Air Changes per Hour (ACH)

Air Changes per Hour (ACH)

Synonym(s): Air Change Rate (ACH/ac/hr), Air Exchange Rate, ACPH

Definition and Explanation:

Air Changes per Hour (ACH) is a fundamental metric in the field of ventilation, quantifying the rate at which the entire volume of air within an enclosed space is replaced with fresh outdoor air, or recirculated and conditioned air, within a one-hour period. It's a key indicator of ventilation effectiveness and is typically expressed as a numerical value, such as "3 ACH."

To put it simply, if a room has an ACH of 1, it implies that over the course of one hour, the entire air content of that room is theoretically exchanged with new air. An ACH of 2 means the air is replaced twice within that hour, and so on. The calculation for ACH is straightforward:

Air Changes per Hour (ACH) formula

While air flow rates are commonly measured in cubic metres per hour (m3/h) or litres per second (l/s), the resultant ACH value is dimensionless, signifying a rate of air exchange.

Practical Examples and UK Application:

For the UK house building, residential retrofit, home renovation, and extension sectors, comprehending ACH is paramount for ensuring superior indoor air quality, effective condensation management, and optimised energy efficiency.

New Builds:

For instance, Approved Document F (Ventilation) of the Building Regulations for England mandates minimum ventilation rates for new dwellings. Although it doesn't directly prescribe a single ACH value for an entire home, it sets specific requirements for intermittent extract ventilation in wet rooms (such as kitchens and bathrooms) and continuous background ventilation throughout the living spaces. A meticulously designed new build will aim for a calculated ACH that successfully meets these regulatory benchmarks, often achieved through a combination of trickle vents, intermittent extract fans, and, increasingly, Mechanical Ventilation with Heat Recovery (MVHR) systems. A typical general ventilation target for a contemporary, airtight UK home often falls within the range of 0.3 to 0.5 ACH, depending on the specific ventilation strategy employed. [Source: www.gov.uk/government/publications/approved-document-f-ventilation]

Residential Retrofit:

In existing homes, particularly older, less airtight properties, the actual ACH can be significantly high due to uncontrolled air infiltration, leading to considerable heat loss and occupant discomfort. Conversely, making a home excessively airtight without implementing adequate controlled ventilation can precipitate problems like condensation, mould growth, and compromised indoor air quality. During a deep retrofit, the best practice involves aiming for a lower, more controlled ACH (achieved through enhanced draught-proofing and insulation) while concurrently installing highly effective mechanical ventilation systems, such as MVHR. This strategic approach allows for substantial improvements in energy efficiency without adversely affecting the quality of the indoor environment.

Home Extensions:

When planning a home extension, the ventilation strategy for the newly added space must be carefully integrated with that of the existing dwelling. Building Regulations extend to extensions, necessitating that the new portion adheres to current ventilation standards. If the extension substantially increases the internal volume of the property, the overall ACH for the combined dwelling needs re-evaluation to ensure sufficient fresh air provision and effective removal of contaminants. For example, a kitchen extension will necessitate specific extract ventilation rates as stipulated by Approved Document F.

Why is ACH Important?

  • Indoor Air Quality (IAQ): A sufficient ACH is instrumental in the effective removal of indoor air pollutants, including carbon dioxide (CO2), Volatile Organic Compounds (VOCs) emanating from furnishings or cleaning products, and various allergens. A low ACH can result in stale indoor air, reduced occupant comfort, and potential health ramifications.

  • Moisture Control: Inadequate ventilation (characterised by a low ACH) can lead to an accumulation of moisture generated from everyday activities (e.g., cooking, showering, breathing). This can result in condensation, dampness, and ultimately, mould growth, issues particularly prevalent in the older UK housing stock.

  • Energy Efficiency: While a high ACH caused by uncontrolled infiltration is inherently energy inefficient (as it allows heated or cooled air to escape), an excessively low ACH in airtight homes lacking mechanical ventilation can lead to the aforementioned IAQ and moisture-related issues. The overarching objective is to achieve a controlled ACH that strikes an optimal balance between maintaining excellent indoor air quality and maximising energy conservation.

Additional Essential Terms for Understanding Building Ventilation:

Mechanical Ventilation with Heat Recovery (MVHR)

A sophisticated, balanced ventilation system that continuously extracts stale, moist air from "wet rooms" (e.g., kitchens, bathrooms, utility rooms) while simultaneously supplying filtered, fresh air to "habitable rooms" (e.g., bedrooms, living rooms). A critical feature of MVHR is its ability to recover a substantial proportion of the heat energy from the extracted stale air and transfer it to the incoming fresh air stream, significantly enhancing overall energy efficiency.

