Formula & worked example
Ve=Vs(ρcold/ρhot−1); Vtank=Ve/[1−Pfill,abs/Prelief,abs].
A 500 L loop expanding 2.5% creates 12.5 L expansion volume before pressure acceptance is applied.
HVAC design calculator · Updated Sep 17, 2026
Hydronic Expansion Tank Calculator — Water Expansion, Acceptance & Tank Volume
Estimate closed-loop hydronic expansion-tank acceptance and nominal tank volume from system water volume, cold/hot density, fill pressure and relief pressure.
Inputs
Live result
Water expansion 12.320329 L (2.4640657% of system volume)
Pressure acceptance fraction 0.37376191 (37.376191%)
Minimum ideal tank volume 32.963039 L
Tank volume with margin 36.259343 L
Assumptions & limits
- Pressures are converted to absolute before applying the ideal-gas acceptance relation.
- Use manufacturer sizing methods for diaphragm precharge, glycol mixtures, code margins and special systems.
Frequently asked questions
What does this Hydronic Expansion Tank Calculator calculate?
Estimate closed-loop hydronic expansion-tank acceptance and nominal tank volume from system water volume, cold/hot density, fill pressure and relief pressure.
Can I use the result for final HVAC selection?
Use the result as a transparent engineering cross-check. Confirm final selections against measured design conditions, manufacturer data, applicable standards and local code.
Why are the assumptions shown?
HVAC shortcuts can look precise while hiding density, efficiency, free-area or pressure assumptions. MAXScanner keeps the governing inputs visible so the result can be audited.