UK building regulations.
System 1 emphasizes natural ventilation, aided by intermittent extract fans and background ventilators, aiming to maintain indoor air quality by allowing fresh air to circulate.
System 2, also known as Passive Stack Ventilation, was removed from Building Regulations in the 2023 amendment.
System 3, previously known as Continuous Mechanical Extract Ventilation (MEV) and d-MEV, involves continuous mechanical extraction of air to ensure a consistent flow of fresh air while expelling stale air from Building 1.
System 4, now referred to as Mechanical Ventilation with Heat Recovery (MVHR), focuses on energy efficiency by using heat exchangers to recover warmth from outgoing air and transfer it to incoming fresh air.
Due to new dwellings becoming more airtight, intermittent extract fans require a large number of trickle vents to be fitted to the windows to enable the fan to extract the higher rates required. The extract rate of the intermittent fan is often reduced. System 1 fans are often installed with flexible ducting, which offers around 7 times greater pressure than rigid ducts. The Ventilation Guide states that no more than 1.5m of duct should be fitted to an axial fan. For a centrifugal type of fan, it is 6m for an airflow of up to 30l/s and 3m for airflows of 31l/s-60l/s. Fans are to be tested to achieve the correct installed airflow.
System 1 involves intermittent extract fans fitted to wet rooms and trickle vents to windows in all rooms, as referenced in Table 1.7 of Document F.
The MEV/DMEV extract system has a lower extract rate but benefits from a low trickle rate determined by the number of bedrooms or a proportion of the floor area (floor area X 0.3 = l/s). Running at a low level 24/7 generally moves a lot more air than intermittent fans. The trickle rate can be achieved by several trickle fans (DMEV) fitted to the wet rooms or a centralized fan (MEV) ducted to all wet rooms. Trickle ventilators should not be fitted in wet rooms and should provide a minimum of 4000mm2 area for each habitable room in the dwelling. Additionally, the total number of adequately sized trickle vents should be equal to the number of bedrooms plus 2 ventilators (1 bedroomed dwelling to have 3 trickle vents, 2 beds to have 4 trickle vents, etc).
The single dwelling trickle rate for a 1-bed dwelling over 24 hours is 19l/s X 3.6 = 684m3/hr X 24hr = 1641.6 m3/day.
System 4, also known as MVHR, extracts air from all the wet rooms and supplies fresh air to all the habitable rooms in the dwelling. Trickle vents are not required to be fitted in dwellings with MVHR.
The extracted air is passed over a heat exchanger with the supply air to transfer the heat from the extracted air to the supply air. If the air temperature outside is 0°C and the internal temperature is 20°C with an MVHR unit with 90% heat exchange efficiency, the supply air temperature will be around 18°C. It’s important to note that MVHR is not air conditioning or primary heating, as is commonly mistaken.
MVHR systems should have a summer bypass that diverts the air around the heat exchanger in the summer to supply fresh air into the building at the outside temperature to help reduce the temperature, with the greatest effect at night time when the outside air is cooler. MVHR is very beneficial if you are building an airtight dwelling, as it supplies air to the habitable rooms and extracts from all the wet rooms, forcing the dwelling to breathe. The air supplied to the dwelling is filtered fresh air that becomes drier after being warmed up by the heat exchanger, giving the air capacity to absorb moisture generated within the dwelling through different activities such as washing, cooking, and breathing.
MVHR offers the greatest benefit to air quality within a dwelling, health, and moisture control. MVHR requires a small amount of maintenance as the system has a supply filter and extract filter, and these filters MUST be changed or cleaned regularly. If the filters become blocked and are not maintained, the motors will get strained and run warmer than normal, causing the bearings to dry out and fail. This is an expensive issue to fix, often resulting in the MVHR unit being completely changed depending on the manufacturer.
For more information on ventilation in homes, please consult Document F of the current building regulations.