A Guide to Ventilation and Draught

Following on from last month’s missive relating to the dangers of CO poisoning my A guide to ventilation and draughtthoughts move on to a related issue. Ventilation, and the impact of any insufficiency.

As stated previously, blocked flues are a clear cause of fume emission into the home, but there are other common occurrences which are difficult to immediately identify. Open flue appliances, such as a solid fuel stove, require an adequate supply of air to ensure the chimney can function as intended. Natural draught occurs as a consequence of the barometric differential between high pressure at ground level and the lower pressure present at the chimney terminal. In domestic properties this pressure difference is quite small but is sufficient to induce a movement of air and therefore flue gasses. Remember, Physics demand that air always moves from high to low pressure; however, it takes little interference to ‘spoil’ the draught.

Fortunately, heat from the fire increases the buoyancy of the gasses to further improve the velocity of the rising flue gasses, however an ingress of cold air into a defective flue will effectively negate this effect. Additionally, the higher the chimney terminal the greater the pressure difference, thus a multi-storey property will always provide better draught conditions than a short flue system fitted to a single storey flat roof extension.

As a consequence, the importance of a correctly designed and constructed chimney is important. All good appliance manufacturers recommend minimum draught levels for their products to operate safely and efficiently. To
Ventilation and Draught (or lack thereof) attempt an installation of any appliance without first measuring the draught available is folly. A draught gauge is essential for this purpose but I am aware of many installers who do not own such an item, or know how to use one!

A guide to ventilation and draught 2Of course, for adequate draught to be available you must first ensure there is sufficient ventilation into the room so that the required amount of air-flow can pass through the fire-box and into the chimney. Naturally, adequate amounts of oxygen are required for full combustion of the fuel to achieve manufacturer’s rated output, however in the context of this article excess air is also needed to allow the correct flow of all flue gasses to atmosphere. Insufficient air will undoubtedly result in fumes escaping into the room, especially when the fire-door is opened for refuelling. A chimney connected to an air-tight room will not function at all.

Current Building Regulations require that all new properties are built to achieve minimal ingress of air into the property. Therefore we must contend with high performance construction, windows and doors designed to minimise heat loss and limit cool ambient air entering the building. Unfortunately, such measures present a challenge to customers and installers alike when considering fitting a stove or similar appliance with a natural draught flue and provision of additional ventilation is essential.

Many stove manufacturers have addressed this problem by marketing a range of appliance with direct air supply to the fire, and no doubt we will see further developments of this design as time passes. However, there remain many conventional designs of stove on the market for which the guidance contained in Table 1 of ADJ apply. Clearly, there will be many instances of older properties where adventitious ventilation levels are available and where Building Regulations Guidance should be followed, likewise installation instructions must be adhered to if a direct air supply stove is to be fitted. However, installers must take care to observe whether customers have unwittingly taken steps to draught-proof their home. It may follow that solid fuel appliances with rated outputs below 5kW will need an external air supply into the room regardless of traditional ADJ guidance.

A Guide to Ventilation and Draught 3Lastly, you will have read elsewhere about the adverse effect of an air pressure drop across a building due to the effect of prevailing winds in some geographic areas. In such instances air may be drawn out of a room via wall mounted ventilation if the room is subject to a low pressure zone. Sadly, there is little one can do about intermittent wind direction and any consequential air pressure except warn the customer not to use the fire in such instances. Opening windows on the windward side of the house may help to ease the problem if the house design is favourable however.

Perversely, I have in the past taken issue with cavity wall insulation contractors who have insisted on fitting a wall mounted vent before leaving the property, regardless of the appliance type or rated output. They seem to be of the opinion that filling cavities somehow reduces the level of ventilation into the building. Of course, no air should be entering the property via the cavities. Clearly, ventilation is an issue much misunderstood.


Jim Lambeth

 

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