Residential ventilation

The UK situation

For many years we have been accustomed to providing some form of ventilation in two particular areas of a home. These par­ticular applications are characterised by high levels of humidity and are as follows:

• Bathroom extract

• Cooker extract

Under previous conditions, these two areas were perhaps unique in British houses and therefore required the provision of extract ventilation with or without recirculation.

In recent years, under UK Government encouragement, the air tightness of domestic residences has improved considerably, to the extent that trickle ventilation openings and airbricks now have to be incorporated into new houses to improve the natural ventilation performance.

The situation elsewhere

In the USAand Commonwealth countries such as Australia and New Zealand, the situation is somewhat different and mechani­cal ventilation systems have been developed to overcome problems associated with mould growth due to the high ambi­ent humidities at certain parts of the daily cycle.

The dwelling should be as airtight as practicable to ensure eco­nomic operation. Currently, the achievable lower limit is a mean, background air infiltration rate of 0.2 air changes per hour (ach) for a conventional brick-built cavity-walled house, but this can be bettered in a timber-frame construction.

Generally speaking, a house with a mean natural air infiltration of 0.2 ach will have a higher rate if “pumped up” by an external fan in accordance with the accepted method of test. At an ap­plied pressure difference of 50 Pa this should not however exceed 0.4 ach.

Air flowrate and air distribution

The total extract air flowrate during normal operation of the sys­tem, should be equivalentto between 0.5 and 0.7 ach based on the whole dwelling volume, less an allowance for background natural infiltration.

A facility to boost the air extract rate from the kitchen during pe­riods of cooking, and from the bathroom during washing is highly desirable. It is suggested that an increase in extract air­flow rate of 50% in a single room, or 25% for the whole system, would be a reasonable minimum.


A master on/off switch should be mounted on or near the fan unit to isolate the system electrically during cleaning and main­tenance. A variable fan-speed control facility to boost the air ex­
tract rate from the kitchen during periods of cooking, and from the bathroom during washing is highly desirable.

Noise generated by continuous, operation should be con­trolled. Suggested reasonable levels are shown in Table 21.3. The CIBSE Guide B5, is also worth studying. These levels are for normal operation; they do not apply to systems under boost operation. They should not be exceeded anywhere that is nor­mally accessible to occupants. The noise should be steady and contain no distinguishable tonal or impulsive characteristics. Noise caused by air flowing in the ductwork, (regenerated noise) can be reduced by minimising velocities and pressure loves across duct components and by having duct terminals, branches and bends well spaced.

Room classification

Suggested design background noise level dbA


Below 30

Living rooms

Below 35

Dining rooms

Below 35

Kitchens, bathrooms, utility rooms, circulation areas

Below 45

Table 21.3 Background noise levels from mechanical services

Fan siting

The fan may be sited anywhere in the dwelling provided there is adequate access for cleaning and maintenance, and that any noise produced by the system will not disturb the occupants or their neighbours.

Dwelling characteristics

With mechanical ventilation, outdoor air must be admitted into the dwelling to replace extracted air. Dwellings which are not airtight will have a combination of controlled ventilation, pro­vided by the fan system, and uncontrolled infiltration through air leakage paths all over the structure. With a leakage rate of 4 ach at 50 Pa, it is probably sufficient to rely on structural air leak­age. If the bottom edges of internal doors clear the floor surface by 5 to 8 mm, there is likely to be a sufficient opening for air movement.


Ventilation ducts should be sized to give average air velocities below 4 m/s during normal operation to minimise noise. Higher air velocities are usually acceptable for boost operation.

Non-circular ducting of equivalent cross-sectional area may also be used. Size the duct system and terminal fittings, taking into account the airflow rate and pressure performance avail­able from the fan unit. All joints should be sealed properly to avoid air leaks into or out of the ducts. Vertical exhaust ducts should be fitted with condensate traps.

Horizontal exhaust ducts should slope away from fans to pre­vent condensate running back and so reduce the risk of con­tacting live parts. Where ducts pass through an outside wall or attic floor, they should be carefully sealed to the building enve­lope to maintain the air tightness of the dwelling.

