Ventilation Options

Put simply, ventilation is the removal of ‘stale’ air from inside a building and its replacement with ‘fresh’ air from out­side. Adequate ventilation is essential in your home to maintain a healthy environment but also to prevent the build-up of excess levels of humidity and to provide air for fuel-burning appliances. A good ventilation system will help remove cooking smells, allergens and other irritants, such as tobacco smoke, and make your home a considerably more pleas­ant, healthy and comfortable place to live.

Buildings are ventilated through a com­bina­tion of air infiltration and purpose-provid­ed ventilation. Infiltration is the uncontrollable air exchange between inside and outside a build­ing through a range of air leakage paths in the building structure i.e. drafts. Purpose-provided ventilation is the controllable air exchange by means of a range of natural and/or mechan­ical devices such as trickle vents in windows and electric extract fans. The current aim of the Building Regulations is to minimise uncon­trol­lable infiltration ven­ti­lation in favour of con­trol­lable ventilation in order to help improve energy efficiency.

The Building Regulations

The different types of ventilation required in the home are currently dealt with by three parts of the Building Regulations: Part F deals with ventilation to the living space, Part C ventilating the structure and Part J the provi­sion of air for fuel-burning appliances, includ­ing fires and stoves.

In September 2005, new measures were announced by the Deputy Prime Minister that intend to make buildings more energy efficient, potentially reducing carbon dioxide emissions by as much as one million tons per year. These new measures will come into effect on April 6th and, taken together with the 2002 Building Regulations, aim to improve standards by 40%. In preparation for the changes, the Government has published interim versions of the approved documents for Part F (ventilation) and Part L (energy efficiency).

What are the Key Changes?

• The new Part F (ventilation) document is more performance based and gives guidance on how required measures can be achieved, giving an overview of a number of ventilation solutions

• The changes in Part L (energy efficiency) require buildings to be better sealed and more airtight, so the new Part F provisions are designed to ventilate buildings having air permeability down to 3 m³/h/m² at 50 Pa

• Guidance has been given for the ventilation of basements in houses

• Replacement windows should be fitted with trickle ventilators, or an equivalent background ventilation opening should be provided in the same room e.g. an airbrick

• More guidance has been given for domestic mechanical and natural ventilation systems

• Requirement F2, regarding ventilating the structure, has been moved to Part C

• Appendixes give guidance on passive stack ventilation system design and installation, good practice on the installation of fans in dwellings and on minimising ingress of external pollutants into buildings in urban areas.

Part L Pressure Testing

The changes to the Building Regulations tie Parts L and F closer together. This is because ventilation is rapidly becoming the greatest source of heat loss in new dwellings as the standard of insulation continues to improve. The revised Part L places a great emphasis on making buildings as airtight as possible and, from April, air pressure leakage testing will become a mandatory requirement for all constructions, including dwellings, to measure air permeability and show any unacceptable leakage. This in turn places a greater impor­tance on controlled, purpose-provided ventila­tion as opposed to infiltration.

Pressure testing will cost around £4-500 per property, but can potentially be avoided for self-builders if it can be demonstrated that, during that 12-month period, a dwelling of the same type constructed by the same builder has been pressure tested and achieved the required design air permeability, or more likely, by using a value of 15m³/(h.m²) when calculating the Dwelling Carbon Emissions Rate. The effect of using this worst-case value would then have to be compensated for by improving standards of energy efficiency else­where in the dwelling. Options for compensating include improving the elemental U-values in floors, walls, roof and windows, a better boiler SEDBUK rating or the inclusion of renewables such as solar panels, a heat pump and photovoltaics.

Which Ventilation Solution is Best for Me?

The right ventilation solution for your project will depend largely on the amount you are prepared to spend, how much system main­te­nance you consider acceptable and the level of importance you place on energy efficiency. You also need to consider whether you want to filter, warm or cool incoming replacement air to give a more controllable indoor climate. Aesthetics is another con­sideration: some people hate the sight of trickle ventilators in windows and the noise of individual extract fans in bathrooms, both of which can be eliminated with a whole-house system. In some areas noise pollution will be a factor and there are ventilation options that reduce this problem.

For households with allergy sufferers a system with electrostatic filters can bring great benefits. If you are building a new house, it will also depend on how airtight and energy efficient the building is, as this will be a big factor in determining the efficiency of features such as heat recovery. These systems can be highly effective but require the property to be relatively airtight and so are best suited to new builds, especially construction systems that tend to be inherently airtight such as timber frame, structural insulated panels and insulated concrete formwork.

