The water supply is the most expensive component of a sprinkler system, and the one most often "made to work". A supply that looks adequate in the calculation can fall short on site for three different reasons: volume, flow or duration.
1. Tank volume
The design assumes a defined flow for a defined duration for a defined hazard class. The volume is the product of those three. Three deviations appear on site:
- Dead volume. Water at the tank floor below the suction inlet, and the depth that must be kept above the inlet to prevent vortexing, cannot be used. The effective volume is less than the total; EN 12845 9.3.2.1 calculates it as the volume between the normal water level and the low water level (Table 12 gives the distance from the suction pipe to the low water level).
- Shared use. Where a tank serves both fire and process or irrigation, the volume reserved for fire must be physically protected — by a separate suction level or a separate compartment. For pre-calculated systems EN 12845 9.3.2.2 requires that volume to be reserved solely for the sprinkler system.
- No level monitoring. The tank is assumed full but never verified. EN 12845 9.3.2.1 requires an externally readable water level indicator on tanks other than open reservoirs; wiring a level alarm to the panel is good practice as well.
2. Infill rate
EN 12845 9.3.4 permits a reduced capacity tank on these conditions: the inflow comes from a town main and is automatic, via at least two mechanical float valves, and the failure of one float valve does not impair the required infill rate; the effective capacity is not below Table 11; the inflow can be checked and the arrangement is accessible for inspection. That solution rests on the assumption that the infill line actually delivers that flow. What to check on site:
- Has the infill flow been measured, or only assumed?
- Does mains pressure vary through the day and by season?
- Do the float valves (at least two under EN 12845) open fully, or are they obstructed; are they checked annually (20.3.4.4)?
- Is the infill line shared with other consumers?
- Does infill still work during a power failure?
The infill assumption is the most fragile one. Where tank volume has been reduced in reliance on the infill rate, every interruption to that line directly shortens the protection duration. The line must be monitored as part of the fire system.
3. Direct connection to mains pressure
Where the system is fed directly from the town main, the design rests on the pressure and flow measured on the day. Over time:
- Development in the area grows and pressure falls at peak hours.
- Mains maintenance or a network change alters the direction of supply.
- The measurement was taken at the wrong time of day (at night, at low demand).
A mains flow test should therefore be done at the least favourable time and repeated periodically. For pumps drawing directly from a town main, EN 12845 10.7.4 likewise requires the test to be carried out at the time of maximum demand on the main. A design resting on a single measurement can be invalid within a few years.
4. Suction side problems
The tank can be full and the pump sound, and water can still fail to reach the pump:
- A blocked suction strainer.
- A partly closed suction valve.
- A concentric reducer used instead of an eccentric one, or an eccentric reducer fitted upside down (an air pocket); EN 12845 10.6.2.1 requires the taper pipe at the pump suction to have a horizontal top side.
- No anti-vortex plate, or a clearance between the suction inlet and the low water level smaller than EN 12845 Table 12 requires — the pump draws air.
- An undersized suction line, with high velocity and excessive loss.
- Available NPSH (NPSHa) not exceeding the required NPSH (NPSHr) by an adequate margin, causing cavitation. EN 12845 10.6.2.1 requires a margin of at least 1 m at maximum pump flow.
5. Errors in the duration calculation
Three common mistakes:
- Hose reel and hydrant flow not added. Where they operate at the same time as the sprinklers, the duration must be calculated on the total flow.
- A hazard class change not reflected in the duration. The facility has moved to storage but the tank volume still reflects the old class.
- Deluge or water curtain flow forgotten. These operate on every nozzle and grow the total demand quickly.
How to verify
| Check | Method |
|---|---|
| Effective tank volume | Volume between the normal water level and the low water level (EN 12845 9.3.5) |
| Infill rate | Real flow derived from a measured fill time |
| Mains performance | Flow test at the least favourable hour (static, residual, flow) |
| Pump performance | Annual full-load flow test (EN 12845 20.3.4.2) and comparison of the curve with the design |
| Suction health | Suction gauge reading and strainer inspection |
| Total demand | Sum of sprinkler, hose reel, hydrant and special system flows |
If a shortfall is found
The order matters: verify the demand first (is the hazard class still correct?), then measure the supply, then look for a solution. The usual solution order is: eliminate losses (blockage, partly closed valve) → improve the infill line → increase tank volume → replace the pump. Starting with the most expensive option is usually unnecessary.
Frequently Asked Questions
Why is the effective tank volume less than the total?
Water below the suction inlet, and the depth that must be kept above the inlet to prevent vortexing, cannot be used. Under EN 12845 9.3.2.1 and 9.3.5 the effective capacity is the volume between the normal water level and the low water level; the distance from the suction pipe to the low water level is given in Table 12.
Why is relying on the infill rate risky?
Where tank volume was reduced in reliance on the infill rate, any interruption to that line directly shortens the protection duration. The flow must be measured and the line monitored.
When should a mains flow test be done?
At the least favourable hour — when demand is highest — and repeated periodically. A single measurement taken at night is misleading.
What is the first step if the supply falls short?
Verify the demand first (is the hazard class still correct?), then measure the supply. The solution order is: eliminate losses, improve the infill line, increase tank volume, and only then replace the pump.
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Download MEP Calc on the App StoreNFPA 13 (2025) · NFPA 20 (2025) · NFPA 25 · NFPA 241 · EN 12845:2015+A2:2026 · FM Global DS 10-3, DS 2-0. The findings here are typical defect patterns defined in the standards and survey guidance, not an account of events at any particular site.