Low Energy House - Mechanical Heat Recovery - Ventilation Systems

Houses now achieve high levels of airtightness as a result of the robust detailing of new houses and the comprehensive draught proofing of existing houses. In order to save heat energy, effective mechanical heat recovery ventilation has become a priority

Mechanical Heat Recovery Ventilation

Ventilation is required in houses for several reasons:

• Provision of outside air for breathing
• Dilution and removal of airborne pollutants including odours
• Control of excess humidity from water vapour in the inside air
• Provision of air for fuel burning appliances

Conventional Extract

When conventional extract fans are employed they extract warm air from the wet rooms in the house. This warm air is replaced by cooler air that comes into the habitable rooms through trickle vents. By this method, the heat energy is lost in cold weather and it has to be replaced by a conventional heating system.

Mechanical Heat Recovery Ventilation

Heat Recovery Ventilation is the process of exchanging the heat energy contained in air that is extracted from a dwelling and transferring it to the incoming replacement air. Mechanical heat recovery ventilation systems can comprise either a central extract system or individual room fans (or a combination of both). In both cases the fans should operate at their normal minimum rate so as to be as quiet as possible and not to discourage their use.

Single Room Mechanical Heat Recovery Ventilation

In smaller dwellings, where most of the rooms are on outside walls, single unit heat recovery fans can be fitted in each room. These fans combine an extract fan, a fresh air supply fan and a heat exchanger which allow the heat from the extracted air to be transferred to the incoming fresh air.

Care should be taken when choosing individual room mechanical heat recovery ventilation units, to ensure that the fans will operate effectively in windy conditions. Measures should be taken to minimise wind effects when any extract vent is located on the side of the house with the prevailing wind.

Whole House Mechanical Ventilation with Heat Recovery

In larger houses or dwellings with large internal spaces, a ducted mechanical heat recovery system is more appropriate. A ventilation unit is usually fitted in the loft space where it is connected via ductwork to ceiling vents.

The central ventilation unit is fitted with two fans. One fan extracts wet, warm air through these vents from the bathrooms, kitchen and utility room and transfers it to the central ventilation unit from where it is extracted to the outside of the house. The other fan draws fresh filtered air from outside and supplies it through ceiling vents in the habitable rooms of the house (bedrooms and living rooms).

During the process the heat exchanger within the appliance transfers between 80 and 95 per cent of the heat from the outgoing stale air to the incoming fresh air.

Advantages of Mechanical Heat Recovery Ventilation

If these two types of system are properly designed they guarantee sufficient ventilation with reduced heat loss and improved comfort conditions. They can also help to combat dust mites and improve conditions for asthma sufferers.

Heat Exchangers in Mechanical Heat Recovery Ventilation

The heat exchangers in domestic heat recovery systems are usually of the fixed plate type. They consist of metal plates over which the incoming air and the outgoing air flow. The airflows are hermetically separated so that they cannot mix. Fixed plate heat exchangers have no moving parts and are generally configured in one of two main ways:

Mechanical Heat Recovery Ventilation - Cross Flow Heat Exchangers

• A cross flow heat exchanger consists of parallel layers of plates that are kept separate and sealed. The outgoing heats the plate as it passes over it. The cold incoming air is heated as it passes over the other side of the plate. The ducts on either side of the plate are hermetically sealed to separate the two airflows from one another so that the can never mix. Heat efficiencies of up to 75 per cent can be achieved with a cross flow heat exchanger.

Mechanical Heat Recovery Ventilation - Counter Flow Heat Exchangers

• Counter flow heat exchangers work on the same principle as cross flow heat exchangers but they are slightly more sophisticated. They have a matrix of horizontal and vertical plates which create multiple ducts and provide more opportunity for heat to be exchanged. Counter flow heat exchangers can achieve up to 95 per cent efficiency.

Mechanical Heat Recovery Ventilation systems use a small amount of electricity in operating the intake and extract fans. For low energy houses, the power for these fans can be provided by supplementary electricity supplies from solar pv systems. Additionally, the heating of the intake air can be supplemented by the use of a ground source heat pump