SettingsSettings for this calculationUS
The hydronic system's design flow rate through the separator.
Use the total circulator flow rate at the point in the system where the air/dirt separator is installed.
The maximum velocity through the separator's connection for effective air and dirt removal — commonly around 0.3 m/s (about 1 ft/s).
Air and dirt separators rely on low velocity to give entrained air bubbles and particulates time to separate out — commonly around 0.3 m/s (roughly 1 ft/s), well below typical pipe velocities elsewhere in the system.
Minimum connection diameter
5.73 in
Effective air/dirt separation requires low velocity at the separator itself — confirm the manufacturer's rated maximum velocity and connection size for your specific separator model rather than assuming a generic value.
They open the calculator with your figures already in it
Hydronic Air/Dirt Separator Sizing Calculator: 5.73 in — 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)
- Minimum connection diameter = √(4×system flow rate ÷ (π×maximum velocity)), sizing the separator's pipe connection to keep flow velocity low enough (commonly under ~0.3-0.6 m/s) for effective air and particulate separation
Inputs used
- System Flow Rate
- 79 gal/min
- Maximum Velocity for Effective Separation
- 0.98 ft/s
Confidence note: Effective air/dirt separation requires low velocity at the separator itself — confirm the manufacturer's rated maximum velocity and connection size for your specific separator model rather than assuming a generic value.
What this calculation does not cover
- Position beats size here. Air leaves solution where the water is hottest and the pressure lowest — on most systems the boiler or chiller outlet, upstream of the circulator. Put the same separator on the cool return and it sees air still dissolved and passes it straight through. No connection diameter compensates for the wrong location; the low velocity sized here only earns its keep once the bubbles actually exist.
- This is a connection diameter, not a pressure drop. Every separator adds head to the circuit — the vessel, the coalescing medium inside it, and the expansion and contraction at each end — and none of that comes back from a velocity formula. A pump selected on pipe friction alone can land short of its duty point once the separator, the strainer and the dirt the separator will eventually be holding are all counted in.
- The dirt half is a different problem from the air half. What blocks a modern condensing boiler's heat exchanger is magnetite — fine ferrous sludge that a gravity pocket catches poorly and a magnet catches well — and no velocity calculation tells you whether you need the magnetic version. Nor does it size the dirt pocket or set how often it is flushed: a separator that is never blown down is a reservoir, not a filter.
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This result is a specification — 5.73 in — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
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
- Minimum connection diameter = √(4×system flow rate ÷ (π×maximum velocity)), sizing the separator's pipe connection to keep flow velocity low enough (commonly under ~0.3-0.6 m/s) for effective air and particulate separation
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