A pressure tank that is too small can make an otherwise capable pump short-cycle, overheat and wear out early. A tank that is unnecessarily large takes up space and adds cost without always improving the system. Knowing how to size a pressure tank comes down to matching its usable water volume to the pump’s flow rate, pressure settings and expected demand.
For a domestic pressure system, farm supply, irrigation line or commercial installation, the tank is not simply a storage vessel. It gives the system a controlled amount of water between pump starts, helping maintain pressure while protecting the pump motor, switchgear and pipework.
What a pressure tank actually does
Most modern pressure systems use a diaphragm or bladder pressure tank. Compressed air sits on one side of the bladder and water enters the other. As water is drawn from the system, the compressed air pushes water out until pressure falls to the pump’s cut-in setting. The pump starts, refills the tank and stops again at its cut-out setting.
The useful amount of water supplied between those two pressure points is called drawdown. This is the number that matters when choosing a tank. A 100-litre tank does not provide 100 litres of drawdown. Depending on the pressure settings and air charge, it may only deliver a fraction of its stated tank volume.
A correctly sized tank reduces pump starts. That is particularly important for bore pumps, surface pressure pumps and submersible pumps, where frequent starts can shorten motor and pressure-switch life.
The information needed to size a pressure tank
Before selecting a tank, confirm the pump’s flow rate in litres per minute and the pressure switch settings. You also need to decide how long the pump should run each time it starts.
For most fixed-speed domestic and light commercial pressure pumps, aim for a minimum pump run time of one minute. Larger pumps, deep bore installations and higher-duty applications may benefit from longer cycles, often two minutes or more. The right target depends on the motor, duty pattern and manufacturer recommendations.
You will need:
- Pump flow at the actual operating pressure, in litres per minute
- Pressure switch cut-in and cut-out settings
- Desired minimum pump run time
- The pressure tank manufacturer’s drawdown chart
How to size a pressure tank using drawdown
Start by calculating the drawdown volume required:
Required drawdown (L) = pump flow (L/min) × desired run time (min)
If a pump delivers 55 L/min at the required pressure and you want it to run for at least one minute, the tank needs to supply approximately 55 litres of drawdown before the pump starts again.
The next step is to find a tank that provides at least that drawdown at your cut-in and cut-out pressures. Manufacturers publish drawdown tables for each tank size. These tables account for the changing air volume within the tank and are far more reliable than estimating from nominal tank capacity.
For example, a tank operating on a 200-350 kPa pressure range may provide a different drawdown volume than the same tank running at 300-450 kPa. Higher pressures generally mean a smaller proportion of the tank is available as drawdown, so the tank selection must suit the actual pressure settings.
A practical sizing example
Assume a house or small farm system has a pressure pump supplying 70 L/min at its normal working pressure. The pressure switch is set to cut in at 250 kPa and cut out at 400 kPa. The target is a one-minute minimum run time.
The required drawdown is 70 litres: 70 L/min × 1 minute.
You would then check the drawdown chart for compatible pressure tanks at 250-400 kPa. If a 200-litre tank provides 58 litres at those settings, it is too small for the target. If a 250-litre tank provides 74 litres, that is the appropriate starting point.
Where demand is irregular, choosing the next size up is often sensible. It allows for actual pump performance being lower than the catalogue figure, pressure losses in long pipe runs and future changes such as an additional dwelling, trough line or irrigation zone.
Set the air pre-charge correctly
The tank’s air pre-charge is as important as its size. For a standard pressure-switch system, set the pre-charge to approximately 10-20 kPa below the cut-in pressure. If the pump starts at 250 kPa, a typical pre-charge is around 230-240 kPa.
Check pre-charge only when the system is isolated, depressurised and completely drained of water. Testing it while the tank is under water pressure will give a misleading result.
If the pre-charge is too high, little or no water may enter the tank before the pump cuts in. If it is too low, the bladder can be overstressed and the usable drawdown may fall. An annual pressure check is good maintenance practice, particularly where a system cycles frequently or operates in a hot pump shed.
Tank size depends on the type of pressure system
A conventional fixed-speed pump with a pressure switch needs a tank sized for adequate drawdown. This is the most common situation for household water systems, rainwater transfer pumps, bore supplies and many small commercial systems.
Variable-speed and constant-pressure pumps are different. These systems use electronic controls to vary pump speed as demand changes. They usually require a small pressure tank to stabilise the system and prevent nuisance cycling, but the tank is not normally sized to provide a full minute of drawdown. Always follow the pump controller manufacturer’s tank-volume and pre-charge requirements.
Pressure tanks are also not a substitute for water storage. A large tank may reduce cycling, but it cannot solve a rainwater tank running dry, a bore with limited yield or an undersized supply line. Where a bore produces water slowly, consider a break tank or storage arrangement that lets the bore recover while a separate pressure pump meets peak demand.
Common pressure tank sizing mistakes
The most common mistake is selecting a tank based only on its total litre capacity. Nominal capacity is useful for comparing models, but drawdown at the working pressure is the selection figure.
Another issue is matching the tank to the property size rather than the pump. A modest household with a high-flow pump may need a larger pressure tank than a larger home using a lower-flow pump. Tank sizing follows pump flow and control settings first, then practical demand patterns.
Short cycling should not automatically be blamed on tank size either. A waterlogged tank, failed bladder, incorrect pre-charge, faulty pressure switch, leaking toilet cistern, leaking pipework or blocked non-return valve can all cause rapid cycling. Diagnose the system before replacing equipment.
It is also worth allowing for the installation. Large steel tanks are heavy once connected and need a level, supported base with enough clearance to inspect the air valve and service fittings. In coastal, agricultural and industrial areas, choose tank materials and fittings suited to the water quality and local environment.
When to seek technical advice
Straightforward replacements are usually easy when the existing pump flow, pressure settings and tank drawdown are known. More complex systems need closer assessment. This includes high-flow irrigation pumps, multiple-dwelling supplies, bore systems, firefighting arrangements, commercial booster sets and pumps controlled by variable-speed drives.
Foundation Pumps can help confirm pump duty, pressure range and suitable tank drawdown before equipment is selected. That is particularly useful where an old tank has failed and the original installation was never properly matched.
A pressure tank is a relatively simple component, but it has a direct effect on how hard your pump works every day. Choose it from the drawdown chart, set the pre-charge accurately and give the pump enough run time to do its job properly.