Terminal fittings

The terminal fittings should be durable, easy to clean and capa­ble of passing the required airflow rate, at the available pres­sure drop, without generating excessive noise. Extract grilles should incorporate a dust filter and, if fitted in a cooker hood, a grease filter.

Extract grilles should be positioned so that they clear air from as much of the room as practical, ideally as far from the internal door and as high as possible. Above a shower cubicle or bath is a good place in the bathroom.

The extract-air outlet should always be outside the building (for example through a roof, wall or soffit) and away from any noise-sensitive areas such as bedroom windows. Outlet fittings should have a louvre, cowl or similar device to prevent rain, birds, large insects and rodents from entering the duct system.

Fire precautions

Ducting within the kitchen and connected to a cooker hood should be made of a non-flammable material such as steel. A fire damper is essential in all installations to close off the cooker hood’s air extraction opening; it should be as close as possible to the hood.

Fire dampers are desirable in other kitchen extract terminals. Quick-acting fire dampers, such as a spring-loaded or gravity operated flap with thermal release, are preferred. If metal ex­tract ducting is used, a fire damper and/or thermal fan cut-out switch should be fitted in the air extract system upstream of the fan unit. If plastic ducting is used for extract, fire dampers must, in addition, be fitted wherever the duct passes through any floor and through those ceilings and internal walls, which are re­quired to be fire resisting. Fire regulations may impose requirements additional to those given here.

Cleaning and maintenance

Cleaning intervals depend largely upon the location and effec­tiveness of the system’s air filters. Air filters and extract grilles will probably need to be cleaned at least two or three times a year. Fan impellers can be inspected, and cleaned if necessary annually. Cooker hood grease filters may need cleaning monthly to prevent contamination of the ductwork. The main source of information on cleaning and maintenance is the manufacturer’s instructions.

Window opening and summer operation

Fan systems are usually operated continuously because they provide most of the ventilation in the dwelling. All habitable and service rooms on an external wall (except sanitary accommo­dation separate from a bathroom) should have one or more rapid ventilation openings, such as an openable window.

The fan and motor unit

Readers, who diligently read Chapter 13, will have noted that the efficiency of small AC electric motors can be disastrously low. There has therefore been a definite move to DC motorised fans for these systems. Modern electronic design has enabled a 240 volt single phase 50 Hz AC to be easily transformed to 12 or 24 volt DC.

Direct current motors run cooler, prolonging the life of insulating materials, lubricants and bearings. Electronically commutated DC motors can have the control module mounted “on board” to reduce the power and control wiring to a minimum, (Figure 21.22).

Residential ventilation

Figure 21.22 Typical DC fan Courtesy of Vent-Axia Ltd

Fan mounting boxes

Figure 21.23 Whole house ventilation system Courtesy of Vent-Axia Ltd

figure 21.23 whole house ventilation system courtesy of vent-axia ltd

Figure 21.24 Typical loft top heat recovery installation Courtesy of Vent-Axia Ltd

figure 21.24 typical loft top heat recovery installation courtesy of vent-axia ltd
Fans can be mounted in boxes, which can if necessary be acoustically lined. The sides of the box can be provided with a number of spigots to which the ducting from the various rooms can be attached. Filters can also be included and a further spigot on the outlet side enables the discharge ducting to be fitted. Figure 21.23 illustrates a typical ventilation system instal­lation.

Heat recovery

Air-to-air heat exchangers have now been developed in the small sizes required for residential ventilation. These enable the heat which would otherwise be lost to atmosphere to be re­covered and re-used for house heating. Typical installations can be disassembled and assembled on walls, ceilings and loft areas of roof voids with limited access. (Figure 21.24)


Whilst it might be thought that the provision of such systems will increase the household electricity bill, this is far from the case. By careful control, overall usage can fall as controlled ventila­tion, lower leakage and heat recovery all contribute to improv­ing household efficiency.

Posted in Fans Ventilation A Practical Guide