About this calculator
A home with mechanical ventilation — typically a heat-recovery unit (MVHR) — supplies fresh air to the living room and bedrooms and extracts stale, moist air from the kitchen, bathroom and WC. This calculator works out how much air each room needs, balances supply against extract, and gives the air change rate and the capacity of unit to look for.
Which rules. The Lithuanian requirements (STR 2.09.02:2005 Annex 1 and the hygiene norm HN 42:2009) could not be verified for this calculator, so it uses the default values for residential buildings in EN 16798-1 Annex B (in Lithuania LST EN 16798-1). Your ventilation designer may have to apply the Lithuanian figures instead; they can differ.
Formula
EN 16798-1 Annex B defaults (l/s):
Supply:
| Cat. | Per m² of dwelling | Per person | Per person, bedroom |
|---|---|---|---|
| I | 0.49 (≈ 0.7 1/h) | 10 | 10 |
| II | 0.42 (≈ 0.6 1/h) | 7 | 8 |
| III | 0.35 (≈ 0.5 1/h) | 4 | 4 |
Extract:
| Cat. | Kitchen | Bathroom | WC |
|---|---|---|---|
| I | 28 | 20 | 14 |
| II | 20 | 15 | 10 |
| III | 14 | 10 | 7 |
S = max(Σ people × q_person,
A_dwelling × q_m²) supply
E = Σ wet-room extract rates extract
Q = max(S, E) supply = extract
room supply = people's need
+ (Q − Σ need) × A_room / A_habitable
room extract = rate × Q / E
n = Q / (A_dwelling × h) air changes, 1/h
unit = Q × (1 + reserve)
The whole-dwelling and per-person rows were read in EN 16798-1 Table B.11 as reproduced by the EU JRC; the bedroom and extract rows were read in REHVA Journal's reproduction of the draft standard's Annex B. How the surplus air is shared between rooms is this calculator's own choice: every habitable room keeps at least its per-person air, the rest goes by floor area.
Worked example
A 74 m² two-bedroom flat, category II: living room 25 m² (2 people), bedrooms 14 m² (2) and 10 m² (1), kitchen, bathroom, WC and an 8 m² hall:
people: 2 × 7 + 2 × 8 + 1 × 8 = 38 l/s
area: 74 × 0.42 = 31.1 l/s → supply needed 38 l/s
extract: 20 + 15 + 10 = 45 l/s → design flow 45 l/s = 162 m³/h
living room 14 + 7 × 25/49 = 17.6 l/s = 63 m³/h
bedroom 1 16 + 7 × 14/49 = 18.0 l/s = 65 m³/h
bedroom 2 8 + 7 × 10/49 = 9.4 l/s = 34 m³/h
kitchen 72, bathroom 54, WC 36 m³/h
n = 162 / (74 × 2.6) = 0.84 1/h; unit with 20 % reserve: 194 m³/h
Here the extract from the wet rooms governs, so the bedrooms and living room get somewhat more than the people alone would need.
FAQ
Which category should I choose? Category II is EN 16798-1's normal level for new buildings. Category I is for higher expectations; III is a moderate level, intended mainly for existing buildings.
Why is supply equal to extract? A heat-recovery unit runs balanced: what it blows in it also draws out, and the heat exchanger needs both streams. The calculator therefore takes the larger of the two needs for both.
What capacity should the unit have? At least the design flow, plus a reserve — at full speed units are noisy and less efficient, and the ducts add pressure loss. The 20 % default is a planning margin you choose, not a figure from a standard; check the unit's fan curve at your duct system's pressure drop.
Is the kitchen hood included? No. A cooker hood is usually a separate extract run only while cooking; the kitchen figure here is the continuous ventilation extract.
Assumptions and limits
- EN 16798-1 Annex B default values, not the Lithuanian STR/HN figures, which could not be verified. Check them with your designer.
- The extract rates and the bedroom rate were verified only in a reproduction of the draft standard.
- Supply in habitable rooms, extract from kitchen, bathroom and WC; halls and corridors only count towards the floor area (transfer air).
- Room shares of surplus air are this calculator's own rule.
- No duct sizing, pressure loss, boost mode, cooker hood, or infiltration allowance is included.