Ventilation Rates
Standard Airtightness Method
Where neither a measured air permeability or design air change rate is known, the whole-building infiltration rate is assumed based on the minimum ventilation rate required for the number of bedrooms in the property.
This data is derived from CIBSE Domestic Heating Design Guide (2021) Table 1.7. The ventilation rate (in L/s) is converted to an whole-house air change rate (ACH or 1/h) based on the combined internal air volume of all rooms.
| Number of Bedrooms | Expected Occupancy | Ventilation Rate (L/s) |
|---|---|---|
| 1 bedroom | 2 | 13 |
| 2 bedrooms | 3 | 17 |
| 3 bedrooms | 4 | 21 |
| 4 bedrooms | 5 | 25 |
| 5 bedrooms | 6 | 29 |
For each known occupant above the expected occupancy, 4 L/s is added to the ventilation rate. Finally, the ventilation rate will be checked to make sure it is not less than 0.3 L/s per m² of floor area.
NOTE
Refer to CIBSE Domestic Heating Design Guide (2021) Section 1.7.1.1 for more information.
Ventilation Systems
Where a mechanical ventilation system exists, the supply and extract ventilation rate will vary based on the type of system.
The supply air temperature will be the design external air temperature in all cases except MVHR, where the heat recovery percentage will be applied relative to the extract air temperature.
The data is derived from Standard Assessment Procedure (10.2) Section 2.6.1 and converted to a airflow volume based on the total internal air volume of the ventilation zone.
| Ventilation System | Supply Flow Rate | Extract Flow Rate |
|---|---|---|
| Mechanical Ventilation (MV) | 0.5 ACH | 0.5 ACH |
| Mechanical Extract Ventilation (MEV) | - | 0.5 ACH |
| Mechanical Ventilation with Heat Recovery (MVHR) | 0.5 ACH | 0.5 ACH |
| Positive Input Ventilation (PIV) | 20 m³/h | - |
NOTE
Refer to Standard Assessment Procedure (10.2) Section 2.6.1, for more information.
Minimum Room Air Change Rates
For each heated room, the default minimum air change rate (ACH) used in calculating ventilation heat loss is determined by the room type and property age band.
This data is derived from CIBSE Domestic Heating Design Guide (2021) Table 3.8.
| Room Type | Minimum ACH (Age Band A-I) | Minimum ACH (Age Band J) | Minimum ACH (Age Band K onwards) |
|---|---|---|---|
| Bathroom | 3.0 | 1.5 | 0.5 |
| Bedroom | 1.0 | 1.0 | 0.5 |
| Bedroom with en-suite | 2.0 | 1.5 | 1.0 |
| Bedroom/study | 1.5 | 1.5 | 0.5 |
| Breakfast room | 1.5 | 1.0 | 0.5 |
| Cloakroom/WC | 2.0 | 1.5 | 1.5 |
| Dining room | 1.5 | 1.0 | 0.5 |
| Dressing room | 1.5 | 1.0 | 0.5 |
| Family/breakfast room | 2.0 | 1.5 | 0.5 |
| Games room | 1.5 | 1.0 | 0.5 |
| Hall | 2.0 | 1.0 | 0.5 |
| Internal room/corridor | 0.0 | 0.0 | 0.0 |
| Kitchen | 2.0 | 1.5 | 0.5 |
| Landing | 2.0 | 1.0 | 0.5 |
| Lounge/sitting room | 1.5 | 1.0 | 0.5 |
| Living room | 1.5 | 1.0 | 0.5 |
| Shower room | 3.0 | 1.5 | 0.5 |
| Store room | 1.0 | 0.5 | 0.5 |
| Study | 1.5 | 1.5 | 0.5 |
| Toilet | 3.0 | 1.5 | 1.5 |
| Utility room | 3.0 | 2.0 | 0.5 |
NOTE
Refer to CIBSE Domestic Heating Design Guide (2021) Section 3.5.4.1 for more information.
Design Flow Rates of Ventilation Devices
When unknown, the design flow rate of ventilation devices will be determined by the device type based on the table below. This data is sourced from Standard Assessment Procedure (10.2) Table 2.1.
| Ventilation Device Type | Design Flow Rate (m³/h) |
|---|---|
| Chimney or flue (closed fire) | 10 |
| Chimney or flue (solid fuel burner) | 20 |
| Chimney or flue (other heater) | 35 |
| Chimney (open) | 80 |
| Chimney (blocked) | 20 |
| Other open flue (vertical duct) | 20 |
| Intermittent extract fan | 10 |
| Flueless gas fire | 40 |
NOTE
Refer to Standard Assessment Procedure (10.2) Table 2.1, for more information.