Ventilation unit and ductwork

Ventilation unit location is very important to both the efficiency and also the long term usability of the system. Ducts connecting to outside contain air at the outside temperature, so are effectively outside walls – we use insulated ducts but can’t possibly insulate them to the standard of the wall. The ducts therefore need to be as short as possible, basically the unit needs to be located adjoining an external wall, or failing that, within 2m of one. It is possible to install the unit in a frost free location outside the thermal envelope, eg, a garage, but the connecting ducts still need to be as short as possible and of course the external terminals have to be outside the garage.

Access is needed to change fi lters (every 3–6 months, depending on how dirty the outside air is), so you should be able to walk up to the unit and reach it without needing a ladder. Some units are too noisy to have in a living room or bedroom, so the ideal is a utility room or WC/cloakroom. Allow space for ducts and ancillary equipment when allocating space – look at previous installations for guidance.

Condensate drainage is needed from the ventilation unit – this should be to the internal soil pipe system via a trap and not just straight to outside, otherwise it could freeze and the unit would flood.

External terminals can be close together – often vertically one above the other with exhaust 600mm below intake is a good solution to minimise wall use on a narrow frontage house. The position of the intake terminal is important for air quality – avoid locating near smells or pollution, eg, bins or car park, and avoid intake through the roof in order to avoid bringing in hot air in summer – even north roofs can get hot. Put the intake 2m or more above ground level to minimise intake of particulates. It is better to have the exhaust on the same wall as the intake, to minimise impact of wind pressure difference. Using a roof exhaust causes problems of condensate from the damp exhaust air collecting in the duct and needs special drainage.

Ductwork can either be rigid – normally steel – in a branched configuration, or newer ‘semi-rigid’ systems which use plastic ducts that can be bent round corners, but still have a smooth inner bore. For the latter, the ducts are arranged radially running one or two to each room from supply and extract distribution manifolds. ‘Flexible’ ductwork, like old tumble drier hoses, is not acceptable – it gets squashed and has a high resistance to airflow. All distribution ductwork has to be within the airtight thermal envelope – no exceptions.

Ductwork size is determined initially on air velocity to avoid regenerating noise, and the requirement is <2m/s generally, and up to 3m/s for the main ducts at the MVHR and connections to outside. With these velocities the ductwork system will have low pressure loss, though for larger houses and non-domestic projects, full design is advised (with specialist ductwork design software – the system supplier should be able to do this) to ensure the system remains balanced despite longer duct runs. Higher pressure loss increases fan energy consumption and the test figures for PHI certified MVHR units only apply up to a maximum system pressure of 100Pa. This should only be exceeded in large nondomestic projects when additional calculation is needed to determine the fan energy usage in PHPP in accordance with actual system pressure loss.

Noise control is needed to make the system unobtrusive – noisy systems get turned down or off. Include proper silencers on supply and exhaust – these are big (900mm long and 100mm larger diameter than the ductwork) so if possible fi t them between ceiling joists. Branched systems may also require cross-talk attenuators on the supply to prevent noise transmission between rooms via the duct.

The Passivhaus ventilation protocol worksheet gives design figures of 25 dB(A) in living and bedrooms and 35 dB(A) in the room containing the MVHR fan unit. These figures are used to calculate attenuation required – they are too low to accurately measure, at least during the day on a busy building site. Once the system is running you shouldn’t be able to hear the ventilation in bedrooms or living rooms.

Room terminals are different for supply and extract, and there are different terminals for wall supply, directional ceiling supply, and all-round ceiling supply. The position in the room depends on the terminal type: directional terminals are often used to throw air in from the doorway; standard terminals need to be near the middle of the room (but avoid locating over beds) – in front of windows is usually good as the terminals are then unlikely to be obstructed by high furniture. Extract terminals can be put on a wall or ceiling; always put these on the far side of the room from the door.

In housing, door undercuts are the normal way to allow air to flow out of supply rooms and into extract rooms. The minimum undercut is 10mm above final floor finish.


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