Toilet Ventilation Example



Design a ventilation system for the toilets in an Auditoria as shown in the accompanying drawings.


The Male Toilet and Female Toilet should each have separate extract ventilation systems to avoid ‘cross-talk’ and spread of smoke within the ductwork system.


The mechanical ventilation rate from Table 3.1 CIBSE Guide B2 (2001) is at least 3 air changes per hour.


For the purposes of this exercise use a value of 8 AC/h.







The volume of the Male Toilet is:                  4.5 x 3.0 x 2.7 m = 36.45 m3


The ventilation rate in m3/h  is:  


Ventilation Rate (m3/h)           =       Room volume (m3)  x  air change rate (ac/h)


Ventilation Rate (m3/h )          =       36.45 m3      x        8 AC/h       


=       291.6  m3/h.                       Divide by 3600


The ventilation rate is      =   0.081 m3/s.


A simple method of duct sizing would be:        Duct area   =  volume flow rate   /   air velocity.


An appropriate maximum air velocity for a toilet is 4 m/s.        (See Ventilation -  Ventilation Design section)


Therefore duct area         =       0.081   /  4   =       0.02025 m2.


If a square duct is used then the duct size is:           ( 0.02025 )0.5             =    0.142 m x 0.142 m


The nearest standard size is 0.150 m x 0.150 m               or, 150mm x 150mm.

The drawing below shows the ductwork layout for the Toilets.

The flexible ductwork may be sized using the same technique as above.

Volume flow rate through each flexible          =       0.081 m3/s  /  2      =       0.041 m3/s.


A simple method of duct sizing would be:    Duct area  =  volume flow rate   /   air velocity.

An appropriate air velocity for a flexible in this situation is 4 m/s


Therefore duct area         =       0.041   /  4   =       0.01025 m2.

Cross sectional area        =       p . d2 / 4                d        =       ( 4 . CSA  /  p )0.5

Therefore flexible diameter        =       ( 4  x  0.01025   /   p )0.5              =       0.114 m       =       114 mm diameter.


The nearest standard size of PVC flexible is  125mm.

The flexible size should be smaller than the duct to which it is attached to enable a boot or branch to be spot welded into position, as shown below.



















The Female Toilet may be sized using the same method but since the physical dimensions are so close to the Male Toilet then the same duct sizes can be used, this also standardizes the complete ventilation system so that the two fans are also identical.



To choose suitable fans for this example the duct pressure drop and fittings pressure drop would be calculated.

If the total pressure drop is assumed to be 200 Pa and the flow rate is 0.081 m3/s (81 litres/s) then a suitable catalogue can be used to pick the fan.

See  Go to Other Fans section.

A tube fan or boxed fan can be used for this project.