Sizes a sealed-system expansion vessel: Vv = Ve / (1 − P0/Pmax), where Ve = Vs × e.
Tender-stage only. Verify with vessel manufacturer data and project spec.
For design guidance only. Always verify with a qualified engineer and vessel manufacturer data.
About this expansion vessel sizing calculator
This free expansion vessel sizing calculator sizes a sealed-system expansion vessel for a heating circuit from its water volume, operating temperatures and system pressures. It is built for UK building-services engineers, heating designers and contractors who need a quick, defensible vessel selection at tender stage. Enter the total system water volume (Vs), the minimum and maximum temperatures, any glycol mix, the static height, cold-fill margin, pre-charge and safety-valve setting, and it returns the expanded volume, acceptance fraction and the calculated vessel volume, then suggests the next standard vessel size up.
How the vessel size is calculated
Sizing follows the CIBSE Guide B1 / BS 7074-1 sealed-system method. The expanded water volume is Ve = Vs × e, where the expansion factor e is derived automatically from the minimum and maximum temperatures via the system water density (Tanaka 2001 / TSD formula), or entered manually. A glycol mix uplifts e per CIBSE Guide C Table 4.2. The acceptance fraction is 1 − P0(abs) / Pmax(abs), with the pre-charge P0 taken at (static height ÷ 10.2) + cold-fill margin and Pmax set at the safety-valve pressure minus the below-SV margin. The required vessel volume is then Vv = Ve / acceptance. Results are indicative for design only. Always verify against vessel manufacturer data and project spec.
Frequently asked questions
How is the expansion factor worked out?
In auto mode the calculator derives the expansion factor from the change in water density between your minimum and maximum temperatures (Tanaka 2001 / TSD formula), cross-checked against the CIBSE Guide B1 / BS 7074-1 tables. You can also enter the factor manually, typical heating systems sit around 0.03–0.05.
What pre-charge pressure does it use?
By default the pre-charge (P0) is set to the cold-fill pressure, calculated as static height ÷ 10.2 plus your cold-fill margin (1 bar ≈ 10.2 m of water head). You can override this with a custom value; setting P0 below cold fill triggers a waterlog warning.
Does it account for glycol?
Yes. Selecting a 25%, 35% or 50% ethylene-glycol mix applies an uplift to the water expansion factor per CIBSE Guide C Table 4.2, increasing the calculated expanded volume and vessel size accordingly.