Air Changes per Hour (ACH) is a measurement of how many times the air within a defined space, such as a room or building, is replaced with outside air in one hour.
ACH is a critical metric used in assessing indoor air quality and ventilation efficiency. It quantifies the effectiveness of ventilation systems in removing indoor pollutants and moisture by comparing the volume of air supplied to the space with the volume of the space itself. A higher ACH indicates a greater rate of air exchange, which can lead to improved air quality but may also result in higher energy costs if not managed properly.
Residential Ventilation
In a typical UK home, maintaining an ACH of 0.5 to 1.0 is often recommended for bedrooms and living areas to ensure sufficient fresh air supply without excessive heat loss. For kitchens and bathrooms, higher ACH values may be required to effectively remove moisture and cooking odours.
These recommendations are aligned with the Building Regulations Part F, which emphasises adequate ventilation to ensure health and safety.
Building Regulations Compliance
The Building Regulations Part F outlines specific ventilation rates, suggesting that new dwellings should achieve a minimum of 0.3 litres per second per square metre of floor area for whole dwelling ventilation. This calculation helps determine the necessary ACH to meet health and safety standards.
Energy Efficiency Considerations
Balancing ACH and Energy Use
While higher ACH can enhance indoor air quality, it is vital to balance ventilation with energy efficiency. Homes with high ACH may experience increased heating or cooling demands, leading to higher energy bills.
The regulations encourage the use of energy-efficient systems, such as:
- Heat Recovery Ventilation (HRV): Captures heat from outgoing air to pre-warm incoming air, reducing energy costs.
- Mechanical Ventilation with Heat Recovery (MVHR): Provides continuous ventilation while recovering heat.
Retrofitting Older Homes
When retrofitting older homes, it’s essential to assess existing ACH levels to determine if additional ventilation is needed. Significant air sealing to improve energy efficiency may necessitate a mechanical ventilation system to maintain adequate ACH without compromising indoor air quality.
Air Changes per Hour (ACH) – Key FAQs
How to convert ACH to Litres per Second (L/s)
How to convert Litres per Second (L/s) to ACH
Why is ACH critical for health?
- Poor ventilation (ACH < 0.5) traps pollutants like VOCs, mould, and CO₂.
- Health risks:
- CO₂ levels > 1,000 ppm reduce cognitive performance by 15-25% (Harvard Study).
- 40% of childhood asthma cases are linked to indoor air pollutants worsened by low ACH (EPA).
What ACH standards apply in the UK vs. globally?
- UK: 0.3 L/s per m² (Part F) ≈ 0.5 ACH for living spaces.
- U.S. (ASHRAE): 0.35 ACH for homes; 4-6 ACH for kitchens.
- Scandinavia: Enforces 1.0 ACH minimum due to airtight construction.
Can improving ACH save energy?
Yes, with the right systems:
- MVHR systems recover 80-95% of heat from exhaust air.
- Retrofitting with MVHR cuts heating costs by £220/year while maintaining 0.8-1.2 ACH (ORNL Study).
- Without heat recovery, 1.0 ACH adds £150–£300/year in heating costs.
How do I calculate required ACH using CO₂ levels?
Use Harvard’s CO₂ Calculator:
- Example: A 20m² bedroom with 2 occupants needs 1.5 ACH to keep CO₂ < 800 ppm.
Does retrofitting older homes work?
Yes:
- A 1930s UK terraced house retrofitted with MVHR:
- ACH improved from 0.3 (leaky) to 0.7 (controlled).
- Energy use dropped 18%, with a 5-year ROI on installation costs.
What’s the ideal ACH for UK homes?
- Bedrooms/Living rooms: 0.5–1.0 ACH.
- Kitchens/Bathrooms: Up to 15 ACH (to remove moisture/odours).
What is the minimum ventilation rates needed for a utility room?
What is the minimum ventilation rates needed for a kitchen?
What are the regulations for bathroom ventilation?
What is the minimum ventilation rates needed for a toilet?