Commercial Boiler Flues

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.

Flueing of heating plant has 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 1860’s firing coke or coal. These boilers were connected to chimneys discharging at the top od the building. This design stayed right through to the late 1950’s when coal gave way to oil and the fuel of choice due to the Clean Air Act. Coal stores where fitted with steel oil tanks and oil burners were fitted to the front of the existing coal firing boilers.

During the 1960’s cast iron and steel boiler design changed to be more suited to oil firing, giving higher efficiencies and better control. In the 1970’s saw the introduction of North Sea gas and the slow conversion of the oil and existing coal/coke fired boilers over to natural gas. From the late 1960’s also saw the introduction of the purpose-built modular gas boilers, usually cast iron atmospheric.

The one thing that was not changed or adapted was the chimneys. This started to cause problems as coal and oil both had high sulphur content while burning gas had a high moisture content that could condense in the chimney, causing the formation od sulphuric acid that would damage the chimney. When BS6644 was first released it then stipulated all gas fired boilers must be connected to a suitable flue or liner to protect the brickwork in the chimney. As the earlier gas boilers were not condensing the flue gas temperature was kept above 150⁰C to minimise condensing in the boiler and flue. Most flues were now made of some form of stainless steel and were designed for a negative pressure as the high flue gas temperature would cause the flue gasses to rapidly rise out of the chimney. The flue headers and liners were designed for this and were not sealed, often pop-riveted together because for a correctly sized flue was very unlikely to leak or spill.

From the late 1970’s 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 has to be shutdown. Another issue is noise from the fan can exit the building and be a nuisance to the area near the boiler room.

Another development was the Monodraught system which brough fresh and combustion air from an airspace around the chimney down into the plantroom, this meant that the Boileroom 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 plantroom doors into the building and no other openings into other parts of the building to ensure airflow. Often these system resulted in hot plantrooms and though they are still allowed the original company that gave the system its name does not make them any more.

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 will be 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 have to be condensing due to the ErP directive then nearly all systems will have to have a flue system designed to match the boilers and comply to all the regulations. One of the major changes is that some boilers can be flues to low level. The Clean Air Act sets this to plant with a total input of under 150kW but the latest IGE UP10 Edition 4 has raised this to plant with a total input of 333kW. There is a very strict check sheet which has to be complied with for low level discharge to be acceptable. We can advise on which type of flue will meet your requirements. For larger system 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 then we can select plume reduction boxes on the discharge, but these are expensive.

There are many different options for flueing, 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 for.

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 high level are also very common. This system the boilers take combustion air from the plantroom 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. 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.

The flue discharge position will be dependant on the total kW rating of the plant, it is important on the height of the flue in relationship to 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 rising up chimneys or internal risers. For riser’s twin wall is usually used for both fire protection and minimise heat transfer into the building. For flues running outside twinwall flues again is 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 manufacture 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 have to 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 to all current regulations