MVHR systems are increasingly becoming a standard in new, highly airtight UK homes and are a cornerstone of deep residential retrofits, enabling properties to achieve stringent energy efficiency targets while consistently delivering superior indoor air quality. [Source: www.bre.ac.uk/service/mvhr].

Info
Conventionally, MVHR systems are centralised, i.e. they have a central air handling unit (AHU) with ducting routed around the property extracting from the wet rooms, and supplying outside air to the habitable rooms, however more compact single-room MVHR systems are becoming more popular for refurbishment or extension projects due to the ductless through-wall design.

Trickle Vents

These are small, discreet, and controllable openings typically integrated into window frames or designed as discreet wall-mounted sleeves. Their primary function is to provide continuous, low-level background ventilation. Trickle vents are a ubiquitous feature in UK housing, frequently employed to meet the requirements of Approved Document F for both background ventilation and purge ventilation.

They are essential for ensuring a basic, continuous movement of air, helping to mitigate condensation and dilute airborne pollutants in homes that rely on natural ventilation or intermittent mechanical extract systems.

Alert
It is important to be aware that the fenestration industry commonly believes that trickle vents are mandatory for new windows under the requirements of Approved Document F.  This is a misconception, and in fact, trickle vents are only one of several routes to comply.  Mechanical ventilation with heat recovery, for example, is a far superior and controlled method of ensuring adequate Indoor Air Quality (IAQ).

Purge Ventilation

This refers to the rapid and intensive removal of air from a room, typically achieved by fully opening windows (cross-ventilation) or by operating high-capacity, intermittent extract fans. The purpose is to swiftly dilute and remove acutely high concentrations of pollutants, odours, or moisture.

A mandatory requirement within UK Building Regulations for habitable rooms, purge ventilation ensures the capability for rapid air change when necessary, for example, to clear cooking fumes, quickly reduce high humidity after showering, or in response to indoor air quality issues. [Source: www.gov.uk/government/publications/approved-document-f-ventilation]

Infiltration and Exfiltration

Infiltration: This describes the uncontrolled and often undesirable flow of outdoor air into a building. It occurs inadvertently through unintended gaps, cracks, and various imperfections in the building's external fabric (e.g., around windows, doors, or where services penetrate the walls).

Exfiltration: Conversely, this is the uncontrolled flow of indoor air out of a building through similar unintentional openings in the building envelope.

In the UK, particularly in older housing stock, high rates of uncontrolled infiltration are a common problem, leading to draughts, significant heat loss, and reduced thermal comfort. Modern construction practices strongly emphasise creating highly airtight buildings to minimise infiltration, which in turn makes the provision of controlled mechanical ventilation systems even more critical for maintaining optimal indoor environments.

Particulate Matter (PM2.5/PM10)

These terms refer to microscopic solid particles or liquid droplets suspended in the air. PM2.5 specifically denotes particles with a diameter of less than 2.5 micrometres, while PM10 refers to particles less than 10 micrometres. Both categories represent significant indoor air pollutants that can have adverse impacts on human health.

Effective ventilation strategies, particularly those incorporating robust air filtration (as found in many MVHR systems), are crucial for substantially reducing indoor concentrations of particulate matter, especially in urban areas or properties situated in close proximity to busy transport routes.

Volatile Organic Compounds (VOCs)

These are organic chemicals characterised by a high vapour pressure at ambient room temperatures, meaning they readily evaporate and become airborne. VOCs are emitted from a diverse range of sources within buildings, including various construction materials, new furnishings, paints, adhesives, and cleaning products.

Adequate ventilation, ensuring a sufficient ACH, is indispensable for diluting and effectively removing VOCs from indoor environments, thereby mitigating their contribution to poor indoor air quality and potential health concerns.

Condensation

This is the physical process whereby water vapour present in the air transforms into liquid water upon contact with a surface that is at or below the dew point temperature. In building contexts, condensation commonly manifests on cold surfaces such as window panes, external walls (particularly in poorly insulated areas), or within unheated, poorly ventilated spaces like lofts and crawl spaces.

Condensation is a pervasive issue in many UK homes, frequently exacerbated by insufficient ventilation (a low ACH) and elevated levels of indoor humidity. Implementing proper and well-designed ventilation strategies is a critical measure for effectively managing condensation and, consequently, preventing the growth of mould and associated damage to the building fabric.


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