Hydronic System Water and Glycol Volume
Fill volume of a hydronic system and its water / glycol split.
Example
You enter
- Total pipe length (ft) 500
- Gallons per foot (gal/ft) 0.025
- Terminal / emitter volume (gal) 8
- Boiler + buffer tank volume (gal) 5
- Glycol fraction (0-1) 0.3
You get
- System volume 25.5
- Glycol charge 7.7 gal
- Water 17.9 gal
No published worked example is on file for this calculation. It is checked against a hand-worked derivation of its method and an independent reference test.
Details, formula, and sources
Scopesystem = pipe length x gallons per foot + terminals + boiler; glycol = system x fraction; water = system - glycol. 500 ft of 3/4 in copper (0.025 gal/ft) plus 8 gal of terminals and a 5 gal boiler is 25.5 gal, so a 30% glycol charge is 7.65 gal of glycol in 17.85 gal of water; a harsher 50% climate takes 12.75 gal of glycol. This fill sizes the expansion tank (expansion-tank) and the glycol order (glycol-mix gives the ratio). Distinct from radiant loop sizing.
Formula or tablepipe_gal = pipe_length_ft x gal_per_ft; system_gal = pipe_gal + terminal_gal + boiler_tank_gal; glycol_gal = system_gal x glycol_fraction; water_gal = system_gal - glycol_gal.
Source and editionHydronic system-volume identity by name (pipe plus terminals and boiler; split by the glycol fraction); first-principles arithmetic.
Free accessThe volume arithmetic is public first-principles; the gallons per foot comes from the pipe size and the fraction from the freeze target.
What governsEstimate. AHJ and licensed professional govern.
API field namespipe_length_ft, gal_per_ft, terminal_gal, boiler_tank_gal, glycol_fraction, system_gal, glycol_gal, water_gal
- Gallons per foot from the pipe size (~0.025 gal/ft for 3/4 in Type L copper; PEX and steel differ); the pipe schedule governspipe dimensions
- Glycol fraction from the freeze-protection target (glycol-mix gives the ratio)glycol-mix / freeze target