There is a way to cut CO2 and NOx gases by modifying the flue, as Andrew Brown explains
Fan dilution systems are available for a range of appliances from modern high efficiency condensing to gas blown and atmospheric boilers.

The idea of diluting the products of combustion is to reduce the carbon dioxide (CO2) content at the discharge to below 1 per cent, limit carbon monoxide (CO) to 50ppm, and cut mono-nitrogen oxides (NOx) to 5ppm.

There are several design considerations to be taken into account when looking at the possible installation of a fan dilution system.

Design considerations
These include the sizing of the fan unit, dilution header, inlet and discharge plenum boxes as well as the final discharge height of gases, the flue's proximity to adjacent buildings and whether there are openable windows or fresh air vent pipes.

These design considerations can be found in the Clean Air Act Memorandum, IGEUP/10 or BS6644 along with other provisions with respect to the final discharge height which is calculated based on the total kW input load of the plant.

Good practice dictates that you consider two possible methods when designing a fan dilution system. In the first, air is taken directly from the outside. In the second, fresh air is taken from within the plant area.

Generally, the favoured option is to take the air direct from the outside via a louvre situated on the external face of the building or above roof level.

But there are also further important considerations you need to take into account:

The inlet and discharge openings should be, as far as possible, apart and located on the same building face to prevent external wind conditions affecting the overall operation and performance of the dilution system.

Inter-connection section from the boiler to the main dilution header should be fitted with a manual volume control damper and, if deemed necessary by the appliance manufacture, allow a draft stabiliser device which will ensure a safe stable operation.

The system should incorporate sufficient noise control.

Suitable ductwork must be manufactured from stainless steel with a requirement for a watertight system when serving high efficiency condensing boilers.

Where a condensing application is considered, it is normal for pluming to take place and a reduced external temperature will accelerate the process. Nevertheless, this may lead to complaints if the discharge point is not positioned correctly.

The dilution fan must incorporate a proving device, normally a pressure switch, which interlocks with the boiler control.

Progressive chimney and flue suppliers such as my own will typically offer a design, supply and installation service for flue dilution systems and a tailor-made package to site conditions and customer requirements.

As well as a product portfolio that covers a full range of commercial and industrial solutions across a selection of fuels and applications, we also offer other products and services. These include CCTV chimney surveys/reports, chimney cleaning and repairs, chimney and flue modifications, special fabrications plus accessories such as inline and exhaust fans, chimney liners and draft aid devices.

// The author is managing director of Airtherm Engineering //