PHAM NEWS | APRIL 2026 Water Heating & Cylinders E
xpansion vessels play a vital role in maintaining safe and stable pressure within sealed
heating and hot water systems. While they are often hidden away within plant rooms or appliance casings, their function is critical to protecting system components, improving reliability, and ensuring long- term performance.
Why expansion occurs Water expands when it is heated. Although commonly misunderstood, water is at its most dense at around 4°C. As system temperatures rise above this point, the volume of water increases. In a sealed system, this expansion has nowhere to escape, which leads to a rapid increase in pressure. Without a means of
accommodating this change in volume, excessive pressure can cause safety relief valves to discharge, stress system components, and in severe cases lead to leaks or mechanical failure. The expansion vessel is designed specifically to manage this process. An expansion vessel provides
a controlled space for expanded water to move into as system temperature rises. Internally, the vessel is divided into two chambers by a flexible diaphragm or bladder. One side contains system water, while the other contains air (or nitrogen) that is pre-charged to a set pressure. Under normal, cold system
conditions, the vessel contains very little water and is predominantly filled with compressed air. As the system heats up and water expands, the excess volume enters the vessel and compresses the air behind the diaphragm. When the system cools and water volume reduces, the compressed air pushes the water back into the system, maintaining stable pressure throughout the heating cycle.
Different designs Expansion vessels are commonly manufactured using either diaphragm or bladder designs. Diaphragm vessels
use a fixed membrane bonded around the vessel circumference. These are typically compact, cost- effective, and widely used in domestic and light commercial heating systems. Bladder vessels contain a
removable internal bladder that How expansion vessels work
As the name implies, expansion vessels are designed to take expansion within a water heating system, thereby preventing damage to components or pipes blowing off. But how do they work and what are the different types available? Stephen Elsmore from Altecnic has the answers.
fully separates water from the air charge. This design is often preferred for larger systems or potable water applications, as it reduces the risk of air absorption into the system water. Both designs perform the
same core function, with selection depending on application, system size, and regulatory requirements. There are three main types
Stephen Elsmore Senior technical support, Altecnic
of expansion vessels: heating, potable or flow through. Heating vessels are designed
to allow the water to sit within the metallic body and there is then a diaphragm which holds the pre-charge. Potable vessels work using
Although relatively simple in design, expansion vessels are a key safety and performance component in modern sealed systems
the same method however the water sits within a rubber bladder, this is due to the fact that the potable water (wholesome water) cannot sit against the metal due to it being oxygenated water. Flow through vessels are
designed in a similar way to potable vessels however there is a special ‘mushroom’ shaped insert which encourages flow through the vessel and not allowing any stagnation. However, there are also flow through vessels where there is a top and bottom connection and the water flows directly through the vessel.
Vessel sizing Correct vessel sizing is essential for effective pressure control. An undersized expansion vessel will fill too quickly, leading to frequent pressure spikes and safety valve discharge. An oversized vessel,
while less harmful, may result in unnecessary cost and installation space requirements. Sizing calculations typically
take into account: • Total system water volume • Minimum and maximum system temperatures • Safety relief valve (SRV) setting • Expansion vessel pre-charge pressure • Whether the system is heating or potable water Accurate sizing ensures
the vessel can absorb the full expansion volume without exceeding safe pressure limits.
Pre-charge pressure The pre-charge pressure is the air pressure within the vessel before it is connected to the system. This must be correctly matched to the system’s cold fill pressure. If the pre-charge is too low, the vessel will partially fill even when the system is cold, reducing its effective expansion capacity. If it is too high, water may not enter the vessel at all,
rendering it ineffective. Checking and setting the
correct pre-charge pressure during commissioning is therefore essential to reliable operation.
Common issues Several common issues can prevent an expansion vessel from working as intended: • Incorrect vessel placement, such as installing it on the wrong side of a pump, can lead to unstable pressure readings. • Loss of air charge over time reduces the vessel’s ability to absorb expansion. • Incorrect pre-charge settings can cause repeated pressure faults or safety valve discharge. • Incorrect vessel type being used for potable or heating applications. These faults often present as
fluctuating system pressure, frequent topping-up, or dripping safety relief valves. Although relatively simple in
design, expansion vessels are a key safety and performance component in modern sealed systems. When correctly selected,
installed, and commissioned, they protect equipment, maintain stable operating pressure, and contribute to the long-term reliability of heating and hot water installations. ◼
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