Pressure Tanks

A pressure tank stores treated water so it is waiting when you open the faucet. Inside is a bladder with an air charge behind it; the air is what pushes the water out. That construction is also why the number on the label is not the number of litres you get.

Nominal volume and usable volume

An 8-litre tank holds roughly 5–6 litres of usable water; a 12-litre tank around 8–9. The difference is the space the air charge occupies. When sizing, work from usable volume — an under-sink system feeding a household of four is comfortable with a 12-litre tank rather than an 8.

Air pre-charge

The pre-charge is set with the tank empty, typically around 0.5 bar for domestic tanks. If output drops to a trickle after a few glasses, the pre-charge has usually leaked away — it is checked with a tyre gauge on the Schrader valve and is a far more common cause of poor performance than a failing membrane.

40 and 80 litre tanks

Larger tanks serve cafés, small offices and systems where draw is concentrated into short periods. They need floor space and a supported position; they are not a drop-in replacement for an under-sink tank.

Tanks are durable spare parts and carry a 2-year warranty.