Chimney Flue

Commercial Flues

(Includes commercial boiler flue regulations for 2020)

The primary regulations covering the installations of commercial flues are the Clean Air Act 1956/9, the Institute of Gas Engineers UP10 Edition 4 and the British Standards BS6644:2011, 6230:2011 and 6896:2011. These regulations must also consider the building regulations and any listed or historic house regulations. Local Building Control can advise and ensure changes to existing buildings comply with all these regulations as well as issuing wavers under certain conditions. Flues in a heating plant have changed over the years to meet the requirements of the boilers and fuel used to fire them. Modern cast iron hot water boilers started to appear from the 1860s firing coke or coal. These boilers were connected to chimneys discharging at the top of the building. This design stayed right through to the late 1950s when coal gave way to oil and the fuel of choice due to the Clean Air Act. Coal stores were fitted with steel oil tanks and oil burners were fitted to the front of the existing coal firing boilers. When buildings first had heating boilers a brick chimney was the normal solution as the boilers were originally firing on coal or coke and in the 1960s these were slowly changed to firing oil and the late 1960s saw the introduction of the purpose-built modular gas boilers, usually cast iron atmospheric.

All these fuels had high sulphur content and some was deposited on the walls of the chimney. From the 1970s and 1980s, these boilers were either converted to fire on gas or replaced with gas-fired boilers. As 10% of the flue gases from gas boilers is H2O, when the exhaust gas temperature drops below the dew point of 150℃ this causes water droplets to drop out of the exhaust gas. This water will then mix with the sulphur deposits to make sulphuric acid which will damage the chimney lining and mortar, this could allow flue gases to enter the building.

The regulations of commercial boilers flues since 1956 has come under The Clean Air. This covers all heating plants over 150kW input. Since then the Institute of Gas Engineers have issued guidance on flues and ventilation in their document IGE UP10, now in its fourth edition. In the late 1980s, the British Standards brought out the BS6644 to guide commercial installation and BS5544 for domestic installations, these cover flues, ventilation, gas pipework and safety equipment to ensure the safe operation of the boiler plant. The Institute of Gas Engineers documents allowed variations to the Clean Air Act with flue dilution systems being allowed for systems up to 6MW and more recently to cover low-level discharges for systems up to 333kW input.

A major change was that all gas boilers chimneys had to be lined and made of a suitable material for the boilers connected to it. With power flame or atmospheric gas boilers this would usually be a stainless steel liner of a grade 430 or 304 grade and sometimes 316 grade. 430 grade was allowed but as it is a coated steel it was prone to rust if the stainless steel coating was damaged. These flues on non-condensing boilers were designed to worked under a negative pressure as the flue gases temperature was very high (150-240 ℃) causing a significant draft in the flue. This meant the flue did not need to be sealed and often fitted with stabilisers and installed in sections just pop-riveted together because a correctly sized flue is unlikely to leak or spill.

The new condensing gas boilers work on a different principle, as the flue gas temperature is very low it has little lift and relies on a positive pressure supplied by the boiler fan to remove the flue gasses. This means that the flue must be sealed, and to ensure the flue can deal with the acidic condensate, made of 316-grade stainless steel. For some applications plastic flues are suitable but they are unlikely to be suitable for outside use as they are not UV protected.

From the late 1970s, a discharge at low level became popular using flue dilution systems. These systems mixed fresh air with the flue gasses to dilute the concentration of CO² in the discharge to under 1% by volume. There are strict regulations on the height from the ground of the discharge, depending on the boiler output. The advantage is that the system does not need a chimney to roof level, but the flue diameter is big due to the volume of fresh air that needs to be moved to create this dilution. The systems often have a single large fan which if it fails then the heating must be shutdown. Another issue is noise from the fan can exit the building and be a nuisance to the area near the boiler room.
A new development was the Monodraught system which brought fresh and combustion air from an airspace around the chimney down into the plant room. This meant that the boiler room did not need to be on an outside wall or required fanned air. To comply with the regulations these systems had to have interlocks on plant room doors into the building and no other openings into other parts of the building to ensure airflow. Often these systems resulted in hot plant rooms and though they are still allowed, the original company that gave the system its name no longer makes them.

