Chimney & Flues

When you are looking to change or update a heating plant on of the most important considerations is how the boilers will disperse their emissions.

When buildings first had heating boilers a brick chimney was the normal solution, the boilers were originally firing on coal or coke, in the sixties these were slowly changed to firing oil. All these fuels had high sulphur content which some was deposited on the walls of the chimney. From the seventies and eighties 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 degree C 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 plant 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 1980’s 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 degree C) 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.

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.

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 plantroom. 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 10-11litres of condensate per hour, there will be condensate in the flue that will need to be drained. You can use plastic pipes inside the plantroom for this and these drains must have traps fitted that are designed to deal with the maximum flue pressures. The flue must be designed to slope about 3 degrees to allow condensate to run to a drain point, as most condensate flue joints are made of silicon it is important to ensure condensate does not build up in the flue over these joints as then can become brittle over time and fail

The local Water Company must be consulted on how the condensate can be disposed off. 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 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.

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 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

Most modern wall or frame mounted boilers can be flued at low level if the total net input is under 333kW. The discharge position must comply with a check list that is printed in the IGE UP10 Edition 4

As part of our survey we can advise on the best flueing solution for your new or refurbished  heating and hot water systems.

Posted in Chimneys & Flues.