Ventsizer Sheet User Guide

The Ventisizer sheet is a basic estimation guide to work out the number of vents needed for a given risk volume and peak room pressure.

The AFP Ventsizer app can also be used but the sheet offers an instant guide.

1. Choose the gas to be used 2. Use the column for either 250 or 500 pascals peak pressure. If in doubt estimate based on 250pa 3. For 60 second inert gas discharges use row 5 to 13 and for 120 seconds 18 to 28 4. Look for the room volume and corresponding vent

Example 1

A room of 200m3 volume using IG-55 with a peak pressure of 250pa.

Following column B down to row 11 you can see that 1 x SHX UN 700 will cover a room volume of between 165 and 320m3.

Example 2

A room of 1200m3 using IG541 with a peak pressure of 500pa.

Following column N down to row 13 shows 1 x SHX UN 1000 will vent for a max volume of 1075m3 leaving 125m3 to still be vented. Follow column N down to row 9 shows 1 x SHX UN 500 will vent for between 95 and 265m3.

Therefore as a guide for this risk of a volume of 1200m3 1 x SHX UN 1000 and 1 x SHX UN 500 is required.


Use the same process for chemical gasses. Only the negative values have been used as they are the largest and all that is needed to estimate the vents required.

As with the Ventsizer app the figures produced are for estimation purposes only. Confirmation of the Free Vent Area required for any risk should be provided by the gas suppliers calculations.

Click here for Vent Sizer Chart: Here