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 below the suction connection at the tank floor cannot be used. The effective volume is less than the total, and the calculation must allow for it.
- 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.
- No level monitoring. The tank is assumed full but never verified. A level alarm must be wired to the panel.
2. Infill rate
In EN 12845 and similar approaches, a smaller tank may be used where a reliable and adequate infill supply exists. That 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?
- Does the infill (float) valve open fully, or is it obstructed?
- 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. 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 the wrong way (an air pocket).
- No anti-vortex plate — the pump draws air.
- An undersized suction line, with high velocity and excessive loss.
- NPSHa falling below NPSHr, causing cavitation.
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 suction connection level and the overflow level |
| 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 flow test 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 connection cannot be used. The effective volume is that between the suction level and the overflow level, and the calculation must allow for it.
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.
SprinkCalc — Fire Sprinkler Design Across Three Standards
SprinkCalc covers hazard classification, design density and area, K-factor selection, water demand and hydraulic calculations for NFPA 13, FM Global and BS EN 12845 in a single iOS app, and exports a professional PDF report.
Download SprinkCalc on the App Store
MEP Calc — 86+ Engineering Calculators
MEP Calc bundles 86+ engineering modules in one iOS app: 21 fire calculations plus heating, cooling, HVAC, plumbing, steam and natural gas.
Download MEP Calc on the App StoreNFPA 13 (2025) · NFPA 20 (2025) · NFPA 25 · NFPA 241 · BS EN 12845:2015+A1:2019 · 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.