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Since 1998
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UK Heating & Pressure Engineering
Accurately size expansion vessels for central heating systems, combi boilers, system boilers, and unvented hot water cylinders — using the correct engineering formula.
An expansion vessel (also called a pressure vessel or expansion tank) is a sealed metal cylinder containing a rubber diaphragm or bladder. One side holds pressurised nitrogen gas; the other side is connected to the heating or hot water system. As water heats up and expands, it compresses into the vessel — preventing dangerous pressure spikes that would otherwise open your pressure relief valve or damage system components.
Water is nearly incompressible. When cold water at 10°C is heated to 80°C, its volume increases by approximately 2.9–4%. In a sealed system — whether a combi boiler central heating circuit or an unvented (mains pressure) hot water cylinder — there is nowhere for that extra volume to go without raising pressure. Without an expansion vessel, pressure can climb from a normal 1 bar to over 3 bar in minutes, triggering the pressure relief valve and wasting water.
Older gravity-fed open-vented systems used a header tank in the loft to absorb expansion. Modern sealed systems — virtually all combi boilers, system boilers, and unvented cylinders — have no open vent. Every sealed system must have a correctly sized expansion vessel. Undersizing is the single most common cause of recurring boiler pressure problems and pressure relief valve discharge.
Get it right and the system maintains stable pressure between 1 and 2.5 bar whether cold or hot. Get it wrong — too small — and pressure spikes to 3 bar every time the boiler fires. Too large and the vessel is wasted cost and space. The correct size depends on the total system water volume, operating temperature range, fill pressure, and maximum allowable pressure.
Use the tabs below to calculate the correct expansion vessel size for a central heating system or a domestic hot water (DHW) unvented cylinder. Enter the values for your system and click Calculate to get your recommended vessel size.
The total volume of water in the circuit. For heating: count all radiators, pipework, and the boiler heat exchanger. For hot water: this is the cylinder capacity. Typical domestic heating systems hold 50–150 litres. You can estimate using a heating system water volume calculator.
The cold fill pressure is the static pressure when the system is cold — typically 1.0–1.5 bar for domestic heating. The vessel pre-charge pressure should match the fill pressure (factory-set at 1.5 bar, but must be adjusted on site). These are critical inputs — incorrect values lead to wrong vessel sizing.
The setting of the pressure relief valve (PRV) — typically 3 bar for domestic boilers and 6 bar for unvented cylinders. The vessel must absorb all expansion before pressure reaches this limit. Always use the actual PRV setting, not an assumed figure.
The maximum temperature the water reaches. For central heating this is typically 80°C (flow temperature). For condensing boilers on weather compensation it may be 60–70°C. For unvented DHW cylinders, the set temperature is typically 60–65°C. Higher temperatures mean more expansion and a larger vessel is needed.
The standard method for sizing an expansion vessel in the UK is based on the acceptance volume method, as described in CIBSE Guide B and referenced in BS EN 12828. Here is a full breakdown.
Water density changes with temperature. The expansion volume is:
For simplicity, expansion coefficients (e) are often used instead of density ratios:
The acceptance volume is the volume of water the expansion vessel can actually receive. It depends on the vessel size, the pre-charge pressure, and the maximum pressure:
The vessel acceptance volume must equal or exceed the expansion volume. Rearranging:
| Cold Fill Temp (°C) | Max Temp (°C) | Coefficient (e) | Expansion per 100L | Application |
|---|---|---|---|---|
| 10 | 60 | 0.0171 | 1.71 L | DHW cylinders, low-temp heating |
| 10 | 65 | 0.0200 | 2.00 L | DHW cylinders |
| 10 | 70 | 0.0228 | 2.28 L | Heating, weather compensation |
| 10 | 80 | 0.0289 | 2.89 L | Standard central heating |
| 10 | 90 | 0.0360 | 3.60 L | High-temperature heating |
| 10 | 5 | 0.0001 | 0.01 L | Chilled water (cooling) |
The tables below give recommended vessel sizes for common system volumes at typical UK domestic and commercial pressures. These assume an 80°C maximum flow temperature, 10°C cold fill, 1 bar fill pressure, 3 bar relief valve setting (domestic), with a 10% safety margin.
