Using Method 2 – System 1
Determine the total equivalent ventilator area ¬ see tables below for a dwelling with any design air permeability. The lower table may be followed for a dwelling designed to an air permeability greater than 5 m3/(h.m2) at 50 Pa which recommends less ventilation provisions.
Follow (a) or (b) as appropriate depending on the number of storeys –
a) for multi¬storey dwellings, and single storey dwellings more than four storeys above ground level – use the total equivalent ventilator area from Step 1; or
b) for single storey dwellings up to and including the fourth storey above ground level – add a further 10000 mm2 to the total equivalent ventilator area from Step 1, preferably shared between several rooms.
System 2 – Passive stack ventilation (PSV)
Step 1 – Determine the total equivalent ventilator area from the tables above (¬ see Table 2.3 for a dwelling with any design air permeability). Alternatively, the guidance in Table 2.4 may be followed for a dwelling with a designed air permeability greater than 5 m3/(h.m2) at 50 Pa which recommends less ventilation provisions,
Step 2 – Make an allowance for the total air flow through all PSV units. As an approximation assume each PSV unit provides an equivalent area of 3000 mm2.
The equivalent ventilator area required for the dwelling is the value found in Step 1 less that calculated from Step 2.
Background vent locations for PSV
Background ventilators should be located in all rooms with external walls except the rooms where a PSV is located, There should be background ventilation of at least 5000 mm2 equivalent area in each habitable room and 2500 mm2 equivalent area in each wet room not containing a PSV.
Background ventilators should be located to avoid draughts, typically 1.7 m above floor level.
System 3 – Continuous mechanical extract (MEV)
For any design air permeability, controllable background ventilators, having a minimum equivalent area of 2500 mm2, should be fitted in each room other than in rooms from which air is extracted.
As an alternative, where the designed air permeability is greater than 5 m3/(h.m2) at 50 Pa background ventilators are not necessary.
Where background ventilators are fitted they should be:
a) located to avoid draughts,
b) Typically, 1.7 m above floor level
c) A minimum of 0.5 m away from fans fitted in the same room
d) Manually adjustable or automatically controlled.
System 4 – Continuous mechanical supply and extract with heat recovery (MVHR)
No background or trickle ventilators required