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The total water volume held in the closed hydronic loop, including boiler/chiller, piping, and terminal units.
Add up the water content of the boiler or chiller, all piping in the loop, and every terminal unit (radiators, fan coils, baseboard, etc.) to get the total system water volume in liters.
The fraction by which the system water expands in volume across its full operating temperature swing.
Water expands as it heats; the exact fraction depends on the system's minimum fill temperature and maximum operating temperature. A hydronic heating system's larger temperature range typically gives a higher expansion fraction than a domestic water heater.
The fraction of the tank's total volume that is actually usable to accept expanded water, based on the tank's fill and relief pressures.
A diaphragm or bladder tank can't accept its full nominal volume as expansion water — the acceptance factor accounts for the pressure cushion reserved between the system's fill pressure and its relief valve setting. Around 0.6 is a common planning figure for a typical diaphragm tank.
Minimum expansion tank total (nominal) volume
8.67 gal
This is a simplified sizing approximation — final expansion tank sizing should follow the ASHRAE/hydronic system design method (or the tank manufacturer's sizing software) accounting for actual fill pressure, relief valve setting, and system operating temperature range.
They open the calculator with your figures already in it
Hydronic Closed-Loop Expansion Tank Sizing Calculator: 8.67 gal — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Simplified hydronic expansion tank sizing: required volume = system water volume × water expansion fraction (over the system's operating temperature range) ÷ tank acceptance factor, distinct from a domestic water heater's thermal expansion tank sizing since a hydronic system has a much larger water volume and different operating temperature range
Inputs used
- Total Hydronic System Water Volume
- 130 gal
- Water Volumetric Expansion Fraction (Over Operating Temp Range)
- 0.04
- Tank Acceptance Factor (Typically ~0.6 for a Diaphragm Tank)
- 0.6
Confidence note: This is a simplified sizing approximation — final expansion tank sizing should follow the ASHRAE/hydronic system design method (or the tank manufacturer's sizing software) accounting for actual fill pressure, relief valve setting, and system operating temperature range.
What this calculation does not cover
- The expansion fractions offered here are water. A loop charged with 30 to 50 percent propylene or ethylene glycol expands appreciably more over the same temperature swing, and the fluid supplier publishes the figure for the concentration you are running. Size a glycol system on a water fraction and the relief valve opens at design temperature and dumps the glycol you paid for — the system then makes up with plain water and quietly loses its freeze protection.
- A tank of the right volume does nothing if it is precharged wrong. The air side must be set to the system's fill pressure at the tank's own elevation, with the water side empty, before the loop is filled. A tank shipped at 12 psi (83 kPa) and hung on a system filled to 25 psi (172 kPa) has already spent most of its acceptance volume before the water is even warm, and the acceptance factor above has stopped describing it.
- Where the tank ties into the loop is not a sizing question, but it decides whether the tank works. The connection point is the one place in the system whose pressure the pump cannot change — pipe the tank into the pump's suction side and the pump adds its head to the rest of the loop, put it on the discharge side and the pump subtracts it, which can drop the pressure at the top of the system or at the pump inlet far enough to draw air or cavitate on a tank that was sized correctly.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.1.1
Regulatory standards & verification citations1
- Simplified hydronic expansion tank sizing: required volume = system water volume × water expansion fraction (over the system's operating temperature range) ÷ tank acceptance factor, distinct from a domestic water heater's thermal expansion tank sizing since a hydronic system has a much larger water volume and different operating temperature range
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