Size a solar expansion vessel to swallow the entire collector content as steam, not just the fluid's thermal expansion. Add the collector volume, the expansion volume and a 3 litre reserve, then multiply by the pressure factor. A typical 5 m² domestic array needs 25 litres, against the 12–18 litres a boiler circuit of the same volume would use.
Why is a solar expansion vessel different from a heating one?
Because a solar loop is designed to boil. When the store is full and the pump stops, the absorber keeps absorbing: within minutes the fluid inside it flashes to steam and pushes the remaining liquid out of the collector and down into the pipework. The vessel has to accept that displaced volume as well as normal thermal expansion, or the 6 bar safety valve lifts and the system loses fluid — and with it, frost protection.
A boiler circuit never does this. Its vessel handles the roughly 4–8% volume change of water heated to 80 °C and nothing more. Fit that vessel to a solar loop and you will be topping up after every hot, unused week — the classic cause of a system that loses pressure every summer, and a recurring theme in overheating and stagnation problems.
What is the sizing formula?
The accepted European approach adds three volumes and applies a pressure factor:
V = (V_reserve + V_expansion + V_vapour) × (p_end + 1) / (p_end − p_pre)
| Term | Meaning | Typical value |
|---|---|---|
| V_reserve | Liquid held permanently in the vessel | 3 L, or 0.5% of system volume |
| V_expansion | Thermal expansion of the fluid charge | System volume × 0.08 for glycol at 20→120 °C |
| V_vapour | Collector content plus any pipework above the collector connection | From the collector datasheet, typically 0.8–1.5 L per m² |
| p_end | Final pressure — safety valve setting minus 10% | 5.4 bar with a 6 bar valve |
| p_pre | Vessel pre-charge | Operating pressure minus 0.3 bar |
The expansion vessel glossary entry covers the component itself; what matters here is that V_vapour is the term boiler practice omits, and it is often a third of the total.
Worked example: 5 m² flat plates, 8 m static height
| Input | Value |
|---|---|
| System fluid volume (collectors, pipes, coil) | 40 L |
| Collector content | 5 L |
| Static height | 8 m |
| Operating pressure (1.0 + 0.1 × 8) | 1.8 bar |
| Pre-charge (1.8 − 0.3) | 1.5 bar |
| End pressure (6 bar valve − 10%) | 5.4 bar |
V_reserve = 3 L. V_expansion = 40 × 0.08 = 3.2 L. V_vapour = 5 L plus about 1 L of pipe above the connection = 6 L.
Sum = 12.2 L. Pressure factor = (5.4 + 1) / (5.4 − 1.5) = 1.64.
V = 12.2 × 1.64 = 20 L → select the next standard size, 25 L.
For a 10 m² combisystem array the same calculation typically lands on 40–50 L. Rounding up is always the cheap decision: an oversized vessel costs a few euros more, while an undersized one costs a glycol charge every summer.
What pre-charge and fill pressure go with it?
Set the pre-charge with the vessel isolated and the fluid side drained, using a tyre gauge and a hand pump. Then fill the loop to roughly 0.1–0.3 bar above the operating pressure so the vessel holds its reserve volume — around 1.9–2.0 bar in the example above.
Two supporting details decide whether the vessel survives:
- Protect the diaphragm from steam. Fit a pre-vessel, or leave two to three metres of uninsulated copper between the tee and the vessel, so vapour condenses before it reaches the rubber. Diaphragms cook above roughly 70 °C sustained.
- Mount it so hot fluid does not convect in. Solar vessels are normally hung below their connection point, with the pipe rising to the tee.
How do I check an existing vessel is still right?
Once a year, and after any hot spell that pushed the collectors into stagnation:
- Read the cold system pressure and compare with the design fill pressure.
- Isolate the vessel, drain its fluid side and check the pre-charge with a gauge; fluid at the Schrader valve means a failed diaphragm.
- Look for dried glycol under the safety valve discharge — evidence the vessel was too small or too flat to absorb the last stagnation.
- Re-run the sizing calculation if collectors have been added; enlarging an array without enlarging the vessel guarantees a summer of top-ups.
Specify vessel capacity from the collector content of the actual array rather than a rule of thumb, because the vapour term scales with collector type: evacuated tubes hold less fluid per m² than flat plates, but reach higher stagnation temperatures, so the two effects have to be calculated, not assumed. It is also the cheapest insurance in the whole loop — the vessel is what keeps the antifreeze inside the system.
Frequently asked questions
What size expansion vessel does a solar hot water system need?
Roughly 18–25 litres for a 4–6 m² domestic array, and 40–50 litres for a 10 m² combisystem. The exact figure comes from adding the collector content, the fluid's expansion and a 3 litre reserve, then applying the pressure factor set by the safety valve and the vessel pre-charge.
Can I use a normal heating expansion vessel on a solar loop?
No. Standard heating vessels are sized only for thermal expansion and their diaphragms are not rated for the temperatures a stagnating collector produces. Use a solar-rated vessel, sized to accept the whole collector content as steam.
What pre-charge pressure should a solar expansion vessel have?
About 0.3 bar below the system's operating pressure, which is itself roughly 1 bar plus 0.1 bar per metre of static height. For a two-storey house that means a pre-charge near 1.5 bar and a cold fill pressure near 2 bar.
How do I know my expansion vessel is too small?
The system loses pressure after every hot, low-use period, and you find dried glycol crystals below the safety valve discharge. A vessel with a correct pre-charge and an intact diaphragm that still cannot prevent this is simply undersized for the collector volume.