An alternative method of ventilation which has found favour with many Local Authorities and Housing Associations is Positive Pressure Ventilation. This is an extension of the intake ventilation concept (as heat recovery is a version of a combined system). This rate of ventilation is very low, half an air change per hour (0.5 ash) and is designed to be continuous. Sensors within the unit change the actual rate according to the ambient air temperature.

The concept of this method of ventilation is that outside air is usually drier than the inside air. BS5925, 1991 Section 4.5 (Control of Internal Humidity) cites “The contribution made by ventilation is to lower the moisture content of the internal air by dilution with the outside air which normally has a lower moisture content”. The amount of moisture that air can hold is dependent on the air temperature, the hotter the air, the more moisture it can hold, and vice versa.

Relative Humidity (RH) is the ratio between the moisture content of air at a certain temperature, and the maximum moisture it could contain at that temperature, expressed as a percentage.

Saturation of the air takes place at 100%RH, i.e. the air contains as much moisture as it can hold. This element and other similar expressions (dew point, condensation, etc.) are detailed in the section on Condensation.

Positive Pressure Ventilation introduces outside dryer air into the dwelling where it is mixed with the internal air, lowering the total moisture content, and gently removing the moist air, either through natural leakage or via specified devices that open and close automatically depending on temperature, moisture content or both.

The most recent development in this type of unit incorporates provision for external sensor control when the air movement rate is increased to cope with higher moisture production periods, and provision for drying out new and substantially refurbished properties for a timed and self-cancelling 14 day period. This again is highlighted in BS5925 Section 4.5. “It should be noted that in newly constructed buildings large quantities of moisture are released from the fabric as the building dries out.

Consideration should be given during this drying out period to the question of whether additional ventilation should be provided”. The amount of water used in the construction of a traditionally built 3-bedroom house can be as high as 4 tonnes.

The air at ceiling level is usually up to 8°C hotter than at lower levels, as hot air naturally rises. This can sometimes be very visible in rooms where heavy smoking occurs with insufficient ventilation. The hot air rises to ceiling level, creating a barrier for the cigarette smoke, which visibly hangs some distance below ceiling level, forming a band of smog at about head height. In the domestic environment positive pressure ventilation mixes this wasted heat at high level, with the dryer outside air and circulates it, firstly across the room at ceiling level, then to the rest of the room volume, without causing cold draughts.

As dry air costs less to heat than moist air, and the room heat is de-stratified, this form of ventilation can reduce heating bills, despite bringing in a small amount of outside air which may be below the internal air temperature at certain times of the year.

As the fans are designed for continuous operation, other than two or three months during high summer when condensation is not a problem; most units incorporate latest technology, low voltage, brushless DC motors.

These motors are much more efficient than usual mains voltage AC motors; up to 80% against mains motor 20%. The reduction in running expenses is considerable. The actual cost of using this type of ventilation equates to about a penny per day throughout the year. During many months of the year, the outside air is tempered by a higher loft temperature (where the unit is sited). This, again, can effect a reduction in heating bills. To avoid problems with external and internal dust, efficient filters are incorporated, usually to EU5 or higher filtration levels.

This method of ventilation together with thermostatic radiator valves can reduce heating costs quite considerably for most of the year. Versions of this type of unit for use in dwellings without lofts (flats, maisonettes, etc.) are also available. These units are usually wall mounted and have a smaller air volume performance commensurate with the smaller dwelling size anticipated.

Which ventilation system is used can be a matter of personal choice, either by the occupier, owner, specifier or even contractor. However, a logical choice is often more dependent on the actual size and layout of the dwelling and, once again, the lifestyle of the occupier.

So called ‘dry’ occupation, where the people are out for most of the day, results in two peaks of moisture generation. This is normally between 0630 and 0800 when they shower and dress for the day and again between 1830 and 2000 when evening meals are prepared, laundry is done and maybe showers are taken again before an evening out. It is usual for central heating to be used only during these two peak periods. In colder weather, this could mean that the building does not reach a high temperature before the heating switches off, depending on the thermal mass and insulation.

Unit ventilation, with sensor operation and manual override, may be most suitable for this type of lifestyle. The sensor will ensure that any residual moisture is exhausted while the building is unoccupied.

A combination occupation where the breadwinner leaves and returns as above, leaving the partner at home, say to care for children or an elderly relative, requires a different ventilation system. Provided that the home has some form of central heating that stays on during the day, either individual room or whole house heat recovery units can offer a condensation solution. Permanent trickle ventilation will cope with the more general and continuous moisture generation during the day and sensors in the kitchen and bathroom will boost the unit to a higher speed for any short duration required. The energy recovered from the exhausted air will mean a relatively short payback time, typically eighteen months or so.

For households where the occupants are indoors most of the day, morning and evening peaks are not as evident. Positive pressure ventilation can offer a simple solution for this lifestyle as the unit is running continuously. This prevents moisture build-up and condensation occurrence, but this can still switch to a higher speed should it be necessary.

For larger dwellings and families, a combination of continuous heat recovery or positive pressure, plus individual multipoint or sensor operated unit extractor fans in kitchen and bathrooms, will cover all eventualities.

Compact positive pressure fans are an ideal and economical means of ventilation for smaller flats,especially if the occupier is a single parent or elderly.

The economy made in reducing heating costs by redistributing and circulating existing heating, together with the low cost installation of a single unit and only one penetration of the building fabric, has found favour with owners and occupiers alike.