Warm Air Central Heating

Grilles & registers » Diffusers Sizing Guidance

It is recommended that when ordering Registers, Grilles or Diffusers that you check with our sales department to ensure you have understood this guidance.

'Nominal sizes' are the dimensions of the duct opening into which a register, grille or diffuser will fit.

a) Diffuser sizes are based on the duct that they will cover; therefore the best method of sizing is to remove the Diffuser and to measure the duct. Measurements are normally taken in inches: length (longest) A x width B as diagram below. (Note: standard diffusers are usually 10,12 or 14 inches in length and 2 or 4 inches wide).

b) Should removal of the Diffuser not be possible at the time of measuring, measure the opening / grille area of the Diffuser. Measurements are again taken length (longest) A x width B as diagram below, then add approximately ¾in to the length and ¼in to the width to find the nominal size of the replacement Diffuser. (Note: standard diffusers are usually 10,12 or 14 inches in length and 2 or 4 inches wide).