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Expansion Tank Selection

Expansion tank - article

Expansion Tank Sizing Calculator per PN-B-02414

Professional calculator for sizing membrane expansion vessels in closed central heating systems. Performs the complete calculation procedure from Polish standard PN-B-02414:1999 — from usable volume Vu through initial working pressure pR to the final selection from standard tank sizes.

Designed for HVAC designers, installers and homeowners planning a boiler room or a heating system upgrade. Results can be exported as an A4 PDF ready to be attached to design documentation.

How to use the calculator in 3 steps

1

Enter the total system water content V in m³. Estimate from the building's design heat demand (NOT the boiler's nominal power, which is usually oversized): typically 50–70 W/m² × floor area = design power, then design power × 12–15 l/kW for panel radiators. For a 150 m² post-2005 house this gives about 150–200 l. Don't forget to add a buffer tank or hydraulic separator if present.

2

Enter the system supply temperature — typically 70–80 °C for panel radiators, 35–45 °C for underfloor heating. The calculator automatically reads the specific volume increase Δv from the water property tables for that temperature.

3

Set the remaining parameters: maximum pressure (usually 3 bar — the safety valve opening setting), tank pre-charge pressure (factory value 1.5 bar) and operational leakage (0.5–1% per the standard). Results update automatically.

What the expansion tank calculator returns

The calculator returns the full set of values from PN-B-02414 plus the final selection from standard commercial tank sizes:

  • Vu — minimum usable volume of the tank, i.e. the amount of water the tank must accept due to thermal expansion of the system.
  • Vn — minimum total tank volume, accounting for maximum and pre-charge pressures.
  • VuR — usable volume including the operational reserve for routine water losses.
  • pR — initial working pressure of the cold system, to be set on the boiler room manometer.
  • VnR — total tank volume with operational reserve. This value drives the selection from the standard size range (8, 12, 18, 25, 35, 50, 80, 100, 140, 200, 250, 300, 400, 500, 600, 800, 1000, 1500 dm³).

Input fields — what to enter and where to get it

System volume V [m³]

Total water content of the entire heating system: boiler or heat exchanger, pipes, radiators, buffer tank, hydraulic separator. In practice estimate from the building's design heat demand (not from boiler rated power): for a post-2005 house take 50–70 W/m² × floor area, then design power × 12–15 l/kW for panel radiators. For a 150 m² house this gives about 150–200 l.

Supply temperature [°C]

Design water temperature at supply. For older radiator systems 75–80 °C, for new systems with condensing boilers 55–70 °C, for underfloor heating 35–45 °C. The calculator accepts integer values in the 41–100 °C range.

Maximum pressure [bar]

Safety valve opening setting, typically 3 bar for residential systems and 6 bar for industrial boiler rooms. The value must match the actual safety valve installed and must not exceed the design pressure of the most pressure-sensitive component in the system.

Tank pre-charge pressure [bar]

Factory pre-charge (typically 1.5 bar) or adjusted to system statics. The standard requires: p ≥ 0.1 × h + 0.2, where h is system height in metres. For a two-storey building of 6 m height the minimum is 0.8 bar.

Operational leakage E [%]

Reserve for routine water losses in the system: air bleeding, seal leaks, degassing. Standard PN-B-02414 recommends 0.5–1%. For new tight systems use 0.5%; for older systems or those with many automatic air bleeders — 1%.

Warning: CH tank ≠ DHW tank

This calculator sizes a tank for a closed central heating system, not for domestic hot water. DHW expansion tanks require a hygienic certificate and use different membrane materials. Never use a CH tank for drinking water or vice versa — a standard EPDM membrane without a hygienic certificate may release substances affecting water quality.

Frequently asked questions about expansion vessel sizing

How do you size a diaphragm expansion vessel for a heating system?

Sizing per PN-B-02414 starts from the water volume of the system V and the supply temperature, from which the useful volume Vu is determined (the water taken up by thermal expansion of the system content). Then, accounting for the maximum pressure (the safety valve setting) and the vessel's pre-charge pressure, the total volume Vn and VnR with operating reserve are calculated — and it is VnR that is used to select a vessel from the commercial range.

What pre-charge pressure should you set in the vessel?

The pre-charge pressure must cover the water column of the system: the standard requires p ≥ 0.1 × h + 0.2 [bar], where h is the system height in metres. For a building 6 m tall this gives a minimum of about 0.8 bar; vessels are factory-set to about 1.5 bar. The pressure is checked and set cold, on the gas side of the vessel disconnected from the system.

How large an expansion vessel for a single-family house?

It depends on the system water content, not directly on floor area. For a house of about 150 m² built after 2005, the system volume is typically 150–200 l, which usually leads to an 18–25 l vessel (radiators, 80 °C supply). Underfloor heating at a lower temperature produces a smaller volume increase. The calculator computes the exact size once you enter V, the temperature and the pressures.

Can a heating expansion vessel be used for domestic hot water?

No. Domestic-hot-water vessels require a hygiene certificate (PZH) and a diaphragm made of a material approved for contact with drinking water. The diaphragm of a heating vessel (usually non-certified EPDM) can impair water quality. Dedicated, certified vessels are used for domestic hot water.

Where do you get the water volume of the system?

Preferably from the design; in practice it is estimated from the building's design heat demand (not from the boiler's nominal output, which is often oversized). For a building after 2005 assume 50–70 W/m², then about 12–15 l/kW for panel radiators. A buffer tank or hydraulic separator, if present, must be added.

Why an operating reserve for water losses?

A system loses water through venting, degassing and minor leaks, so PN-B-02414 recommends a reserve of 0.5–1% of the volume. For new, tight installations 0.5% is enough; for older ones or those with many automatic air vents, 1% is assumed. The reserve prevents too-frequent topping up of the system.

Related calculators for boiler room design

When designing a complete boiler room or upgrading a heating system, our other calculators may also be useful:

Safety valve sizing per PN-B-02414Panel radiator sizing per PN-EN 442Heating pipe diameter sizingWater underfloor heating sizing

Want the theory and full derivation?

A full guide with step-by-step formula derivation, a worked example for a 150 m² house, a Δv table for different temperatures, a description of tank types (CH/DHW/solar), installation tips and the most common mistakes — in our article:

Expansion vessel sizing — formulas, PN-B-02414, calculator
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