Expansion Tank Sizing Calculator (Metric)

At typical UK domestic settings — 80°C maximum flow temperature, 1.0 bar pre-charge, 3.0 bar pressure-relief valve, 10% safety margin — a 150-litre system needs a minimum vessel of about 9.5 litres, which rounds up to a standard 12-litre vessel. The exact figure changes with your actual pressures and maximum temperature, so use the calculator above with your own system's values rather than relying on this example alone.

The most reliable method is to check the boiler manufacturer's installation manual or the original system design/commissioning paperwork, which often states the total system water content. If that is not available, add up each radiator's rated water content plus an estimate for pipework (roughly 0.1-0.2 litres per metre of 15-22mm pipe) and any cylinder coil volume. Some installers simply measure how many litres it takes to refill the system from empty using a filling loop and a measuring container.

The industry-standard method (based on BS EN 12828) is: expansion volume = system water volume × expansion percentage (from a temperature table); acceptance factor = 1 − (pre-charge pressure + 1) ÷ (maximum system pressure + 1), with both pressures in bar absolute; minimum vessel volume = expansion volume ÷ acceptance factor. A safety margin (commonly 10%) is then added before rounding up to a real commercial vessel size.

A diaphragm or bladder expansion vessel only has part of its total volume usable for accepting water, because some of that volume is needed to compress the vessel's pre-charged air/nitrogen cushion from the pre-charge pressure up to the maximum system pressure. This usable fraction is the acceptance factor — at typical domestic settings (1 bar pre-charge, 3 bar PRV) only 50% of the vessel's total volume is actually usable for water, so the vessel itself must be roughly double the raw expansion volume.

The vessel's pre-charge is normally set to match the system's cold fill pressure, commonly 1.0-1.5 bar for a typical UK two-storey house, and slightly higher for taller properties. Always check the specific boiler and vessel manufacturer's instructions, and pre-charge is normally checked and adjusted with the system fully drained and depressurised, using a bicycle-pump-style gauge on the vessel's Schrader valve.

3 bar is the standard PRV setting on the vast majority of UK domestic sealed central heating systems, and is the default this calculator uses. Some older or non-standard installations use different settings, so always check the value printed on the PRV itself or in the boiler's installation manual rather than assuming 3 bar.

This happens when the pre-charge pressure is close to the maximum system (PRV) pressure — for example a 2.5 bar pre-charge against a 3.0 bar PRV leaves very little usable acceptance volume in the vessel, so a much bigger vessel is needed to absorb the same expansion. In practice this is often a sign the pre-charge pressure has been set too high, or that a higher-rated PRV should be considered — review the pressures with a qualified engineer rather than simply buying an oversized vessel.

Not directly with the default settings. Unvented (mains-fed) hot water cylinders use a different type of expansion vessel with mains cold-water pressure typically 3-4 bar and a PRV set much higher, around 6-8 bar, plus different temperature assumptions. You can still use the same underlying formula by entering your cylinder's actual mains and PRV pressures, but the sensible defaults on this page are set up for sealed central heating, not potable water systems.

An oversized vessel is not usually dangerous — it simply absorbs the expansion volume with margin to spare — but it costs more, takes up more space, and very occasionally an excessively oversized vessel can indicate the pre-charge pressure was set incorrectly (too low), which itself is worth checking, since an under-charged vessel can accept water too early and lose some of its usable capacity.

Most manufacturers recommend checking the pre-charge annually, ideally during an annual boiler service, since the air/nitrogen charge can slowly leak over years even with no visible fault. A telltale sign of a failed or under-charged vessel is the system losing pressure repeatedly, or the PRV discharging water during normal heating cycles.

An undersized vessel cannot fully absorb the water's expansion as the system heats up, so system pressure rises further than intended and the pressure-relief valve may begin to weep or fully discharge water during normal operation. Repeated PRV discharge is one of the most common calls a heating engineer receives for "system keeps losing pressure" and is frequently traced back to an undersized or failed expansion vessel rather than a leak elsewhere.

No — they are unrelated. The safety margin in this calculator (default 10%) is a sizing buffer added to the calculated minimum vessel volume to allow for measurement uncertainty in your system's actual water content and normal manufacturing tolerances. The pressure-relief valve is a separate physical safety device that protects the system from over-pressure regardless of how the vessel was sized.

Underfloor heating circuits typically hold significantly more water per square metre of heated area than an equivalent radiator circuit, because of the length of pipe involved, so the total system water volume figure you enter should include the full underfloor circuit content — this is usually the single biggest contributor to system volume being higher than expected on properties with underfloor heating, not a separate factor in the formula itself.

Vessel sizing must cover the worst realistic case, not the everyday average, because the pressure-relief valve has to be protected even on the hottest heat-up cycle the system will ever see, for example a cold snap that pushes a weather-compensating boiler to its highest possible flow temperature. Sizing to a lower typical temperature would undersize the vessel for that worst case.

The BS EN 12828 method and the manufacturer expansion-percentage table used here are the same European standard used across the UK and much of the EU, so the underlying calculation applies broadly. However, typical pre-charge and PRV pressure conventions, and standard commercial vessel sizes, can vary by country and manufacturer — always check your local installation standards and the specific products available to you.