Dwelling ventilation,
room by room.
Enter the rooms and get what the Spanish building code's DB-HS3 requires: each room's airflow, where air comes in and goes out, the area of every opening and the size of the ducts. With the table behind every figure.
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A three-bedroom flat.
Three bedrooms, a living-dining room, a kitchen and two bathrooms. Each band is as thick as its airflow: air comes in through the dry rooms, crosses the home through the transfer openings and leaves through the wet rooms.
Home airflow
33.0
l/s · 119 m³/h
Cooker hood
50
l/s · 2.1.4
Mechanical ventilation
83
cm² · 4.1
Hybrid ventilation
1 × 400
cm² · T-2 · 4.2
Table 2.1 adds up to 26 l/s of intake in the dry rooms, but in a home with 3 or more bedrooms the wet rooms must extract at least 33 l/s. What goes out has to come in, so the intakes rise in proportion and the home ventilates 33.0 l/s (119 m³/h).
Mechanical ventilationEach section needs 2.5·q cm² (formula 4.1): 83 cm² for the 33.0 l/s of the main duct. The cooker hood has its own: 125 cm² for 50 l/s.
Hybrid ventilationIn Madrid, at 657 m, the thermal zone is X (table 4.4); with 5 storeys discharging into the duct, the draught class is T-2 (table 4.3) and the main duct needs 1 × 400 cm² (table 4.2), each branch at least half of that.
From airflow to duct size.
Each room's airflow
Main bedroom, other bedrooms, living rooms, kitchens, bathrooms and toilets, depending on the number of bedrooms. Storage rooms, garages and waste stores through table 2.2.
What comes in goes out
Air comes in through the dry rooms and leaves through the wet ones. Extraction never falls below the total minimum or the per-room minimum; if it exceeds the intake, the intakes rise in proportion. You can also fix one room's extraction by hand.
Every opening
Effective area of intake and extract openings (4·q) and of transfer openings (70 cm² or 8·q, whichever is larger), room by room.
The ducts
Mechanical ones by formula 4.1, or 4.2 if they run over the roof. Hybrid ones by table 4.2, with the draught class of table 4.3 and the thermal zone of table 4.4, from the province and altitude.
The cooker hood, separately
The cooking zone's extraction runs through a duct independent of the general ventilation, with its own size.
The windows
The minimum openable area of each room: one twentieth of its floor area.
Table 2.1 of DB-HS3.
Minimum airflows for constant-airflow ventilation in habitable rooms, in l/s.
| Dwelling | Main bedroom | Other bedrooms | Living and dining rooms | Wet rooms: total minimum | Wet rooms: per room |
|---|---|---|---|---|---|
| 0 or 1 bedrooms | 8 | — | 6 | 12 | 6 |
| 2 bedrooms | 8 | 4 | 8 | 24 | 7 |
| 3 or more bedrooms | 8 | 4 | 10 | 33 | 8 |
DB-HS3 “Indoor air quality” of the Spanish building code (CTE), current text (Order FOM/588/2017 and Royal Decree 732/2019).
What it does NOT do.
The limitations, before they surprise you. And why.
Constant airflow, not variable
It applies the airflows of table 2.1, which the code accepts as meeting its CO₂ limits. Variable-airflow ventilation, with sensors and the appendix C calculation, is not covered.
One dwelling, not the building
It sizes the ducts of one dwelling. A main duct shared by several floors, garages with carbon monoxide detection and networks with several fans need a whole-building calculation.
It is not the services design
It is a supporting calculation with the table behind every figure. Pressure losses, fan selection and signing the design are the responsible professional's job.
Frequently asked questions.
Table 2.1 of DB-HS3 sets it by room and by number of bedrooms: 8 l/s in the main bedroom, 4 l/s in each other bedroom, and 6, 8 or 10 l/s in living rooms for homes with 0–1, 2, or 3 or more bedrooms. Wet rooms must extract at least 12, 24 or 33 l/s in total, and 6, 7 or 8 l/s per room.
Because air has to flow from dry rooms to wet rooms (3.1.1 a): what comes in through the bedrooms and living room is what leaves through the kitchen and bathrooms. The extract is the largest of the sum of intakes, the total minimum for wet rooms and the sum of per-room minimums; if it exceeds the intakes, these rise in proportion.
Table 4.1: the effective area of an intake or extract opening is 4·q cm², and that of a transfer opening 70 cm² or 8·q, whichever is larger, with q in l/s. The gap under a door counts as a transfer opening.
With mechanical ventilation, 2.5·q cm² per section, or 1.5·q if the ducts run over the roof. With hybrid ventilation it comes from table 4.2, which depends on the draught class of table 4.3, set by the number of storeys and the thermal zone of table 4.4.
No: it applies the Spanish code's tables, and the thermal zones are Spanish provinces.
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Enter your rooms.
Get the ventilation.
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