Natural Ventilation

In this case, the ventilation system is based on two natural means of driving the air flow through planned openings (vents) and unplanned ones (air infiltration):

In Ireland, the air movement resulting from the stack effect is limited due to our relatively mild climate and wind would be the main natural ventilation driver.

The most common approach to natural ventilation in Ireland is to provide a series of vents (through the wall or trickle vents on windows, referred to as background ventilators) and mechanical extract fans in the ‘wet’ rooms (kitchen, bathroom, toilets, utility, etc.). While extract fans are used intermittently to deal with water vapour, smells, etc., it is assumed that the air movement through the vents will provide good cross-ventilation and air renewal throughout the house (see graph below for illustration). 

Source: Irish Building Regulations, TGD Part F, 2009.

Building Regulations in Ireland and the UK specify a minimum area for background ventilators in different rooms and minimum rates of extraction in wet rooms for the application of natural ventilation, as illustrated in the graph above. It also requires various levels of window opening for purge ventilation.
As an alternative to intermittent extract fans in wet rooms, Part F in Ireland and the UK allow for the use of Passive Stack Ventilation (PSV). PSV is a ventilation system using ducts to extract air from wet rooms to the outside by a combination of the natural stack effect and the pressure effects of wind passing over the roof of the building. The ducts connect an extract terminal in the wet rooms, typically at ceiling level, and a terminal on the roof (a slate vent or a ridge tile).

Source: Part F TGD.

Whole-house mechanical extract ventilation

Whole-house mechanical extract ventilation is a system providing air renewal by extracting exhaust air from the wet rooms with a central fan on a continuous basis and supplying fresh air to the dry rooms through air inlets (wall vents or window trickle vents). The transfer of air from the dry rooms to the wet rooms before being extracted ensures air renewal throughout the whole house, including in transition spaces (corridors, stairwells, etc.). A free passage area of a minimum of 7600 mm2 should be provided for the transfer of ventilation air between dry and wet rooms, typically by undercutting doors 10 mm above finished floor levels for a standard door with.

A key advantage of whole-house mechanical extract ventilation is that since ventilation is continuous, the equivalent area of background ventilators (air inlets) required by the Building Regulations (TGD Part F, UK) are much lower (up to 3 times) for continuous extract systems than with the natural ventilation approach with intermittent extracts. With less and smaller wall vents or trickle vents required, the risk of excessive air infiltration on cold, windy days is considerably reduced, so are discomfort and heat losses.  

Mechanical ventilation with heat recovery

Mechanical ventilation with heat recovery (MHRV) is another whole-house ventilation system which supplies fresh air to dry rooms and extracts stale air to wet rooms. Both air flows are ducted and driven by two fans, one on the supply side and one on the extract side. The key element of this system is that it uses a heat exchanger to transfer heat from the warm exhaust air to the fresh air, achieving up to 85% heat recovery. The reduction in heat losses due to ventilation is very significant and occupants comfort is also increased as the air supply is warmed before entering the rooms. The HRV unit which houses the heat exchanger and the fans is also equipped with filters which prevent outside dust entering the system and internal air particles depositing within the unit. Filtering the outdoor air, possibly up to removing pollen and other allergens, can be of great to people suffering of respiratory problems including asthma.

Source: Irish Building Regulations Part F

Since all the air supply is ducted, there is no direct opening to the outside and the risk of cold draughts is eliminated, provided the house is not draughty. It is in fact essential that the house is built airtight (recommended airtightness for new built ≤ 3 m3/m2,hr at 50 Pa) otherwise it is virtually impossible to balance the system (air supply rate = air extract rate) and get it to operate properly. In addition, energy efficiency gains achieved through heat recovery are largely negated by uncontrolled air infiltration.

The HRV unit should be selected to provide the air flow required in the house (nominal flow), on the extract and supply side, and achieve the highest level of heat recovery possible. The Building Regulations Part F (UK and Ireland) require that the system provides for an ongoing air renewal of approximately 0.4 AC/hr in our case-study house, and also specifies the extraction rate to be achieved in wet rooms (50 m3/hr in kitchen, 30 m3/hr in bathroom, 20 m3/hr in WC, etc.). The controls of the unit should allow varying the ventilation rate from minimum (when the house is not occupied) to boost when the house is very busy (party mode). It is also important that the fans are highly efficient to reduce electricity consumption.

Ducts should be kept as short as possible and the number of bends, reductions and other obstructions to air flow should be kept to a minimum to avoid the loss of air pressure and ensure good air distribution throughout the system. Rigid ducts (galvanised steel or PVC) and semi-rigid ducts (ridged polyethylene duct with internal smooth bore) are recommended, and flexible ducts are generally not as they create too much pressure drop and are prone to dirt deposits. The ducting should be planned in such a way that it can be cleaned every few years (5 years maximum). 
Supply and exhaust duct sections from the HRV unit to the outside (assuming the unit is located inside) should be well insulated (50 mm for short duct lengths and 100 mm for duct lengths superior to 5 m), and the insulation material protected against vapour diffusion to avoid condensation on the duct. It is also very important to select a quiet HRV unit and apply all the precautions required to avoid noise pollution (max. 25 dBA in living and sleeping areas). Sound transmission through the ducting from the fans and between rooms should be eliminated by the use of silencers (sound absorbing sections of duct). The unit itself should be installed in utility spaces or in the attic away from bedrooms.

Demand Controlled Ventilation

Demand controlled ventilation (DCV) is a method providing automatic regulation to the ventilation system dependent upon the needs of air change rates (occupancy, pollution loads, etc.) using a suitable sensor and without user intervention i.e. automatic.

This approach can be applied to any of the system types above and has been very popular in Europe for decades. Demand Control is now a common feature of quality s continuous mechanical extract ventilation and passive stack ventilation systems.

With DCV systems, extract and supply air flow rates can be regulated to ventilate where and when needed, achieving an overall reduction in air renewal and associated heat losses. Scientific studies carried out in France demonstrate that DCV can result in at least 30% reduction in ventilation losses compared to natural ventilation with conventional extract fans. It can also reduce the primary energy use of associated fans. Extensive monitoring also shows that demand-controlled mechanical extract ventilation achieves excellent indoor air quality.

For further info on ventilation systems:

Links

• UK Appendix Q: this database provides a useful tool to compare the heat loss reduction and electricity consumption associated with different ventilation systems.
• Ventilation in Passive Houses
• IEA Air Infiltration and Ventilation Centre