The floor area one head protects - and with it the flow and pressure that head needs.

Fewer heads means less pipework, fewer ceiling penetrations and faster installation.

But throwing water further requires more flow and more pressure per head.

So the number of operating heads may fall while the demand from each one rises.

Whether the total water supply requirement goes up or down depends on the building.

It is a calculation rather than an assumption, and the result differs between projects.

Not always - it saves installation cost and may increase water supply cost.

Fewer heads and less pipework is a genuine saving in materials and labour.

A higher per-head demand can push the water supply, pump or tank into a larger size.

In a building where the supply is already generous, the saving is straightforward.

Where the supply is marginal, extended coverage can be the more expensive option.

The comparison has to be made on the whole system rather than on head count.

Very - a maximum coverage area, a minimum pressure, a hazard classification and stated ceiling conditions.

Using a head beyond any of those parameters is not a marginal deviation; the listing does not cover it.

Two heads both described as extended coverage may have quite different approved parameters.

The design must be built on the specific product's data rather than on the category name.

Ceiling slope, obstruction and construction type all appear in the listing conditions.

Substituting another manufacturer's extended coverage head is a design change, not a procurement one.

More demanding, because the water travels further and each head covers more.

A longer throw gives more opportunity for something to interrupt the pattern.

Wider spacing also means a shadowed head leaves a proportionally bigger gap in coverage.

Ceiling services, structural members and light fittings all need checking against the specific listing.

Later alterations are more consequential than with closely spaced standard heads.

In congested ceilings, standard coverage heads are often the more practical choice.

Heads approved by application-specific fire testing rather than by general design rules.

They cover situations standard listings do not address - particular storage arrangements, unusual occupancies or ceiling geometries.

Their approval is tied to the tested application, sometimes very narrowly.

That makes them powerful where they fit and unusable where they do not.

The manufacturer's technical data is the governing document rather than the general standard.

Design assistance from the manufacturer is normal practice for these products.

Within one design area, generally not - the hydraulic calculation assumes a consistent operating pattern.

Different areas of a building can certainly use different head types.

Mixing within an area produces an operating pattern nobody calculated.

Response type must also be consistent within a design area for the same reason.

Where areas adjoin, the boundary between them is a design decision.

Spares stock has to cover every type installed, which is another argument for consistency.

Extended coverage systems are more sensitive to it, because each head needs a minimum pressure to achieve its listed coverage.

A standard head at reduced pressure covers less area but still discharges usefully.

An extended coverage head below its listed minimum pressure does not achieve the coverage the design relies on.

Since the spacing assumes that coverage, the gap between heads becomes real.

Mains pressure changes, pump degradation and new demands on a shared supply all cause this.

Periodic flow testing of the supply is part of the maintenance regime for that reason.