Product group 06
Water expands when heated; in a closed circuit, that expansion needs somewhere to go. The expansion tank is where the system breathes without triggering the safety valve.
Membrane closed expansion tank · gas cushion and water volume
Why it's needed
When the tank is wrongly sized, the system doesn't stop right away; it gets by for months and then causes problems in unexpected places.
When an undersized tank fills up, pressure rises and the safety valve discharges water. As the system cools, pressure drops and water gets topped up again. Every cycle brings fresh oxygen into the circuit.
Continuous water top-up means continuously carrying dissolved oxygen into a closed circuit. The basic condition for protecting a closed circuit from corrosion is keeping the same water in the loop.
The pre-charge pressure on the tank's gas side is set relative to static pressure. A tank with the wrong setting won't do its job even at the correct volume — it either never engages, or stays full from the start.
Pressure keeps changing through heating and cooling cycles. The tank smooths out that variation, putting less strain on fittings and gasket surfaces.
In the membrane type, the water never contacts the nitrogen cushion. This way the gas doesn't dissolve into the water, and the tank itself doesn't turn into a source of air in the circuit.
System water volume, maximum operating temperature, pre-charge pressure and the safety valve's set pressure all go into the calculation together. Change any one of the four and the result changes.
Types
Membrane construction and intended use determine the selection.
The membrane is fixed to the body. The standard, economical solution for heating circuits.
The membrane can be removed and replaced through a flanged cover. Provides long life and easy maintenance.
For domestic water and pressurization lines. Requires a food-grade membrane material.
Selection criteria
Volume can't be calculated without these four data points; the rest determine type and material.
System total water volume
The total water content of the boiler, pipework, radiators or fan coils, and exchangers. Expansion volume is derived directly from this.
Liters
Maximum operating temperature
Determines how much the water will expand. The difference between a 90 °C and a 60 °C regime shows up directly in volume.
°C
Pre-charge (static pressure)
The water column from the tank's elevation up to the highest point. Calculated from building height.
bar
Safety valve set pressure
The system's allowable upper limit. The gap between this and the pre-charge determines the tank's usable volume.
bar
Fluid type
A glycol mix expands differently than water, and its compatibility with the membrane material is checked separately.
Water / % glycol
Membrane material
EPDM is standard on a heating circuit. Domestic water needs a certified food-grade membrane.
EPDM / butyl / food-grade
Mounting style
Wall-hung, legged, or on a stand. At large volumes, floor load capacity should also be checked.
Wall-hung / legged
Connection size and location
The tank connects close to the pump's suction side and the boiler. An isolation valve and a drain should sit on the connection.
DN/threaded size
You don't need all of it. We'll work out anything missing together with you.
Create a quote requestFAQ
The four questions we hear most often at the quote stage.
It's possible to arrive at an approximate volume based on boiler capacity and system type. Radiator heating, fan coils and underfloor heating each carry a different water volume per unit of capacity. Tell us the capacity and installation type, and we'll start from a reasonable estimate and set the safety margin accordingly.
The tank ships from the factory with a standard pre-charge, and that value doesn't have to match your building. Before installation, while the tank is empty, it needs to be set on the gas side to match static pressure. If this step is skipped, the tank won't do its job even at the correct volume — this is the mistake we see most often in the field.
Usually one of three reasons: the volume was undersized, the pre-charge was set incorrectly, or the membrane has ruptured. Checking the membrane is simple — if water comes out of the gas valve, the membrane has failed. We tell the other two apart by calculation.
The membrane is a wear part; its life varies with operating temperature, the number of pressure cycles, and water quality. In fixed-membrane types, the whole tank is replaced once the membrane fails. In bladder-membrane types, only the membrane is replaced — at large volumes, this difference gives a clear long-term advantage.
Next step
Share your system volume and regime, and let's work out the tank volume and pre-charge together.