Fire pumps: electric, diesel, and what the jockey pump is for

If your building has a sprinkler system or a wet riser and it is not fed straight off a town main with enough pressure, there is a fire pump set somewhere: normally in a pump house or a plant room, normally red, and normally the single most expensive thing in the fire protection installation.

It is also the part most often signed off on a test that does not prove anything.

What a pump set is made of

Walk into a typical pump house and you will find four things.

The main pump. A horizontal split case or end suction pump, driven by either an electric motor or a diesel engine, sized to deliver a specific flow at a specific pressure. That pair of numbers is its duty point, and it comes from the hazard class of the building.

The jockey pump. A much smaller vertical multistage pump beside it. Its only job is to hold the system at its standing pressure against the minor losses every installation has: a weeping gland, a slightly passing valve, temperature change in the pipework.

The controller. A cabinet that watches system pressure and starts the pump when it falls past a set point. On a diesel set it also manages two battery banks and a start sequence.

The suction arrangement. Tank, suction valve, strainer, checked monthly under the regime in sprinkler servicing intervals, and on a stored-water system the tank level and its refill.

Why the jockey pump exists

This is worth understanding because it explains a fault most buildings have and nobody reports.

Without a jockey, every small pressure drop starts the main pump. A large pump starting and stopping repeatedly wears its bearings and seals, uses a great deal of power on an electric set, and, worse, makes a real start indistinguishable from routine noise. Nobody investigates a pump that runs all the time.

So the jockey handles the small stuff and the main pump is reserved for real demand.

Which gives you a free diagnostic: if the jockey pump is cutting in every few minutes, the system is losing water somewhere. The jockey is not the problem, it is the thing politely covering up the problem. That is a leak worth finding before it finds you.

Electric against diesel

Neither is better. They fail differently, and the choice is made by the fire strategy and by the insurer.

Electric sets are simpler, quieter, cheaper to install and much cheaper to maintain. They have one dependency, and it is total: if the power goes, so does the pump. Where they are used, the supply is normally required to be taken ahead of the main building isolator so the fire pump does not lose power when someone hits the emergency stop.

Diesel sets carry their own energy. A mains failure, a substation fire, a contractor cutting a supply cable, none of it touches them. The price is a much heavier maintenance regime: fuel condition and quantity, two independent battery banks, a cooling system, an exhaust route, and weekly running to keep it all honest. Diesel that has sat in a tank for three years is not the same fluid that went in.

A great many buildings have both, with a diesel set as the standby duty.

The test that matters, and the test most people get

Here is the part that decides whether the pump is real.

A weekly start test proves the controller senses pressure and the pump starts. It takes two minutes and it is genuinely worth doing. It proves almost nothing about capability.

An annual flow test proves the pump against its performance curve: water is put through it at its rated flow and the pressure it develops is measured and compared with what it is supposed to produce. That is the test that finds a worn impeller, a partly closed suction valve, a blocked strainer, a failed non-return valve or a tank that cannot supply the flow.

We have found pumps that started perfectly every week for six years and could not make sixty per cent of their duty. Every one of them had a shelf of signed weekly logs.

If your annual paperwork does not include measured flow and pressure figures, you have a start test with a bigger invoice. What a real one includes is set out under fire pump testing, and if you want it done properly that is fire pump servicing.

What goes wrong, in order of how often we find it

1. Suction valve not fully open. Closed for work, reopened “mostly”. Instantly caps what the pump can do.
2. Strainer blocked. Especially on stored-water systems and especially after tank work.
3. Batteries down on a diesel set. Two banks exist precisely because one will fail, and both are often flat.
4. Fuel degraded or low. Diesel has a shelf life and tank gauges stick.
5. Jockey masking a leak, running constantly, and nobody logging it.
6. Worn impeller, which nothing except a flow test will reveal.
7. Controller in manual after a maintenance visit, so the automatic start no longer exists.

Number seven is the one that keeps us awake. A pump left in manual looks completely normal, passes a visual inspection, and will not start when it matters.

The short version

• The jockey pump holds pressure; the main pump answers real demand. A busy jockey means a leak.
• Electric is simple and depends on power. Diesel is independent and needs looking after.
• Starting is not working. Insist on measured flow and pressure once a year.
• Check the controller is in automatic every time anybody has been near the pump house.