The first fill of a fire protection pipe network is one of the most critical commissioning stages, directly affecting the long-term integrity, reliability and life safety performance of the system. This guide explains flushing, slow filling, air venting, priming, hydrostatic testing and the start-stop settings under NFPA 20 (2025) and NFPA 24 (2022) — including what must be done and what must never be done on site.
1. The Basic Philosophy of First Fill
The underlying principle is to prevent destructive water hammer through slow, controlled filling, to expel all air from the network, and to bring the system safely into service without ever running the pump dry. NFPA 20 governs how a fire pump and its accessories are installed, tested and commissioned; NFPA 24 governs the installation, flushing and acceptance of the infrastructure mains carrying water from the source to the pump and into the building systems.
2. Preparation and Flushing
Before the first fill, a full flushing test is mandatory to clear construction debris from underground and above-ground pipework. Foreign objects that enter pipes during construction — stones, gravel, timber blocks, plastic, work gloves and sacking — will, if not flushed out, permanently damage the pump impeller, disturb shaft alignment, or block sprinkler heads, alarm valves and check valves. NFPA 20 and NFPA 24 therefore require suction and supply mains to be flushed at high velocity before they are connected to the pump.
Flushing Flow Rates and Velocity Requirements
To dislodge and carry away debris, the velocity in the pipe must be at least 10 ft/s (3.0 m/s), or the flushing flow must be at least 150% of the rated pump capacity. Minimum flushing flows by pipe diameter:
| Nominal size (in) | Nominal size (mm) | Min. flow (gpm) | Min. flow (L/min) |
|---|---|---|---|
| 2" | 50 mm | 150 gpm | 570 L/min |
| 2½" | 65 mm | 229 gpm | 870 L/min |
| 3" | 75 mm | 330 gpm | 1,250 L/min |
| 4" | 100 mm | 590 gpm | 2,240 L/min |
| 5" | 125 mm | 920 gpm | 3,490 L/min |
| 6" | 150 mm | 1,360 gpm | 5,150 L/min |
| 8" | 200 mm | 2,350 gpm | 8,900 L/min |
| 10" | 250 mm | 3,670 gpm | 13,900 L/min |
| 12" | 300 mm | 5,290 gpm | 20,100 L/min |
3. What To Do: Filling and Pressurisation
Once cleanliness has been verified by flushing, the first fill and pressurisation are completed step by step:
1 Fill slowly via the mains or bypass
Filling is always slow and controlled. The preferred sources are the municipal water supply pressure, the pump bypass line, or external filling connections. Suction and discharge valves are opened gradually so the water spreads through the pipework in stages, preventing pressure shocks at pipe junctions.
2 Vent the air progressively
Air trapped in the pipework causes water hammer and component damage. During filling, all air cocks and manual vents are held open and are not closed until an uninterrupted flow of water appears. Centrifugal pumps require an approved automatic float-operated air release valve of at least 0.5 in (12.7 mm) on the casing; vertical shaft turbine pumps require an air release valve of at least 1.5 in (38 mm).
3 Prime the fire pump
Centrifugal fire pumps are not self-priming; the casing and suction line must be completely full of water before the pump can operate. Where the water level is above the pump (flooded suction) this happens automatically. Where the source is below the pump (suction lift), a separate priming tank and an approved foot valve are used. The pump is never run without water.
4 Check alignment and rotation
After filling, the coupling is disconnected and the driver rotation is verified against the arrow on the pump casing with a momentary start test. Shaft alignment between pump and driver is then checked with a dial indicator. Alignment is repeated after the baseplate is grouted, after the first 10 hours of running and after three months of operation, and the equipment is then dowelled to the foundation.
5 Hydrostatic leakage test
Once full, the pipework is tested for 2 hours at not less than 200 psi (13.8 bar), or 50 psi (3.4 bar) above working pressure, whichever is greater. There must be no pressure drop or visible leakage during that period. Permissible leakage on underground mains is calculated with the NFPA 24 formula.
4. What Must Never Be Done During First Fill
Even a small procedural or sequencing error during first fill and commissioning can render very expensive equipment unusable. The most critical prohibitions:
- Never run the main fire pump to fill the system: electric and diesel main pumps are designed for high flow and pressure. Pushing water into empty or air-filled pipework with the main pump produces violent water hammer that can tear off elbows and flanges. Main pumps are only placed on standby once the system is full and proven leak-tight.
- Never use the jockey pump to fill an empty system: jockey pumps are selected for a very low flow, sized only to make up acceptable leakage, and their output is usually less than the flow from a single sprinkler. Trying to fill an empty installation with the jockey runs it continuously for hours, overheats it and can burn out the motor. The jockey is only used to pressurise a system that is already full.
- Never run the pump dry: running without water burns the mechanical seals and wear rings through friction heat within seconds. Even for a rotation or motor check the pump is not turned without water; the rotation test is performed with the coupling removed so the driver turns free.
- Never fit a butterfly valve in the suction line: the disc of a butterfly valve sits in the flow path, creating turbulence and constriction that lead to cavitation and damage at the suction flange. Only a full-bore, outside-screw-and-yoke (OS&Y) gate valve with visible position indication is permitted in the suction line. A butterfly valve may only be located at least 15.3 m (50 ft) or 10 pipe diameters upstream of the suction flange.