If you are on a tight budget the least cost option is probably going to be individual extract fans in wet areas and passive vents (trickle ventilators or airbricks with sliding hit and miss vents), or a positive input ventilation system (PIV). If you are building an eco house with a very low heat requirement then your choice is likely to lie between a passive stack system that uses no direct electricity, a PIV system, or a heat recovery ventilation system (HRV). If you want to be even more ecological, you could opt for a mechanical system that preheats the incoming fresh air via solar panels: when the heat is not required it is diverted to the hot water cylinder.

You may also wish to consider other factors such as whether you want to combine your ventilation with an air conditioning and/or a heating system. If you don’t want to see trickle vents in all of your windows, then you should consider an HRV system, a PIV system or a mechanical whole-house system that operates continuously. If you would like to control allergens, a mechanical system with controlled (filtered) air intake is the best option.

Whichever system you opt for you must include purge (formerly rapid) ventilation, either through natural means (such as open­ing windows to each room) or where there are no windows, a mechanical fan. You might also wish to provide extraction above the kitchen hob. This can be incorporated into a whole-house system but it is generally consid­ered best to keep this separate due to the high levels of grease and moisture prod­uced in kitchens. Kitchen extractors can be ducted to the outside, or can filter out cook­ing smells and return the cleaned air back into the kitchen.

When considering ventilation it is important to remember that the roof structure and any voids beneath suspended floors and ceilings need to be ventilated under Part C of the Building Regs and under Part J ventilation is required for fires and stoves.

Trickle Vents

Trickle vents in windows or airbricks with sliding louvers have to be included to provide background ventilation unless you opt for a system that controls incoming fresh air such as a heat recovery system, a continuous mechanical extract system, or a positive ventilation system. The building will still need any background ventilation required under Part J for combustion.

Heat Recovery Ventilation (HRV)

HRV systems are designed to continuously extract polluted air from moisture- and odour-producing areas such as kitchens and bath­rooms. This is done at a low background level with an occasional boost when required. The air is carried via a network of ducts concealed within the building structure, connected to a central mechanical fan unit, usually located in the loft or other out-of-the-way location. Before filtered air from the outside is supplied to ‘dry’ habitable rooms (bedrooms, living rooms etc.) heat is transferred from the stale air via a heat exchanger in the power unit, with a 70-93% efficiency.

The relative cost-effectiveness of heat recovery will depend on its efficiency level, i.e. how much of the energy consumed for space heating is recycled. In a well-insulated and fully airtight house, the efficiency will be high — whereas in a house with lots of leaks and full of trickle vents and airbricks, its efficiency will be seriously compromised. This is why the Building Regulations are becoming much stricter about air tightness in new buildings.

HRV systems negate the need for individual bathroom extract fans and background vents — with the exception of those required under Part J for combustion. This saving can help to offset the relatively high cost of such systems, which are around £1,500-2,000 plus instal­lation — although this can be under­taken relatively easily by a competent DIYer at first fix stage.

Passive Stack Systems

Passive vents operate on the principle of con­vec­tion — the movement of air via currents created by temperature differences inside and outside the house. Passive vents are located in the same places as extract fans, such as kitchens and bathrooms, but as they have no mechanical fans they are silent and use no direct energy. The vents are simply plastic ducts that run from the ceilings of wet areas, up and out through the roof. Some vents have humidistats that adjust the vent opening and therefore the rate of ventilation, in relation to the humidity level. Passive stack ventilation must be used in conjunction with background ventilation, typically trickle vents in windows. The air that enters the building is unfiltered and at outside air temperature. The idea is that the system is controllable and that the inevitable energy loss through ventilation is mitigated by the energy saved by not using electrically powered mechanical fans.

Continuous Mechanical Extract Ventilation

This is essentially the same as having bath­room extract fans, only instead of individual fans in each wet area, a single central fan is located remotely and ducting is installed to each wet area. This form of ventilation is designed to work at a low background level with an occasional boost when required. It nor­mal­ly removes the need for background ventila­tion by drawing in air through natural leakage points throughout the house, so there is usually no need for trickle vents or airbricks. Continuous ventilation can also help to keep the house cooler on hot days.

