Air Combustion Ventilation For Heating Appliances
Air Combustion Ventilation For Heating Appliances
All you need to know about a new balanced approach
- The Problem
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- Vents need hoods to counteract draughts and rain ingress from head-on winds
- Conventional hoods increase draughts from common upward winds
- Subject to cross winds, conventional hoods significantly reduce Effective Area
- The Solution – ArrestAir-Pro
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- ArrestAir-Pro is a combined grille and hood
- ArrestAir-Pro controls draught irrespective of wind direction
- ArrestAir-Pro does not cause significant reduction of Effective Area in cross winds
For many years simple passive through-the-wall vents – simple holes in the wall with grilles over their ends to keep the wildlife out – have provided “trickle” ventilation by letting fresh air in and stale, damp air out of dwellings. Such vents are also used to provide fresh air for combustion in open-flue gas, oil and solid-fuel boilers and fires. However, the trickle of fresh air blowing into many passive through-the wall vents can so easily become a torrent as the outside wind increases beyond a gentle breeze.
To counteract the resulting draughts indoors, several vents have recently been developed in an attempt to reduce this irritating airflow while simultaneously preserving the statutory requirement for a minimum Effective or Equivalent Free Area of 100 cm2 (BS 5440 Part 2: 2000) for gas vents which may provide fresh air for an open-flue boiler. However, the standard test to determine the Effective Area has been performed only in still outside air.
When the new range of vents (with their open-bottomed hoods designed to ward off head-on winds and prevent the ingress of rain) are tested for their Effective Area subject to winds blowing parallel to the wall, the hoods cause serious problems. Firstly, as the wind is forced to go over a building – as it often is in order to get by it – it frequently approaches at an upward angle and is scooped up by a conventional vent´s hood; this causes as much draught as a head-on wind approaching a hoodless vent as shown overleaf. Secondly, when the wind approaches from the side – as it does in many situations such as between buildings or when the wind is simply in that direction – the Discharge Coefficient (the ratio of its Effective Area with a crossflow to that in still air) of a conventional hooded vent can be reduced by up to 50% (see below).
Most manufactures do not take cross winds into account. However, as shown overleaf, tests funded by the Health and Safety Executive have demonstrated that the ArrestAir-Pro does indeed allow for cross winds and leads the field in controlling draughts while largely preserving its Effective Area irrespective of the direction of wind approach
* From BULMAN, S. and N. CHAN (2004): Development of a ventilator test procedure to take account of external wind conditions. Advantica Limited, Loughborough, pp 99-100. Based on tests by ETHERIDGE, D. and E. Y-H CHIU at the School of the Built Environment of the University of Nottingham funded by the Health and Safety Executive.
Ever since the “greenhouse effect” raised its ugly head, everyone has been aware of the need to conserve energy. They may not be doing it, but they are aware of it! The easiest way to conserve energy is to insulate properties to stop heat escaping. Unfortunately, that also stops moisture vapour escaping – about two gallons per day for the average household – and leads to condensation on cold surfaces.
Passyfiers take advantage of the differential partial moisture vapour pressure that is normally higher inside an occupied building than on the outside. This pressure is completely independent of that in the air. For example, a typically cold foggy British winter´s day (a green light for condensation) may have an outside temperature of 5°C and humidity at 100%, while the inside temperature is at a comfortable 20°C. Keeping the inside relative humidity to 70% generates a difference between inside and outside moisture vapour pressure of 8 millibars, ample to alleviate condensation, all this with little or no heat loss.
The diffusion process is slow compared with mechanical ventilation and would not cope with a copious amount of water vapour produced in a short time scale in kitchens and bathrooms without the additional use of mechanical ventilation. Its main use is in habitable rooms where it operates continuously without noise 24 hours per day and can extract up to 2.3 litres of water per day even at low vapour pressures.