#Expansion Vessel Sizing: How to Size a Vessel for a Sealed Heating System

Water expands when it heats up, and in a sealed system that expansion has nowhere to go except into the expansion vessel. Get the vessel size right and the pressure gauge barely moves between cold and hot. Get it wrong and you're back on site for "the boiler keeps losing pressure" — which is really the PRV doing its job because the vessel can't. This guide walks through sizing a vessel properly: estimate the system volume, work out the expansion, apply the acceptance factor.

#What the vessel actually does

An expansion vessel is a steel shell split by a rubber diaphragm: system water on one side, a pre-charge of air or nitrogen on the other. As the system heats and the water expands, the diaphragm flexes and the air cushion compresses, absorbing the extra volume with only a modest pressure rise. On cooling, the cushion pushes the water back out and the pressure returns to the cold fill figure.

The vessel can only usefully absorb expansion between its pre-charge pressure and the system's maximum working pressure. That usable fraction is the acceptance factor, and it's why a vessel always needs to be several times bigger than the expansion volume itself.

#Symptoms of an undersized or failed vessel

The tell-tale pattern is the same whether the vessel is too small, has lost its charge or has a split diaphragm:

• Pressure climbs steeply as the system heats — from 1 bar cold to 2.5–3 bar hot, instead of a gentle rise
• The PRV discharges once the pressure hits the valve setting (usually 3 bar), often visible as drips from the discharge pipe outside
• The system reads low when cold, gets topped up, and the cycle repeats — and every fresh-water top-up drags in oxygen and accelerates corrosion

Before condemning the vessel, check the charge: with the system side at zero gauge pressure (vessel isolated or system drained), the Schrader valve should show the pre-charge. Water at the Schrader valve means the diaphragm has failed.

#Step 1: estimate the system volume

Vessel sizing scales directly with the water content of the system, so this is the figure to get roughly right.

Rule of thumb: around 10 litres per kW of boiler output is a reasonable ballpark for a typical radiator system — so a 15 kW system is somewhere in the region of 150 litres.

Better: count the radiators. The standard per-radiator figures give a tighter estimate:

Underfloor heating loops, cast iron column radiators and buffer stores all hold significant extra water that the per-radiator figures miss — work those out separately and add them on. Our free system volume calculator does the radiator-count method for you, and the same figure feeds straight into inhibitor dosing.

If you need a precise number, meter the water in on refill.

#Step 2: work out the expansion volume

Water heated from cold (about 10°C) to normal flow temperatures (about 85°C) expands by roughly 4%:

Expansion volume (L) = 0.04 × system volume

So a 125-litre system generates about 5 litres of expansion. That's the volume the vessel has to swallow — but it's not the vessel size, because of the acceptance factor.

#Step 3: apply the acceptance factor

The acceptance factor is the usable fraction of the vessel between fill pressure and maximum working pressure, worked in absolute pressures (gauge + 1):

Minimum vessel size (L) = (0.04 × system volume) ÷ (1 − Pi ÷ Pf)

where Pi = cold fill pressure + 1 and Pf = max working pressure + 1

The max working pressure must sit above the fill pressure and below the PRV setting — with a 3 bar PRV, 2.5 bar is a sensible design maximum, leaving margin so the valve isn't flirting with its lift pressure every heat-up.

Notice what the formula does as the gap between fill and max pressure narrows: the acceptance factor shrinks and the required vessel size rises sharply. A system filled to 1.5 bar needs a noticeably bigger vessel than the same system filled to 1 bar.

#Worked example: 15 kW system

A 15 kW boiler serving 5 singles and 7 doubles, cold fill 1.0 bar, 3 bar PRV:

1. System volume: (5 × 6) + (7 × 10) + 25 = 125 L (the 10 L/kW ballpark said ~150 L; the radiator count is the better figure)
2. Expansion volume: 0.04 × 125 = 5 L
3. Acceptance factor at 1.0 bar fill and 2.5 bar max: Pi = 2.0, Pf = 3.5 → 1 − (2.0 ÷ 3.5) = 0.43
4. Minimum vessel: 5 ÷ 0.43 = 11.7 L → fit the next standard size up, 12 L

Sizing right up at the 3 bar PRV limit (Pf = 4.0, acceptance 0.5) would give a 10 L minimum — which shows why designing to a margin below the PRV, not at it, is the safer habit. A slightly larger vessel is never a problem; an undersized one always is.

Run your own numbers in the free expansion vessel calculator — system volume, fill and max pressure in, minimum vessel size out.

#Pre-charge must match the cold fill (and the cold fill must suit the building)

Two pressures need to line up:

• Cold fill vs static head. The fill pressure has to exceed the static head of the system so the highest point stays positively pressurised — roughly 0.1 bar per metre of height from the gauge to the highest radiator, plus a margin. On most two-storey houses, 1 bar covers it comfortably; a three-storey townhouse may need more, which in turn changes the vessel calculation.
• Vessel pre-charge vs cold fill. The pre-charge should match the cold fill pressure. Charged below fill, the diaphragm is already partly compressed before the system even heats up, cutting usable capacity. Charged well above fill, the vessel won't accept any expansion until the system pressure rises to meet it. Always check and adjust the charge with the system side at zero gauge pressure — checking against a pressurised system gives a false reading.

Recheck the charge at every annual service. Vessels lose air over time, and a two-minute check beats a PRV callback.

#Combi boilers: when the built-in vessel isn't enough

Combis ship with an internal vessel of around 7–10 litres, sized for a typical system of roughly 100–125 litres at 1 bar fill. That's fine for the average install — but add a conservatory radiator or two, or fit the boiler to a large system it wasn't sized for, and the internal vessel comes up short. The symptoms are exactly the undersized-vessel pattern above, often appearing a season after the "quick extra rad" was added.

The fix isn't a bigger boiler vessel — it's an additional external vessel teed into the return, sized with the same calculation using the whole system volume (the internal vessel then contributes its capacity towards the total).

#A note on unvented hot water

Everything above is for sealed heating circuits. Expansion on the potable side of an unvented hot water cylinder is a different job: potable-rated vessels (or air-gap arrangements), sizing to the cylinder manufacturer's specification, and the whole package of safety controls under Building Regulations Part G3 — installed and commissioned by a G3-qualified operative and recorded on an unvented commissioning certificate. Don't size a potable vessel with heating-circuit rules.

Guidance only — always follow the boiler and vessel manufacturer's instructions and current standards, and remember any gas work involved must be carried out by a Gas Safe registered engineer.

#Do it faster with TradePlanr

TradePlanr's free expansion vessel calculator applies the exact formula above — 4% expansion, acceptance factor from your fill and max pressures — and the system volume calculator gets you the litres from a radiator count in seconds. No sign-up needed. When the job's done, issue the paperwork from the same app: commissioning certificates, service records and G3 unvented certificates, signed on screen and sent as PDFs. Free plan to start, then a flat £9.99/month — no per-certificate fees.