Replacement air is at outside temperature and is not filtered. A benefit of such systems is that fan noise can be removed from bath­rooms to wherever the unit is located. A central extract fan system can be a more cost-effective alternative to extract fans and background vents in a larger house that has several bathrooms.

Positive Input Ventilation (PIV)

Positive ventilation uses a mechanical fan to inject fresh air into the house and forces stale air out of the building via natural leakage. The system works continuously and background vents are not normally required, so there is often no need for trickle vents — though sometimes it is necessary to have transfer grilles between rooms to enable ventilation of rooms that do not open directly onto the hall­way. As fresh air is being drawn in mech­anically, it can be filtered to remove pollen and other irritants.

When installed in a cold roof space (an un-insulated loft) positive ventilation can draw its incoming air from the loft space that is naturally warmer than outside air and therefore requires less heating, helping to improve energy efficiency. Systems are also available with two intakes, one from the loft and another directly from outside at soffit level (the underside of the roof overhang) with a thermostatic control unit that mixes the intakes to your chosen setting, according to whether heating or cooling is required. A third option is to have an intake below the roof tiles which can work as a solar air preheater on sunny days and help to cool incoming air at night.

Combining Warm Air Heating and Air Conditioning

Many mechanical, whole-house ventilation systems can electrically pre-warm the incom­ing fresh air, but this is via a small electric element that is only capable of taking the chill off the air. However, systems are available that have ducts into every room through which pre-warmed air is blown as the main source of central heating. This takes care of ventila­tion requirements at the same time via the return ducts.

Warm air heating can be powered directly by electricity, natural gas or LPG, or via indirect heaters (water to air) such as a gas or oil boiler. Alternatively warm air heating can be powered by an electric heat pump, which extracts energy from the air outside the house and from the stale air from inside the house (cooling it to below external air temperature), and uses it to heat the fresh air being brought into the house. The idea is that the value of the heat generated is greater than the cost of the electricity it consumes (electric) so ventila­tion becomes a heat source rather than a source of heat loss.

Comfort cooling can also be added to a whole-house ventilation system. This usually takes the form of an air cooling unit placed alongside the central fan unit, typically in the attic. Alternatively, an external unit can prove less noisy. Air cooling uses a heat pump to cool incoming fresh air. With a split heat pump (one which works both ways) the system can be used to provide warm air in winter and cool air in summer, switching between the two according to changes in the ambient temperature.

Filtering the Air

Mechanical ventilation systems that have controlled air intake can also include air filtration to remove allergens such as pollen. An electrostatic filter can be installed which electrically removes even the smallest par­ticles in the air — this is a separate unit costing around £500 but will significantly improve air quality in the house.

Filtered air can be of great benefit to allergy sufferers but just having good quality ventilation will help reduce allergy problems such as asthma. Regardless of which venti­la­tion system is used, the lower humidity levels reduce the number of house dust mites and fungal spores which are amongst the main allergens for asthma sufferers. Allergy problems can be reduced further by using a whole-house vacuum system that does not recycle vacuumed air within the house.

Ventilation for Noise Polluted Areas

Noise pollution is now graded and the need for appropriate ventilation is taken into consideration by planning departments for plots near a main road, a railway or in a flight path. For Category B sites it is normally recommended that ventilation is designed to avoid the need to open windows. In the past, treatment has meant the use of large unsightly acoustic boxes for input and extract vents. However, whole-house ventilation systems are now recommended as an alter­native, because they operate continuously, unlike input/extract acoustic boxes which are typically switched off.

Central Vacuum Systems

A central vacuum is a built-in appliance that is a good idea to install around the time of ventilation ducting, and is especially useful for asthma and allergy sufferers. Ordinary vacuum cleaners tend to be noisy, unhealthy and awkward, but in contrast central vacuums are quiet, remove dust and have no clumsy vacuum cleaner to carry — a lightweight hose is simply inserted into a convenient vacuum point. Dust is removed via hidden ductwork to the vacuum unit, which is discreetly located away from your living areas. This ducting can be fitted quickly and simply at first fix stage, using the same natural spaces in your home as the HRV, but because the ductwork is only 2” (52mm) in diameter there are several other places that it can be hidden or boxed in. Inlet points are located in central positions such as hallways or landings, sufficient to allow the vacuum hose to reach the whole house. The sockets are installed at second fix along with your electrical sockets and light switches. A 9m hose with two inlets would typically cover a four bedroom property of about 1,500ft² (139m²).