With energy efficiency at the top of the construction agenda, choosing the right ventilation system is crucial. Chris Marriott advises on five key areas

Climate change has become one of the most high-profile issues within the building industry. With around half of the UK’s carbon dioxide emissions being generated by buildings, the industry is making a concerted effort to improve its green credentials.

Electrical and mechanical contractors can play a critical part in a greener approach, which means they have an opportunity to become sustainability experts for the built environment.

The Code for Sustainable Homes has applied to all homes from May 2008 and has been mandatory on all public sector homes for some time. It awards homes a rating from one to six.

Over the next few years, the code will begin to have a significant effect in the market and the hope is that it will provide valuable information to homebuyers on the sustainability performance of different properties, therefore assisting buyers with their choice of home.

In many cases, the client may be interested in maximising their rating above the minimum requirement, because it is better for the environment, as well as creating more valuable properties. The code looks at factors such as carbon emissions, use of water and the indoor environment.

Outside air

Choosing the right ventilation system can have a key influence because, by definition, ventilation can have an adverse affect on a property’s energy efficiency, simply because it introduces outside air into the dwelling.

For maximum energy efficiency, mechanical ventilation with heat recovery (MVHR) can provide a good solution. MVHR works through a series of ducts that collect stale moist air from inside the house from areas such as the kitchen, laundry and bathrooms. This stale contaminated air then passes through the MVHR unit and is exhausted to the outside (see figure 1).

Clean fresh air is simultaneously drawn from the outside and, as the two air streams pass each other, the heat is transferred from the outgoing stale air to the fresh incoming air. There is no mixing of air streams.

MVHR systems are able to capture up to 90% of the heat energy from the outgoing stale air before delivering it as warm, filtered, preconditioned air into the living areas of the property through the ducting.

Passive stack ventilation (PSV) and central mechanical extract ventilation (MEV) systems have low-level energy use and are therefore acceptable to the way industry is moving in terms of energy efficiency and sustainability. Both are far more efficient than standard extractor fans.

PSV systems (figure 2) also avoid adding to the energy consumption of the property, again a key requirement of the code. Movement of air within the building is created by the natural stack effect, which results from the difference in temperature between indoors and outdoors, together with the effect of wind passing over the roof of the building.

Many specifiers and clients prefer PSV systems because its operating principle is very simple, with no mechanical fan. In addition, because the system relies on the natural stack effect it consumes no energy and therefore does not need any electrical connections.

SAP Appendix Q

SAP is the government’s Standard Assessment Procedure for the Energy Rating of Dwellings. It is used to demonstrate compliance with the Building Regulations for dwellings – Part L (England and Wales), Section 6 (Scotland) and Part F (Northern Ireland).

The latest legislation requires that all new dwellings must be 20% more efficient than the previous minimum standards of Part L1a (2002).

SAP Appendix Q allows the energy performance of new technologies and advanced versions of existing technologies to be evaluated for inclusion in SAP assessments. It offers a method, supported by data, to enable building designers to improve their carbon rating, in line with Part L1a requirements, through use of energy-efficient ventilation products.

SAP Appendix Q is supported by a website that lists a range of the most efficient domestic ventilation products. By selecting products from this website, developers and specifiers can improve a building’s energy efficiency and with it achieve a better SAP rating.

SAP Appendix Q assesses the overall energy efficiency of each ventilation product listed on the website by measuring specific fan power and, where applicable, the energy recovered by the ventilation system. Feeding this data into the SAP calculation allows an easy comparison to be made between different ventilation systems.

Client requirements

Building designers and engineers are increasingly aware of the client’s interest in sustainable buildings. Principally this is to meet the Building Regulations, which require that ventilation is provided in all dwellings to restrict the accumulation of moisture that could lead to mould growth and prevent the build-up of pollutants that may he hazardous to health.

In addition, Building Regulations ensure that high levels of thermal efficiency are built into the property.

Many large client companies now have sustainability commitments in the form of Corporate Social Responsibility (CRS) documents. They often include specific and measurable environmental targets such as reducing energy loss and CO2 emissions.

Clients are also set targets for BREEAM and the Code for Sustainable Homes on new and refurbished buildings, which means they are constantly looking at ways of meeting their commitments.

Building type

When selecting the method of ventilation, consider the design of the dwelling and the position and size of rooms that generate the largest amount of moisture such as kitchen, laundry room and bathroom. All these factors will affect the design and type of system that is required, along with the number and position of extract and inlet ducts.

As a general rule, ventilation systems should be placed near local sources of moisture and steam. For larger rooms the ventilation unit extract points should be safely distributed to obtain even ventilation throughout.

The point at which moist air can begin to cause deterioration of fixtures and fittings is 70% relative humidity. In many older properties, natural ventilation often occurred due to draughts and the lack of double glazing.

However, modern buildings can soon develop atmospheres above 70% relative humidity. It is because of the tendency to develop these humid environments that high-performance ventilation systems are needed.

Building use

Each person produces their personal quota of heat, water vapour and carbon dioxide, and the use of the building will often determine the number of people who use it and therefore the level of ventilation needed.

If a home or building is relatively crowded, the amount of fresh air provided by ventilation must be increased. If air space per person is as little as 3 m³, up to 20 air changes per hour are needed. This reduces to around two changes per hour when each person has 12 m³ of air space.

When dealing with a crowded area – a commercial centre or office – it is worth checking that the ventilation system provides adequate air change. This is to prevent stale air from becoming a nuisance. In certain situations the carbon dioxide content of the air can also become an issue, and any concentrations above 10 parts per 10 000 would be a concern.