How to Choose a Booster Pressure Tank (Expansion Tank)? Sizing, Pre-charge and Prices (2026)

Does your booster pump start and stop every time a tap is opened, does the pressure fluctuate, or is water coming out of the tank's air valve? Almost all of these symptoms come from the pressure tank (expansion tank). If the tank is too small or its pre-charge is set wrong, the pump starts dozens of times an hour, the motor overheats and the booster set's life is shortened. This guide explains step by step how to choose an expansion tank, what size you need, the difference between spherical, horizontal and vertical tanks, and how to set the pre-charge pressure.

Short answer: in most cases a 24–50 litre membrane tank is enough for a detached house or single flat, 100 litres for small apartment blocks and 200–300 litres for twin pump booster sets in apartment buildings. The tank's air pre-charge should be set about 0.2–0.3 bar below the pump's cut-in pressure. Isımatik offers Nema EPDM membrane expansion tanks from 24 to 2,000 litres.

Quick Selection Table: What Tank Size for Which System?

The values in the table are for a first selection; the exact volume depends on the pump flow and its cut-in and cut-out pressures. Send us your details and we will do the calculation.

What Is an Expansion (Pressure) Tank and What Does It Do?

An expansion tank is a closed pressure vessel with a rubber membrane inside, holding compressed air on one side of the membrane and water on the other. When the booster pump runs, water fills the tank and compresses the air. When a tap is opened, the first water comes from the tank without the pump starting. As a result:

• The pump does not start for every small draw-off, so the number of starts drops.
• Pressure fluctuations and water hammer in the system are softened.
• The pump and motor last longer and use less energy.

Step 1: Determine the Right Tank Volume

The larger the tank, the less often the pump runs. Three values decide the volume:

• Pump flow: the higher the flow, the larger the tank must be.
• Difference between cut-in and cut-out pressure: a small difference (e.g. 0.8 bar) needs a larger tank.
• Number of pumps: in twin pump sets the pumps alternate, so a larger tank is chosen for the total flow.

According to the Pedrollo catalogue, a 100 litre tank is used with single pump MK-1H sets and 200–300 litres with twin pump MK-2H sets. Ready-to-use Pedrollo Hydrofresh sets come with a 24 or 50 litre tank.

Step 2: Choose the Working Pressure (10 or 16 bar?)

The tank's maximum working pressure must be higher than the booster set's cut-out pressure and the highest pressure in the system.

• 10 bar tank: enough for homes, apartment buildings and most standard booster systems.
• 16 bar tank: preferred for tall buildings, fire-fighting and industrial booster sets, and systems where pressure exceeds 8 bar.

Step 3: Spherical, Horizontal or Vertical Tank?

Step 4: Set the Pre-charge (Air Pressure) Correctly

The tank leaves the factory with a set air pressure, which must be adjusted to your system. The rule is simple: tank pre-charge = pump cut-in pressure – 0.2…0.3 bar.

Example: if the booster set cuts in at 2.5 bar and cuts out at 4 bar, the tank's air pressure should be about 2.2–2.3 bar. Adjust it through the air valve with a tyre pump or compressor while the tank is isolated from the system and completely drained of water. Check the pre-charge at least once a year.

Step 5: Check the Membrane Material and Water Suitability

In drinking and domestic water systems, the membrane surface in contact with water must be hygienic and durable. Nema expansion tanks have an EPDM membrane and can be used in booster sets and domestic water systems up to 90 °C.

Note: closed heating circuits such as central heating and combi boilers use expansion tanks designed specifically for heating systems; the selections in this guide are for booster sets and domestic water systems.

Nema Expansion Tank Prices (November 2026)

At Isımatik, Nema tanks are sold at list price excluding VAT; prices are shown on the site in Turkish lira at the daily exchange rate. All volumes can be selected from the Model option on a single product page.

For the large 750, 1,000, 1,500 and 2,000 litre volumes and the other 16 bar models, see our product page. Prices are list prices at the time of publication; the current price is shown on the product page.

5 Signs of a Failing Tank

1. The pump starts and stops very often: the membrane may have burst or the pre-charge has dropped.
2. Water comes out of the air valve: the membrane is torn; replace the tank or the membrane.
3. The tank is full of water and heavy: the air side has emptied; reset the pre-charge.
4. Pressure fluctuates at the taps: the tank is too small for the system.
5. Rust or condensation on the body: it is time to replace the tank.

Frequently Asked Questions

What pressure should a booster tank have?

The tank's working pressure (10 or 16 bar) must be higher than the system's maximum pressure. The tank's air pre-charge is set 0.2–0.3 bar below the pump's cut-in pressure.

What size tank is enough for a home booster set?

For single flats and detached houses a 24–50 litre tank is usually enough. If use is heavy or the pump cycles often, moving up one size extends the pump's life.

Is there any harm in buying a larger tank?

No. A larger tank makes the pump run less often; the only limits are space and cost. A small tank causes frequent pump starts and early failure.

Are an expansion tank and a pressure tank the same thing?

In everyday use they mean the same thing. The membrane pressure tank used in booster systems is also called an expansion tank.

How long does a pressure tank last?

A membrane tank running at the right pre-charge and checked regularly works for many years without problems. Checking the pre-charge once a year noticeably extends the membrane's life.

If you are replacing the whole booster set, see our booster set selection guide and our Packaged Booster Sets category.