| System Volume | Expansion Volume | Min Vessel | Recommended Vessel | Standard Stock Size | Typical Application |
|---|---|---|---|---|---|
| 50 L | 1.45 L | 2.9 L | 3.2 L | 8 L | Small flat, 4–5 radiators |
| 75 L | 2.17 L | 4.3 L | 4.8 L | 8 L | Mid-size flat, 6–7 rads |
| 100 L | 2.89 L | 5.8 L | 6.4 L | 8 L | 2-bed house |
| 120 L | 3.47 L | 6.9 L | 7.6 L | 8–12 L | 3-bed house |
| 150 L | 4.34 L | 8.7 L | 9.5 L | 12 L | 4-bed house |
| 200 L | 5.78 L | 11.6 L | 12.7 L | 18 L | 5-bed house, large system |
| 300 L | 8.67 L | 17.3 L | 19.1 L | 25 L | Commercial / large domestic |
| 500 L | 14.45 L | 28.9 L | 31.8 L | 35 L | Small commercial plant room |
| System Volume | Expansion Volume | Acceptance Ratio | Min Vessel | Recommended Vessel |
|---|---|---|---|---|
| 500 L | 14.45 L | 0.375 | 38.5 L | 45 L |
| 1,000 L | 28.9 L | 0.375 | 77.1 L | 90 L |
| 2,000 L | 57.8 L | 0.375 | 154 L | 180 L |
| 5,000 L | 144.5 L | 0.375 | 385 L | 450 L |
| 10,000 L | 289 L | 0.375 | 771 L | 900 L |
| Cylinder Size | Expansion Volume | Min Vessel | Recommended Vessel | Common Brand Sizes |
|---|---|---|---|---|
| 120 L | 2.05 L | 4.8 L | 5.3 L | 8 L (Flamco, Reflex) |
| 150 L | 2.57 L | 6.0 L | 6.6 L | 8 L |
| 170 L | 2.91 L | 6.8 L | 7.5 L | 8–12 L |
| 200 L | 3.42 L | 8.0 L | 8.8 L | 12 L |
| 210 L | 3.59 L | 8.4 L | 9.2 L | 12 L |
| 250 L | 4.28 L | 10.0 L | 11.0 L | 12–18 L |
| 300 L | 5.13 L | 12.0 L | 13.2 L | 18 L |
| 400 L | 6.84 L | 16.0 L | 17.6 L | 18–25 L |
In a sealed central heating system, the expansion vessel is typically located in the boiler cupboard or plant room, connected to the return pipework (cold side) near the boiler. The vessel absorbs the increase in water volume each time the boiler fires, keeping system pressure stable.
You need the total system water volume to size the vessel correctly. The most accurate method is to fill the system and measure how much water goes in. Where this isn't practical, use these estimates:
| Component | Approx. Volume | Notes |
|---|---|---|
| Standard single radiator | 3–8 L | Depends on type and size; double panel ≈ 8–15 L |
| 15mm copper pipework | 0.13 L/m | Per linear metre of pipe |
| 22mm copper pipework | 0.32 L/m | Per linear metre of pipe |
| 28mm copper pipework | 0.54 L/m | Per linear metre of pipe |
| Combi boiler heat exchanger | 1–3 L | Check manufacturer's data sheet |
| System boiler + cylinder coil | 2–5 L | Additional DHW coil volume |
| Underfloor heating per room | 2–8 L | Depends on loop length and pipe size |
A widely used installer rule of thumb for domestic UK systems: size the expansion vessel at approximately 6–8% of the total system water volume, with a minimum of 8 litres for any system. This works well for standard 80°C systems with a 1 bar fill and 3 bar PRV, but always verify with the full calculation for any system above 150 litres.
Most modern combi boilers and system boilers have an integral expansion vessel built in. However, the built-in vessel is sized to handle only the water volume within a typical small-to-medium system. If your system is large, or the integral vessel has failed, you will need an external supplementary vessel.
A combi boiler expansion vessel is typically 6–10 litres and is suitable for systems up to approximately 100–130 litres total volume. Most 2 and 3-bed properties fall within this range. If you have a large number of radiators, extensions to your property, or an underfloor heating zone, the integral vessel may be undersized.
