It is the single assembly through which the whole system is fed, controlled, tested and drained - and it is where the system's condition is visible.

The main control valve, alarm valve, gauges, and test and drain connections are all there.

Anyone assessing an unfamiliar installation starts at the riser for that reason.

Riser assemblies are frequently supplied pre-fabricated with the trim already fitted, which reduces installation time and error.

Access to the riser has to be maintained - a riser behind stored goods is a common and serious finding.

Identification signage at the riser is normally required so that responders can find and operate it.

A wet system holds water in the pipework; a dry system holds compressed air, because standing water would freeze.

Wet systems are simpler, faster to operate and used wherever the space is heated.

Dry systems keep water back at a differential valve held shut by air pressure in the pipework.

When a head opens, air escapes, the differential is lost and the valve trips.

That introduces a delay between the head operating and water arriving, which the design has to account for.

Unheated warehouses, loading docks, car parks and external canopies are the usual applications.

It is held closed until a detection system releases it, so the decision comes from electronics rather than from heat at a head.

A deluge system uses open heads and floods the whole protected area when the valve operates.

A preaction system uses closed heads and requires both the detection system to release the valve and a head to open before water discharges.

That double requirement is why preaction is used where accidental discharge would be very costly - data halls, archives and cold stores.

Deluge is used where fire spread would be extremely rapid, such as around flammable liquids.

Both depend on a detection system that is itself maintained and tested.

Because a closed control valve is the most serious defect a system can have and it is invisible without one.

A shut valve leaves an installation that looks entirely normal and will do nothing.

Valves get closed for maintenance and not reopened, or are closed deliberately.

A tamper switch signals valve position to the alarm panel so the condition is known.

Most regimes require supervision of valve position rather than merely recommending it.

Valve position is also a standard item on every routine inspection, precisely because the consequence is total.

They are the system's only way of telling anyone that something has happened.

A waterflow switch detects water moving in the pipework and signals an alarm - it is how a sprinkler operation becomes a fire alarm.

Retard features prevent brief pressure surges from causing false alarms.

Pressure switches monitor air or water pressure and report loss, which on a dry system is an early warning of a leak or a trip.

Without these devices a sprinkler can operate and discharge unnoticed until the water is seen.

They are part of the alarm system as well as the sprinkler system, so their testing usually appears in both regimes.

Because oxygen trapped in dry and preaction pipework corrodes it from the inside, and that has shortened the life of many systems.

Compressed air contains oxygen and moisture, both of which attack steel pipe internally.

The corrosion produces pinhole leaks and, worse, debris that can obstruct heads.

Nitrogen generators supply the pipework with nitrogen instead, which is largely inert and greatly slows the process.

Air maintenance devices are the simpler control, regulating supply pressure and limiting how much fresh air enters.

This is a retrofit as often as a new-build measure, since the problem is usually discovered in an existing system.

It is, but permitted solutions and concentrations are tightly controlled and differ by market.

Antifreeze loops were traditionally used to protect small unheated areas without a full dry system.

Certain solutions were later found capable of igniting under some conditions, which led to significant restrictions.

Permitted formulations, maximum concentrations and listing requirements are now specific and vary between jurisdictions.

Existing systems in some regimes have been required to be tested, diluted or replaced.

Because the rules have changed and continue to differ, this is a question to settle against the current local standard rather than against previous practice.