The clapper presents a much larger area to the air than to the water, so a modest air pressure balances a far higher water pressure.

That area differential is the principle the whole valve is built on.

It means the system can be held closed with air at a fraction of the supply pressure.

When a head opens, air escapes and the differential collapses.

The clapper then trips and water enters the pipework.

Required air pressure is stated by the valve manufacturer and depends on the supply pressure.

Because water has to travel the whole length of empty pipework after the valve trips.

In a wet system water is already at the head; in a dry system it starts at the riser.

Standards limit the permitted time from trip to water delivery.

That limit constrains how large a single dry system may be, which is why big buildings use several.

The delay also means a dry system delivers less water onto the fire in its early moments.

Designs compensate by treating dry systems as slightly less effective than equivalent wet ones.

It trips the valve sooner than escaping air alone would, by admitting air beneath the clapper when it senses the pressure drop.

Air escaping through a single sprinkler takes time to reduce system pressure.

An accelerator detects the drop and destroys the differential deliberately.

An exhauster achieves a similar result by dumping system air rapidly.

Either device shortens the time to water delivery, which is what the standards limit.

They are required on larger systems and are one of the ways a bigger dry system stays compliant.

Enough to make up normal leakage without running continuously - and a compressor that runs constantly is reporting a leak.

Air leaks are normal in a large dry system and are made up automatically.

Air maintenance devices regulate supply pressure and limit how much fresh air enters.

A system losing pressure faster than it can be made up will eventually trip on its own.

False trips flood the pipework and require the whole system to be drained and reset.

Compressor run time is therefore a useful diagnostic that is rarely looked at.

Because it freezes - which is the exact condition the dry system existed to avoid.

After a trip or a test, water remains at low points regardless of grading.

Pipework is graded to drain and low point drains are provided for that purpose.

Draining them is a scheduled task and one of the most commonly forgotten.

Frozen residual water splits pipework and can prevent the system operating at all.

Upright sprinklers are often specified in dry systems because they do not trap water in a drop.

Air means oxygen and moisture, and both attack the pipe internally where nobody can see it.

The combination of wet and dry surfaces inside the pipe accelerates the process.

It produces pinhole leaks and, worse, debris that can obstruct sprinkler heads.

Systems have failed prematurely for this reason across many installations.

Nitrogen generators supply the pipework with an inert gas instead and greatly slow it.

Air maintenance devices are the simpler partial control by limiting fresh air ingress.

By trip testing on a schedule set locally - and a full trip test is disruptive, which is why it is periodic rather than frequent.

A partial flow test exercises the alarm without tripping the valve.

A full trip test proves the valve, the accelerator and the water delivery time.

After a trip test the system must be drained, reset and re-pressurised.

Water delivery time is measured and compared against the design figure.

Frequencies and acceptance criteria are set by the standard in force where the building is.