A worked free-area calculation for a boiler in a cupboard, showing both the high and the low vent, with rates to verify.
When an appliance sits in a cupboard or compartment, it needs ventilation both to supply combustion air and to carry away the heat that builds up in the enclosed space. This worked example shows the shape of the calculation; treat the rates as examples and confirm them in the standard.
Why two vents. A compartment normally has a low vent and a high vent. The low vent admits cooler air for combustion and cooling, and the high vent lets the warmed air escape, so the appliance does not overheat in its box.
Method. A common teaching approach sizes each vent as a fixed effective free area for every kW of appliance input, with one rate when the vents communicate with a room and a smaller rate when they go directly outside. For this example use 10 square centimetres (cm2) per kW to a room.
Step 1 - take the input. Use a boiler of 24 kW input for this example.
Step 2 - size each vent. At 10 cm2 per kW: 24 kW x 10 cm2 = 240 cm2 of effective free area for the low vent, and the same 240 cm2 for the high vent.
Step 3 - remember the difference for vents to outside. If the vents discharge directly outside rather than into a room, a smaller rate is used, so the areas would be smaller; the method is the same but the rate changes.
Step 4 - convert free area to a vent. As always, the grille has to deliver that effective free area after its bars and mesh are accounted for, so choose the vent from the free area stated by the maker, not its overall size. Confirm the correct rates, positions and whether the compartment vents to a room or to outside against the current standard.