Short answer
Diversity is an allowance for the fact that not every appliance runs at full rate at the same time. Applying it reduces the design flow and therefore pipe and meter sizes. It is appropriate for groups of intermittently used appliances such as catering equipment, but should not be applied to plant that genuinely runs together, such as boilers in a cascade on a cold morning.
Where diversity is reasonable
Commercial kitchens with many ranges, ovens and fryers; multi-occupancy buildings with individual appliances; mixed sites where heating and process loads peak at different times. The diversity should be based on knowledge of how the site actually operates.
Where it is not
Boiler plant sized for peak heat load, process loads that run continuously, and any appliance whose safety depends on full pressure at full rate. Boilers sized to meet design-day heat loss will all fire together on the coldest days — exactly when gas pressure is under most strain.
Kitchen example
A kitchen has appliances totalling 250 kW gross. Experience of how this kitchen operates suggests peak simultaneous use around 70–80%. Designing for about 190 kW gross (roughly 18 m³/h) rather than 250 kW (about 23.5 m³/h) can save a pipe size — but the assumption should be agreed with the client and recorded, because a future menu change can invalidate it.
Figures are typical for UK natural gas. The current edition of the relevant IGEM standard and the manufacturer's instructions always take precedence on site.
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