Chimney design – What installers need to know

Chimneys are a part of many UK houses – often still integral to the heating of the home. Mike Heads looks into how chimney design has developed over forty years as well as the process involved in testing their condition.

In the 1960s very few people could afford the installation cost of central heating and fireplaces in every room, including the bedrooms, were used to heat a building in the winter. The most popular fuel at the time was solid fuel – the Clean Air Act of 1956 meant people started to use smokeless fuels in the form of anthracite and coke. This fuel needed a large chimney – often referred to as a 9″ 3 9″ (225mm 3 225mm) brick. Each day it had to be cleaned to remove ash and clinker.

With the introduction of cheap natural gas in the early 1970s, millions of families changed to using a gas fire to heat their homes.

Brick chimneys could still be used for gas fires, but they had to be swept beforehand. However, this did not guarantee that all the soot was removed and, as a result, a catchment space of 12dm3 is stipulated to allow for this.

The draught for these chimney systems reduced the efficiency and restrictors were introduced, which could only be fitted on chimneys with a brick 225mm 3 225mm flue system and a chimney length of approximately 10m – giving a draught of approximately –0.25mb. The restrictors would not be fitted to bungalows as the chimney length would not provide sufficient draught. The fire spigot needed to enter the closure plate by
at least 15mm, and the gap between the chair brick and the spigot needed to be at least 50mm. This chimney system is sometimes referred to as a ‘Class 1’ flue.

In 1975 a new type of flue system arrived – the ‘precast’ flue. The original size for this flue was 63mm 3 207mm (BS 1289: 1975), which gave a cross-sectional area of just over 13,000mm2. The blocks were built into the stretcher bond build of the house, leaving the wall flat and consequently increasing the room area.

The pre-cast system consisted of three ‘starter blocks’ (see Figure 1). These started on the floor, and the next block to be installed was the ‘cover block’ or ‘gather block’ (see Figure 2). This block channelled the products of combustion into the rectangular flue duct. The next set of blocks was installed in a stretcher bond system that maintained the continuity of the flue.

Once into the loft, the transfer block was fitted. This changed the flue system to a circular flue, and the size of the flue for a gas fire increased in 1975 from 100mm to 125mm. This was then connected with twin-wall flue pipe to a ridge terminal. This flue system is referred to as a ‘Class 2’ flue.

At first the wall was plastered directly onto the blocks, but this caused cracking of the plaster. The solution was to use an air space or some form of insulated lining and a dry lined (plasterboard) wall. This meant there was a gap between the plasterboard and the starter blocks, which needed to be filled with a high temperature sealant.

To prevent too much heat from the fire causing cracking of the starter block, a cooler plate was introduced to deflect the heat. This system was notorious for creating problems of downdraught and spillage, and BS 1289: 1986 changed the size to 184 mm 3 90mm, effectively increasing the cross-sectional area to just over 16,500mm2.

The original blocks were joined using cement mortar but sometimes the builder had not removed any excess mortar from the inside of the flue duct (snots). Also, the blocks were often misaligned. These factors reduced the effective cross-sectional area and caused increased resistance to the flow of combustion products. The cement mortar itself often cracked so the joints were made using high temperature silicone sealant.

Ridge terminals cause a considerable amount of frictional resistance and many builders started passing the chimney outside through the roof. See Gas Safe Register Technical Bulletin 038 for more information.

This change in BS 1289 in 1986 meant that chimneys installed between 1975 and 1986 are not to standard and consequently cannot be used for a new installation. When installing a gas fire to a pre-cast chimney/flue system, the appliance manufacturer’s instructions must be followed, firstly to check that the appliance is suitable for installation with precast flues, and secondly to ensure it is suitable for a pre-cast flue to BS 1289: 1986.

When carrying out a flue flow test, a thorough examination in the roof space must be made.

BS 1289-1 has now been superseded by BS EN 1858: 2003 (concrete) flue blocks and BS 1289-2 has now been replaced by BS EN 1806: 2006 (clay) ceramic flue blocks.

Building Regulations Part J 2010 states: “Where a hearth, fireplace (including a chimney box), flue or chimney is provided or extended, information essential to the correct application and use should be permanently displayed in the building. We normally refer to this as a chimney plate, and it should have the following information:

  • Location of the hearth fireplace or flue box.
  • The category of flue and the generic types of appliance that can be installed.
  • Type and size of the flue and the manufacturer’s name.

The chimney plate should be robust, indelibly marked and securely fixed in an unobtrusive but obvious position. The most popular positions are:

  • Next to the electricity consumer unit.
  • Next to the chimney or hearth described.
  • Next to the water supply stopcock.