The vessel's nitrogen pre-charge pressure must match the cold fill pressure of the system. For a system filled to 1 bar, the vessel pre-charge should also be 1 bar. If the pre-charge is too high, the vessel provides almost no acceptance volume when cold. If too low, it fills with water when cold and provides no cushion when the system heats up.
Most vessels are factory-set at 1.5 bar. Before installation, always check and adjust the pre-charge with a tyre-type pressure gauge via the Schrader valve, with the vessel disconnected from the system or the system depressurised.
If the integral boiler vessel has failed or the system is too large for it, a supplementary external expansion vessel can be fitted to the return pipework. The total acceptance volume of both vessels combined must meet the system requirement. In practice, many engineers simply fit an additional 12 or 18 litre vessel alongside the boiler.
A replacement expansion vessel typically costs £15–£60 for the vessel itself (8–18 litres). Labour to fit and repressurise the system typically adds £80–£150, depending on location and complexity. Brands such as Flamco, Reflex, Caleffi, and Watts are commonly used by UK installers. Some boiler manufacturers (Vaillant, Worcester Bosch, Ideal, Potterton) supply own-brand vessels for their appliances.
Unvented hot water cylinders (such as Megaflo, Joule, Gledhill, OSO) operate at mains pressure and must have a dedicated expansion vessel to absorb the expansion of the stored water as it heats up. This is a statutory requirement under Building Regulations Part G and the Unvented Hot Water Regulations.
Unlike heating expansion vessels, DHW expansion vessels must be rated for potable water contact — they have a food-grade EPDM or butyl diaphragm. Do not use a heating vessel on a DHW circuit.
Select your cylinder size to see the recommended vessel size and key parameters:
For a 150 litre unvented cylinder at 3 bar mains pressure, 6 bar PRV, and 60°C set temperature: expansion volume ≈ 2.57 litres, minimum vessel size ≈ 6.0 litres. With the 10% safety margin and rounding to the next standard size, a 8 or 12 litre vessel is correct. Most installers fit a 12 litre vessel for added margin. Pre-charge pressure should be set to match the mains static pressure at the vessel location (typically 2.5–3.5 bar).
For a 200 litre cylinder: expansion volume ≈ 3.42 litres (at 60°C), minimum vessel ≈ 8.0 litres. Standard recommendation: 12 litre vessel. This is the most common size for 200L cylinders and provides a comfortable margin above the minimum requirement.
For a 210 litre cylinder — the most common UK unvented cylinder size, often a Megaflo HE — the expansion volume is approximately 3.59 litres. The minimum calculated vessel is 8.4 litres, making a 12 litre vessel the standard recommendation. Many cylinder manufacturers (including Heatrae Sadia for Megaflo) specify a 12L vessel as part of their approved installation kit.
For a 250 litre cylinder: expansion volume ≈ 4.28 litres, minimum vessel ≈ 10.0 litres. Standard recommendation: 12 or 18 litre vessel. If mains pressure is above 4 bar at the cylinder, the 18 litre vessel is preferred for greater safety margin.
For a 300 litre cylinder: expansion volume ≈ 5.13 litres, minimum vessel ≈ 12.0 litres. Standard recommendation: 18 litre vessel. At this cylinder size and above, always verify the mains cold water pressure with a gauge and use the full calculation rather than a rule of thumb.
Pressure is the most important and most misunderstood aspect of expansion vessel installation. Getting the pressures wrong causes every other symptom — boiler pressure rising, PRV discharge, waterlogged vessels, and system faults.
The pre-charge pressure of the expansion vessel must equal the cold fill pressure of the system. For a standard domestic central heating system filled to 1.0 bar, the vessel pre-charge must be set to 1.0 bar (after any static head adjustment). For an unvented cylinder with 3.5 bar cold mains pressure, the vessel pre-charge must be set to 3.5 bar.
