The Noise Insulation Regulations 1975

Specification for Ventilator System

6.  The ventilator system shall consist of a sound attenuating ventilator unit and an air supply duct protected by a cowl (or grille). The air supply duct and cowl (or grille) may be of separate construction from the ventilator unit or integral with it, but the following conditions shall be complied with—

(a) the air supply duct and cowl (or grille) shall be designed to allow the passage of air between the ventilator unit and the external air at all times and shall be so constructed that when installed in a cavity wall exposed to the weather—

(i) the weather resistance of the cavity is retained, and

(ii) the external cowl (or grille) provides protection against the passage of snow, rain and vermin;

(b) the ventilator unit shall consist of a controlled variable-speed inlet fan with sound attenuating duct and cover, shall be fitted with an easily removable and washable air filter on the inlet side of the sound attenuating duct to afford adequate protection for the acoustic lining, and shall be capable of supplying fresh air to the room directly from outside by means of the supply duct and cowl (or grille);

(c) the ventilator unit shall be securely fixed to the wall in such a position that the air filter can be easily removed and the junction between the ventilator unit and the face of the wall shall be fully sealed by means of compressible strip or other equally effective means;

(d) the air supply duct and cowl (or grille) shall be securely fixed to the wall and the junction between the air supply duct and the internal leaf shall be fully sealed by suitable means;

(e) the ventilator unit shall be electrically safe in operation and maintenance and shall not present a fire hazard;

(f) the ventilator unit (with air filter in position), by itself or with an integral air supply duct and cowl (or grille), shall be capable of giving variable ventilation rates ranging from—

(i) an upper rate of not less than 37 litres per second against a back pressure of 10 newtons per square metre and not less than 31 litres per second against a back pressure of 30 newtons per square metre, to

(ii) a lower rate of between 10 and 17 litres per second against zero back pressure;

and if there is no continuous control of the ventilation rate, the following intermediate settings shall be provided—

• (i) a ventilation rate of more than 31 and less than 33 litres per second against a back pressure of 10 newtons per square metre; and

• (ii) a ventilation rate of more than 21 and less than 26 litres per second against a back pressure of 10 newtons per square metre;

(g) the effective area of the air path through the ventilator system, with the fan switched off and the air filter in position, shall be not less than 3,250 square millimetres and shall be ascertained by measuring the static pressure difference across the ventilator system for various air flow rates through the ventilator system and calculating the effective area from 1270

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(where Q is the measured air flow rate through the ventilator system in litres per second and δp the measured static pressure difference across the ventilator system in newtons per square metre): effective areas shall be calculated for air flow through the ventilator system in both directions and the lower calculated value shall be taken for the effective area;

(h) the ventilator unit, by itself or integral with the air supply duct and cowl (or grille), shall be so constructed that—

(i) when it is in operation in any room the sound level in the room due to the operation of the unit at a ventilation rate of 31 litres per second against a back pressure of 10 newtons per square metre, measured at any point not nearer than 1 metre to the unit or to any of the room surfaces and normalised by the subtraction of

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(where A is the measured sound absorption in the room in square metre units at each ⅓ octave frequency interval from 100 to 3150 Hz), does not exceed 35 decibels on the A scale, and at the maximum ventilation rate of the unit against a back pressure of 30 newtons per square metre does not exceed 40 decibels on the A scale; and

(ii) the sound pressure level difference measured in accordance with British Standard specification for the measurement of sound transmission in buildings published on 31st May 1956 under the number BS2750: 1956 between two rooms separated by a solid wall (of area of not less than 10 square metres and of average mass, calculated over any portion of the wall measuring 1 square metre, of not less than 415 kilogrammes per square metre) incorporating the ventilator unit, normalised by the addition of

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(where A is the measured sound absorption in the receiving room in square metre units at each ⅓ octave frequency), is not less than the figure shown in Table 2 below except for total adverse deviations (at all ⅓ octave frequencies) not exceeding 32 decibels and an adverse deviation at any one ⅓ octave frequency not exceeding 8 decibels.

TABLE 2

⅓ octave frequency band centre (Hertz)	normalised sound pressure level difference (Decibels)
100	30
125	33
160	36
200	39
250	42
315	45
400	48
500	49
630	50
800	51
1000	52
1250	53
1600	53
2000	53
2500	53
3150	53