After 2000 the regulations started to push for higher boiler efficiencies and more and more boilers supplied were condensing. This meant the flue gas temperature was much lower and the natural draught was poor, so they needed a fan to push the gasses out. As the flue gasses condense in the boiler to ensure that the efficiency is maintained there is a significant amount of slightly acid water in the flue gasses. This is the reason than all flues on condensing boilers must work under a pressure and be designed to remove the condensate to conditioners or if allowed to drain. As all commercial boilers under 400kW must be condensing due to the ErP directive, nearly all systems will have to have a flue system designed to match the boilers and comply with all the regulations. One of the major changes is that some boilers can have flues at a low-level discharge. The Clean Air Act sets this to plants with a total input of under 150kW but the latest IGE UP10 Edition 4 has raised this to plants with a total input of 333kW. There is a very strict check sheet which must be complied with for low-level discharge to be acceptable.

We can advise on which type of flue will meet your requirements. For larger flue dilution, systems can still be used up to 6,000kW but again with condensing boilers even the best dilution fans will have a limited life and pluming can be a major problem. If this system is the only option, we can select plume reduction boxes on the discharge.

Flue Options

There are many different flue options and these are often dictated by the boiler manufacturers. The boiler manufacture will stipulate the types of flues, they usually have designations such as B23, C13, C33, C53 etc. Boilers must not be connected to flue systems that they are not CE rated.

The most common flues for wall/frame-mounted boilers are C13 and C33 which are room sealed concentric flues with either horizontal or vertical discharge. For both Wall/frame and floor mounted boilers the B23 flue which connects all the boilers to a single flue system and discharges at a high level are also very common. In this system, the boilers take combustion air from the plant room instead of from directly outside as the C13 and C33 systems. The B23 flues are usually made of high grade 316 Stainless Steel with silicon push-fit joints. The flue must be vertical or sloping, usually about 3⁰, so any condensate can be drained correctly. If puddles of condensate sit over the silicon joints, they can fail in time as they will become brittle. Replacing flues for multiple boilers have to be carefully designed to ensure they work correctly for both full and part loads, so the flue gasses are correctly removed from the boilers, the running boilers must not put too much back pressure on the non-running boilers either otherwise flue gases may enter the plant room. Most boilers are fitted with flue gas non-return valves but there is a limit to what these can deal with.

As a 100kW boiler can produce 10-11 litres of condensate per hour, there will be condensate in the flue that will need to be drained. You can use plastic pipes inside the plant room for this and these drains must have traps fitted that are designed to deal with the maximum flue pressures. The local Water Company must be consulted on how the condensate can be disposed of. As this water is acidic they may request conditioners to be conditioned, particularly if the building uses little water that would help to dilute the condensate or that the drains are unsuitable, ie cast iron or copper that would fail over time due to the acidity of the condensate When an existing installation has been installed with a fan dilution system, great care must be taken to replace the system. If a vertical flue can be fitted then that could be a solution, or if the total input is less than 333kW, discharge at a low level may be possible.

A flue dilution system can be used with condensing boilers if constructed with suitable fans and high grade 316 stainless steel ducting from the boilers to the system discharge. If pluming is an issue then we can fit plume removal box before the discharge point.

The flue discharge position will be dependant on the total kW rating of the plant, the height of the flue must be taken into consideration with other taller structures, nearest building, openable window or fan intake. For horizontal flues, then the height from the ground and distance from adjoining structure will affect the position or if it will be allowed. Flues may require a fire rating and this will generally need a twin wall flue system.

Other flues in common use are plastic flues rated for use on boilers. These are generally only used in plant rooms connecting to stainless steel flues which rise in chimneys or internal risers. For riser’s, twin-wall is usually used for both fire protection and to minimise heat transfer into the building.
For flues running outside, twin-wall flues are again usually used to minimise the chance of condensate freezing in the flue and allow some heat updraft when the boilers are running. Plastic flues are generally not UV protected so must not be used outside as they will become brittle and fail in time.

If the boiler manufacturer supplies the flue, their guidance must be followed for size and run lengths as well as meeting all the relevant regulations. All flues must be pressure tested in position to ensure there are no leaks to meet the latest regulation and the test results must be retained with the plant records.

Sometimes it is easier to keep the existing flue liner which is not suitable for a condensing boiler and fit a laminated furniflex liner down the inside, this can be a quick solution when time is important. This material can be used to line an existing brick or concrete chimneys, as it can be made to measure and still retains the maximum cross-section area of the flue or chimney as it is only a few millimetres thick. The flue must be correctly designed by a specialist or boiler manufacturer and the calculations, method of assessment and conclusions of the design and sizing must be retained with the plant records.

We are here to guide you through the options available to ensure the system works and that it will comply with all current regulations.

Posted in Heating.