Flue/chimney testing

The method of checking a flue is the flue flow test. Prior to completing this test, it is necessary to carry out a visual inspection. The visual inspection should include the following:

  • Check for any blockage or obstructions. A common problem of natural draught chimneys is spider webs, which can become extremely thick. The chimney should be checked to ensure it is continuous throughout its length.
  • Check that it serves only one room or appliance.
  • Check that the terminal is correctly sited and is weather tight between the terminal and the chimney.
  • Are there any dampers or restrictor plates? If so, they should be either removed or fixed in the open position.
  • Is the catchment space the correct size, is it free from debris, and are any gaps in the catchment space sealed from the surrounding structure?
  • Any signs of spillage of an existing chimney should be investigated and any faults rectified.

The flue flow test (smoke test)

After a satisfactory inspection, a flue flow test can be carried out.

  • Make sure there is an adequate air supply for combustion, according to the appliance manufacturer. Close all doors and windows in the room in which the appliance is installed.
  • Carry out a test using a smoke pellet that generates at least 5m3 of smoke with a burn time of at least 30 seconds.
  • For Class 2 chimneys it is likely to need some heat to create a draught – a blowlamp is normally used. Some chimney systems may need much more heat to warm the flue.
  • The pellet is lit and the smoke observed. The chimney must be checked throughout its length, paying particular attention to roof spaces and bedrooms.
  • Termination should be checked to ensure it is coming from the correct terminal.

There should be no significant escape of smoke from the appliance location, no seepage of smoke over the length of the chimney, and the smoke should discharge from the correct terminal.


The flue flow test is to check the integrity of the flue and ensure it is not blocked. Therefore it is not necessary to operate fans during the test.

Spillage test

The spillage test is a test of the appliance and the chimney system, and should be carried out according to the manufacturer’s installation instructions. Usually the appliance is in the normal operating condition, the case on and the fireguard removed.

The manufacturer usually specifies a minimum five minute preheat time. If the installation instructions are not available, the standard procedure is as follows:

In the room

  • Close doors and windows.
  • Close all adjustable vents.
  • Switch off any mechanical ventilation supply to the room other than that required for combustion air for the appliance.
  • Operate any fan and open passive stack ventilation.

The appliance should be at its maximum input rating (open flue range rated boilers should be set to maximum). It is necessary to check that the appliance clears its products of combustion using the method described in the manufacturer’s installation instructions. This is usually a smoke match – before smoke matches were available, spillage was often detected using a cold mirror.

The position of the test is important. For open flue appliances, boilers and water heaters, the test should be at the draught diverter skirt.

For gas fires, it is usual to use a smoke plume tester (some manufacturers supply a testing tool) or you could make one up using some 10mm copper tube.

For a gas fire, the test is at the gas fire canopy, but it is important that the smoke match is in the correct position.

If spillage is detected, turn off the appliance, disconnect then rectify the fault.

If the test is successful, close any passive stack ventilation and repeat the test.

If there is no obvious reason for a spillage test failure, it may be necessary to leave the appliance operating for a further 10 minutes.

If there are fans elsewhere in the building, the tests should be repeated with all internal doors open, all windows, external doors and adjustable vents closed and all fans in operation. If spillage occurs, it may be there is insufficient air entering the room (including the situation when no fan is present). In this case, repeat the test with the window slightly open. If the appliance now clears the products, additional ventilation should be provided.

Many open flue gas fires under 7kW do not need any extra permanent ventilation and are covered by adventitious ventilation. However, where the room includes a solid floor and double glazing, there may be
insufficient ventilation for the appliance flue operation.

Examples of fans that may affect flue operation include:

  • Fans in cooker hoods
  • Wall or window mounted room extract fans
  • Fans in chimneys of open flue appliances, including tumble dryers
  • Circulating fans of warm air heating or air conditioning systems (these may not be gas)
  • Ceiling paddle fans (these can seriously affect inset live fuel effect fires).


If the ground consists of granite, it is often slightly radioactive and a gas called radon can be emitted. To counteract this, fans are often fitted to remove this gas. Therefore, when carrying out a spillage test, any radon fans should be operated.

To sum up

  • Chimney design has changed from the Class 1 to the much more compact precast Class 2 chimney.
  • The original BS 1289 (1975) provided a cross-sectional area of 13,000mm2 but BS 1289 (1986) increased the crosssectional area to 16500mm2.
  • A flue flow (smoke test) is a check of the integrity of the chimney system and to ensure it is not blocked. It should be carried out using a pellet providing 5m3 of smoke and a burn time of at least 30 seconds.
  • A spillage test is completed in the worst conditions – e.g. with windows and doors closed and fans running.