- Never fit non-metallic flexible connectors in the discharge line: non-metallic flexible connections are not permitted on the discharge pipework; they cannot withstand the pressure and fire exposure and can rupture. Non-metallic flexible sections are permitted only in diesel engine cooling water lines, rated to twice the working pressure and certified for 30 minutes fire resistance to ISO 15540.
- Never fit an isolation valve in the pressure sensing line: fitting a shut-off valve in the controller sensing line is strictly prohibited. If it were closed inadvertently the controller could not sense the pressure drop and the pump would fail to start in a real fire. Sensing lines are connected directly and continuously to the controller sensors.
5. Start-Stop Pressure Settings After Filling
Once the system is completely full, vented and proven leak-tight, the controller start-stop settings are made per the NFPA 20 guidance:
- Jockey pump stop = pump churn pressure + minimum static suction pressure of the water supply.
- Jockey pump start = jockey stop − at least 10 psi (0.68 bar).
- Main fire pump start = jockey start − at least 10 psi (0.68 bar) — the NFPA 20 2025 update (it was 5 psi in the 2022 edition). With multiple pumps, each additional pump steps down a further 10 psi so they start in sequence.
Worked Example
| Parameter | Derivation | Value |
|---|---|---|
| Fire pump rating | — | 1000 gpm at 100 psi (churn 115 psi) |
| Water supply (municipal main) | — | Min. static 50 psi / max. static 60 psi |
| Jockey pump stop | Churn + min. static | 115 + 50 = 165 psi (11.37 bar) |
| Jockey pump start | Jockey stop − 10 psi | 165 − 10 = 155 psi (10.68 bar) |
| Main pump start (2025) | Jockey start − 10 psi | 155 − 10 = 145 psi (10.00 bar) |
| Main pump start (2022) | Jockey start − 5 psi | 155 − 5 = 150 psi (10.34 bar) |
| System maximum pressure | Churn + max. static | 115 + 60 = 175 psi (12.07 bar) |
6. Acceptance Testing and Certification
Once filling, flushing and leakage testing are complete, field acceptance tests are witnessed by the authority having jurisdiction, the owner and the contractor. Results are recorded and signed on the formal certificates, which also form the baseline for the periodic inspection regime under NFPA 25:
- Contractor's material and test certificate (NFPA 24): the legal record of underground main installation details, joint types, flushing flow and duration, hydrostatic pressure and leakage measurements. The pipe network cannot be accepted until it is signed.
- Centrifugal fire pump acceptance test form (NFPA 20): records the net pressures produced at churn, 100% of rated flow and 150% peak load, the current and voltage drawn, and the simulated test of every controller alarm, all compared against the manufacturer's curve.
Frequently Asked Questions
Why must a fire system be flushed before the first fill?
Foreign objects that enter the pipework during construction - stones, gravel, timber, plastic and work gloves - will damage the pump impeller, disturb shaft alignment or block sprinkler heads, alarm valves and check valves if they are not removed. NFPA 20 and NFPA 24 therefore require suction and supply mains to be flushed at high velocity before they are connected to the pump.
What flushing flow rate is required for fire mains?
The velocity in the pipe must be at least 10 ft/s (3.0 m/s), or the flushing flow must be at least 150% of the rated pump capacity, in order to dislodge debris. For example a 6 inch (150 mm) main requires a minimum flushing flow of 1,360 gpm (5,150 L/min), and an 8 inch main 2,350 gpm (8,900 L/min).
Can an empty sprinkler system be filled with the jockey pump?
No. The jockey pump is sized only to make up acceptable leakage, and its flow is usually less than that of a single sprinkler. Trying to fill an empty system with it runs the pump continuously for hours, overheats it and can burn out the motor. The jockey is only used to pressurise a system that is already full.
Can the main fire pump be run to fill the system?
No, this is strictly prohibited. Pushing water into empty or air-filled pipework with the main pump produces violent water hammer that can tear off elbows, flanges, fire valves and sprinkler branch lines. Filling is always done slowly from the town main, the bypass line or an external filling connection, and the main pump is only placed on standby once the system is full and tested.
Why does a centrifugal fire pump need priming?
Centrifugal pumps are not self-priming; the casing and suction line must be completely full of water before the pump can operate. Where the water level sits above the pump this happens automatically under flooded suction. Where the source is below the pump, a separate priming tank and an approved foot valve are required, and the pump is never run dry.
At what pressure and for how long is the hydrostatic test carried out?
The system is tested for 2 hours at not less than 200 psi (13.8 bar), or 50 psi (3.4 bar) above working pressure, whichever is greater. There must be no pressure drop or visible leakage during that period. Permissible leakage on underground mains is calculated using the formula given in NFPA 24.
Can a butterfly valve be used in the fire pump suction line?
No. The disc of a butterfly valve sits in the flow path and creates turbulence and constriction that lead to cavitation. NFPA 20 requires a full-bore outside-screw-and-yoke (OS&Y) gate valve with visible position indication in the suction line. A butterfly valve may only be located at least 15.3 m (50 ft) or 10 pipe diameters upstream of the suction flange.
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Download MEP Calc on the App StoreNFPA 20, Standard for the Installation of Stationary Pumps for Fire Protection, 2025 Edition. NFPA 24, Standard for the Installation of Private Fire Service Mains and Their Appurtenances, 2022 Edition. Stationary Fire Pumps and Standpipe Systems Handbook, NFPA. NFPA 25, Standard for the Inspection, Testing and Maintenance of Water-Based Fire Protection Systems.