heat recovery ventilation
some frequently asked questions
- Why is there a need for ventilation?
- What is condensation?
- What are the effects of condensation?
- Are there other problems associated with inadequate ventilation?
- How many air changes are needed for proper ventilation?
- Is it expensive to operate?
- What is the power consumption of the unit?
- How is the air distributed to the rooms?
- Are there any major problems in fitting the system?
- How do I control the system?
- Must I operate the system throughout the year?
- Would a dehumidifier help?
Normal household activities such as cooking, bathing, showering etc., all produce potentially harmful water vapour.
In the past, houses built by traditional methods had open flues to every room, and plenty of air gaps. This allowed a constant flow of air through the house, removing the water vapour before condensation could occur.
Today's energy saving measures, double glazing, cavity wall insulation, and draft proofing, have resulted in tightly sealed houses having no natural ventilation. This means a build up of water vapour until condensation occurs.
The ability to hold water as vapour varies with the air temperature and the amount of water in the air. Condensation will occur when the warm moist air meets a cold surface that causes the air to drop to a temperature below its dew point. At this point, the water vapour in the air will appear as condensation on the cold surface.
The most obvious effect of condensation is the appearance of moisture on windows and other cold impervious surfaces. Whilst irritating, this water can be easily removed.
Much more harmful is the moisture that condenses on absorbent surfaces, such as masonry or timber. Here the water is absorbed into the surface and cannot be easily removed.
A continued build up will take place in a badly ventilated home and will result in spoilt decoration with peeling wallpaper and growth of black mould combined with structural damage to timber by way of wet or dry rot.
Yes. Cooking smells, tobacco smoke, pet smells, toilet odours and generally stale, musty and stuffy atmosphere are all associated with lack of air change in the home, and can be cured by correct ventilation.
In today's well insulated homes, up to 37% of the total heat loss takes place through ventilation air losses; therefore it is most important to achieve a balance between excessive ventilation with consequent heat loss, and sufficient ventilation for a condensation and odour free clean atmosphere.
At present the Building Research Establishment suggests an ideal ventilation rate of between 0.5 and 1.5 air changes per hour.
Countries with greater experience of controlled ventilation tend towards a standard at the lower end of the range. The Swedish standard calls for 0.5 air changes, the Canadian requirement is for 0.5 air changes with a minimum air volume of 18 cubic metres per house per room, and, finally the DIN standard for Germany calls for between 25 and 34 cubic metres per hour per person occupying the dwelling.
All available information points to an air change rate of 0.5 air changes per hour.
On the contrary. HEAT RECOVERY VENTILATION can actually reduce heating costs. It is now generally agreed that to combat moisture production in kitchens and bathrooms, there is a need for mechanical ventilation in the form of fans or cooker hoods.
The heat in the air exhausted by these units is lost completely to the outside.
When replacing kitchen and bathroom extractors with a central system with heat recovery, a substantial amount of the heat is captured and transferred to the incoming fresh air. Energy savings vary with the volume of air moved, ie the size of the house and average outside air temperatures.
As a guide, a typical system operating in Scotland from September to May continuously at 100 cubic metres per hour, would recover 2000 kilowatts with a fan running cost of 800 kilowatts, showing a net saving of 1200 units.
Each centrifugal fan in the unit has a power consumption of 60 watts, therefore the total is 120 watts at maximum operating volume. The fan speed controller is an electronic unit which results in lower speeds using less power, with a consequent reduction in operating costs.
Air distribution takes place through aluminium flexible ducts of 125mm, 100mm and 63mm diameters. These corrugated ducts, of all metal construction, are supplied in 4 metre lengths compressed to 1 metre for ease of transport. They are easily extended by hand to their full length for installation.
Aluminium duct is used, not only for strength and corrosion resistance, but also for safety under fire conditions. The tube has been tested to the requirements of BS 476 PART 8 (Fire Tests on Building Materials and Structures), and maintained its integrity for 15 minutes at temperatures up to 750°C.
With care taken at the planning stage, no problems are normally experienced on installations.
The first important area to bear in mind is the duct runs to the lower floors where it must be remembered that the ducts cannot cross joists, and therefore the correct location of the dropping duct from the loft space is critical.
The other area that warrants special consideration is the connection of the supply and extract ducts to the outside. Here there are three possibilities :
- A roof terminal
- A soffit board grille
- A self closing louvre for installation in a gable end wall
Any one, or combination of these, may be used depending on individual dwelling requirements.
The only control needed on the system is the variable fan speed switch. This unit allows infinite control of the fans from zero to maximum operation volume. It is an electronic unit which means that reducing speed reduces the power consumed, making it economic in use.
It is only important to operate the system when the windows are closed and the house tightly sealed. In fact it would be most uneconomic to operate it under other conditions.
A normal operation period would be from September to May, unless, of course, the windows are kept closed throughout the year for reasons of security or noise, when the system would be needed on a constant basis.
If one wishes to remove air from the kitchen or bathroom.
It is possible to operate the unit on extract only during the summer period, by installing an on/off switch in the fan supply circuit. This is listed as a summer/winter switch.
Yes, it will remove moisture and reduce condensation. But it won't change the air in your home at all, and it won't get rid of stale air smells.
What's more, you would need a dehumidifier in every room. At 300 to 400 watts per unit, you will definitely notice an effect on your electricity bill.
