712 and counterbalance 713 are supported in a fixed relationship to each other i e flap 712 and counterbalance 713 rotate as one unitary part about support 710 counterbalance 713 preferably has a suitable dimension and weight with respect to flap 712 such that at low values of pressure difference between the inlet 701 and outlet 702 the counterbalance 713 serves to hold flap 712 in the position shown in fig9 with the airflow duct 704 fully open a spring 714 is positioned in contact with the counterweight arm 713 but not touching a part of the housing 705 fig9 a as the air pressure increase the flap 712 rotates about support 710 and spring 714 makes contact with the part of the housing wall fig9 b the spring 714 provides a resilient force which serves to resist movement of the counterbalance 713 at even higher values of pressure difference flap 712 further rotates about support 710 causing spring 714 to compress fig9 c advantageously the properties of the spring 714 cause it to exhibit a non linear response the spring properties should also preferably be substantially constant over the operating temperature range e g a temperature range of u2212 20 u00b0 c to 40 u00b0 c for example the spring may be made of metal a ventilation according to fig9 a to c has been tested in accordance with the dutch test standard nen 1087 edition 05 1997 at varying pressure drops across the device x axis the flow rates vs pressure differences are shown in fig7 as can be seen the flow rate remains substantially constant over the range of pressures tested e g between 4 and 7 litres s over a pressure range of 2 to 25.