An expansion tank is a diaphragm pressure vessel that takes up the extra volume of heated water in a closed system and stops pressure from climbing. In booster systems it works as a pressure vessel: it covers small draws and reduces pump cycling.
Contents
- What Does an Expansion Tank Do?
- How Does an Expansion Tank Work?
- Where Are Expansion Tanks Used?
- Open and Closed Expansion Compared
- How Do You Size an Expansion Tank?
- How Do You Set the Pre-Charge Pressure?
- Aquamax Expansion Tank Data
- How Can You Tell an Expansion Tank Has Failed?
- Frequently Asked Questions
What Does an Expansion Tank Do?
An expansion tank makes room for the growing volume of heated water and keeps pressure in a closed system from reaching a dangerous level. Water does not compress; its volume rises as it heats. If that extra water has nowhere to go in a closed circuit, pressure climbs quickly and the safety valve discharges.
The effect is not small: the expansion coefficient for water at 90 °C is about 0.0356, so a system holding 1,000 litres produces about 36 litres of extra water.
The Aquasan catalogue sums up the tank's role as protecting closed heating systems from the high pressure caused by expanding water, and working as a pressure vessel in booster systems (Aquapump catalogue p.36, Turkish edition).
How Does an Expansion Tank Work?
Inside the tank, a flexible diaphragm separates the gas and water sections. Aquamax tanks use a replaceable EPDM diaphragm (Aquapump catalogue p.36).
- Cold system: The gas section holds the pre-charge pressure and the diaphragm pushes water out of the tank.
- Heating up: Expanding water enters the tank, pushes the diaphragm and compresses the gas.
- Cooling down: The compressed gas returns the water to the system and pressure falls back.
The logic is the same in a booster set: water enters the tank while the pump runs, and gas pushes it back into the pipework during use.
Where Are Expansion Tanks Used?
Expansion tanks are used in closed heating and cooling circuits and in booster systems. The catalogue notes for Aquamax tanks are (Aquapump catalogue p.36):
- Suitable for both heating and booster systems.
- In heating systems the tank must be connected to the return line.
- Suitable for systems containing antifreeze.
- Operating temperature is −10 °C to +120 °C.
For the booster side, see what is a booster pump set.
Open and Closed Expansion Compared
An open expansion tank sits at the highest point of the system and is open to the atmosphere; a closed expansion tank works under pressure inside the system.
| Feature | Open tank | Closed tank |
|---|---|---|
| Location | Highest point of the system | Plant room or another suitable point on the system |
| Contact with air | Yes | No, the diaphragm separates them |
| Pressure | Open to atmosphere | Works under pressure |
| Use on booster sets | No | Used as a pressure vessel |
How Do You Size an Expansion Tank?
Tank size comes from the water content of the system, the operating temperature and the system's pressure limits. The common method based on the EN 12828 approach is:
- Expanded water: Vex = Vs × n (Vs is system water content, n is the expansion coefficient)
- Water reserve: Wres = Vs × 0.005
- Pre-charge pressure: P0 = static height (m) / 10 + 0.2 bar
- Final pressure: Pfin = safety valve pressure − 0.5 bar (for valves below 5 bar)
- Tank volume: Vn ≥ (Vex + Wres) × (Pfin + 1) / (Pfin − P0)
Example: a heating system at 70/50 °C holding 1,500 litres, with a static height of 15 metres and a 3 bar safety valve:
| Item | Calculation | Result |
|---|---|---|
| Expanded water (n = 0.0228) | 1,500 × 0.0228 | 34.2 L |
| Water reserve | 1,500 × 0.005 | 7.5 L |
| Pre-charge pressure | 15 / 10 + 0.2 | 1.7 bar |
| Final pressure | 3 − 0.5 | 2.5 bar |
| Tank volume | 41.7 × 3.5 / 0.8 | 182 L, so a 200 L tank |
The example only shows the method. Final sizing should be done by the mechanical designer using the real system values.
How Do You Set the Pre-Charge Pressure?
Pre-charge pressure is the pressure in the gas section when the tank holds no water, and it must be set for the project. Aquamax tanks leave the factory at 4 bar pre-charge (Aquapump catalogue p.36), which does not suit every system.
- Heating systems: Add 0.2 bar to the pressure from the static height: 1.7 bar for 15 metres.
- Booster sets: A common figure in manufacturer manuals is about 90% of the pump cut-in pressure.
- Measuring: Use a gauge at the gas valve with no pressure on the water side; a reading under water pressure misleads.
- Fill pressure: Fill a cold heating system to about 0.3 bar above the pre-charge pressure.
Aquamax Expansion Tank Data
Aquamax closed expansion tanks are made from 24 litres up to 5,000 litres. Key values (Aquapump catalogue p.36):
| Feature | Value |
|---|---|
| Tank volume | 24 – 5,000 L |
| Maximum working pressure | 6 and 10 bar up to 400 L, 10 and 16 bar from 500 L |
| Factory pre-charge | 4 bar |
| Operating temperature | −10 °C / +120 °C |
| Diaphragm | Replaceable EPDM |
| Water connection | 1", 1 1/4", 2", NW65, NW80 |
| Coating | Electrostatic powder coating |
| Manufacturing standard | TS EN 13831 |
Model dimensions are on the 10 bar and 16 bar product pages. See the Certificates page for documents.
How Can You Tell an Expansion Tank Has Failed?
When the tank is not doing its job, system pressure swings quickly with temperature. Look for these signs:
- Pressure climbs fast on heating and the safety valve discharges.
- Pressure drops too far on cooling and the system needs frequent topping up.
- Water comes out of the gas valve, which points to a damaged diaphragm.
- On a booster set, the pump cycles too often.
Check the pre-charge pressure first. Aquamax tanks have a replaceable diaphragm; contact us for replacement.
Frequently Asked Questions
What pressure should an expansion tank be?
Pre-charge depends on the system. In heating systems, add 0.2 bar to the pressure from the static height; for a 15 metre building that is about 1.7 bar. Aquamax tanks leave the factory at 4 bar, so check the value during installation.
Where is an expansion tank connected?
In heating systems it must be connected to the return line (Aquapump catalogue p.36). On a booster set it connects to the set as a pressure vessel.
What size expansion tank do I need?
It is calculated from system water content, operating temperature and pressure limits. In the example in this guide, a system holding 1,500 litres needs a 200 litre tank.
Can the diaphragm be replaced?
Aquamax tanks use a replaceable EPDM diaphragm (Aquapump catalogue p.36). If the tank shell is sound, the diaphragm can be changed.
Is a pressure vessel the same as an expansion tank?
Their construction is similar: both have gas and water sections separated by a diaphragm. Aquamax tanks can be used in heating and booster systems; pre-charge is set to suit the use.
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