These distances are specified on page 31-32 of Approved Document J of the Building Regulations 2010.

In practice, this means that the position of the stove within the building can radically alter the routeing and length of the flue system. This is where a stove survey is essential, as a small change internally to the position of the stove could greatly increase or decrease the cost of the flue.

The Four Bends Rule

A twin wall flue system cannot have more than 4x 45° bends. This might sound generous but you use up two of those bends if you are forced to use the 90° rear access on the stove and you use 2x 45° bends every time you offset around an obstacle (e.g. floor joist). Great care must be taken plan the flue route through a two-storey house. With two floors and a roof to get through, a poorly planned starting position can quickly use up these allowances. The images below show typical internal and external routeing of twin wall flue systems.

: Internal Twin Wall Flue Through Two Storey House

: Internal Twin Wall Flue Through Single Storey House

: External Twin Wall Flue Through Exterior Wall

It is Better to Route Through the House

As previously stated, one of the purposes of the insulation in a twin wall system is to keep the flue gases warm until they exit the terminal. If the combustion gases cool too much they can condense leaving excess deposits or stop rising and fail to draw. External routeing and exposure to the elements are therefore less desirable than running the twin wall through a warm house. If the flue routes through an upstairs bedroom you can easily box it in. That said, leaving it open allows the release of heat into the room through which it passes.

A final consideration must also be given to the potential combination of the above factors. For example, the performance of a 9m high flue that rises straight up through a two-storey house will be vastly superior to an external 4.5m flue with 4 bends. With the later system, it will likely be harder to establish your fire when it’s freezing outside and the flue is stone cold.

Summary

For homes that don’t have a chimney, a twin wall flue system is an excellent solution. They often allow placement of the stove in a variety of positions within your home and generous clearances mean you have the option to box in and hide the flue. While more expensive than utilising and lining an existing chimney, a twin wall flue is considerably cheaper and less restrictive than building a masonry chimney.