| System Type | Cold Fill Pressure | Vessel Pre-charge | Max Pressure (PRV) | Hot System Pressure |
|---|---|---|---|---|
| Domestic CH (combi) | 1.0 bar g | 1.0 bar | 3.0 bar g | 1.5–2.0 bar g |
| Domestic CH (system boiler) | 1.0–1.5 bar g | Same as fill | 3.0 bar g | 1.5–2.5 bar g |
| Unvented DHW (low-rise) | 2.5–3.5 bar g | Same as static cold | 6.0 bar g | 3.5–5.0 bar g |
| Commercial LPHW | 1.5–2.0 bar g | Same as fill | 4.0–6.0 bar g | 2.5–3.5 bar g |
| Chilled water (CHW) | 1.0–1.5 bar g | Same as fill | 4.0–6.0 bar g | Minimal change |
The pre-charge pressure is the nitrogen gas pressure inside the vessel before any water enters. The fill pressure is the pressure of water in the system when cold. They must match. If the pre-charge is higher than the fill pressure, the diaphragm is pushed hard against the water port — the vessel effectively has zero acceptance volume until pressure exceeds the pre-charge. If the pre-charge is lower, the vessel partly fills with water when cold, reducing available acceptance volume.
If the expansion vessel is not at the same height as the pressure gauge, correct for static head. Every 10 metres of height difference equals 1 bar. A vessel located 5 metres below the boiler gauge needs its pre-charge set 0.5 bar higher than the gauge fill pressure reading.
Commercial heating and cooling systems — LPHW (low-pressure hot water), MTHW (medium-temperature hot water), and CHW (chilled water) — require the same fundamental sizing approach, but with several additional considerations.
Chilled water systems operate at low temperatures (typically 6–12°C flow, 12–18°C return). Water expansion is minimal compared with heating, but the vessel is still required to absorb any pressure variations from pump cycling, temperature changes, and glycol concentration effects. For glycol mixtures (common in outdoor pipework to prevent freeze damage), use the glycol-adjusted expansion coefficient — glycol solutions expand more than pure water.
Large commercial systems often require multiple expansion vessels connected in parallel to achieve the required total vessel volume. This is common in plant rooms where individual vessels larger than 500–1,000 litres would be impractical. Each vessel should be fitted with an isolation valve (normally open, lockable) to allow maintenance without system draining.
The acceptance volume ratio is the fraction of the vessel volume that is available to receive expanding water: ratio = 1 − (P_precharge_abs / P_max_abs). Higher operating pressures (commercial systems at 4–6 bar) give better ratios than domestic systems. A commercial vessel at 1.5 bar fill and 4 bar PRV has a ratio of 0.456 — better than a domestic vessel at 1 bar fill and 3 bar PRV (ratio 0.5) simply because the span between fill and max pressures is proportionally larger.
The most common fault. Symptoms: boiler pressure rises rapidly when heating switches on, PRV discharges regularly (look for a small pipe dripping outside), and the system needs frequent topping up. Solution: calculate the correct vessel size and fit a supplementary vessel or replace the existing one.
The rubber diaphragm perishes over time — typically 10–15 years. When it fails, the vessel fills completely with water and provides no cushioning. Symptoms are identical to an undersized vessel. Diagnosis: press the Schrader valve — if water comes out, the diaphragm has failed. The vessel must be replaced.
Nitrogen slowly leaks from the Schrader valve over time. A vessel with insufficient pre-charge has reduced acceptance volume. Check and re-inflate the pre-charge annually as part of boiler servicing. The vessel must be isolated and depressurised (water side) before checking the gas charge.
A vessel pre-charged too high acts as though it is too small — very little acceptance volume until pressure exceeds the pre-charge. A vessel pre-charged too low partly fills with water when cold, reducing its useful volume. Always set pre-charge = cold fill pressure, checked with the system depressurised.
Over time (especially if the diaphragm has a small pinhole), water migrates through the diaphragm and logs the nitrogen side. The vessel becomes almost entirely water-filled and is effectively dead. This is diagnosed by shaking the vessel — a waterlogged vessel is uniformly heavy; a healthy vessel is light at the top (gas side) and heavier at the bottom.
Heating expansion vessels use a standard EPDM diaphragm not rated for potable water. Using a heating vessel on an unvented cylinder degrades the diaphragm rapidly and contaminates the drinking water. Always use a vessel specifically rated for potable water (food-grade diaphragm) on DHW circuits.
Use these complementary tools alongside the expansion vessel calculator to fully design and specify your heating system or hot water installation.