Low Loss Headers — Hydraulic Separators for Multi-Boiler Systems

Technical guide

What is a low loss header?

A low loss header (sometimes called a hydraulic separator or buffer header) is a vertical or horizontal vessel installed between the primary heat source circuit — typically the boilers or chillers — and the secondary distribution circuits that serve emitters, AHUs, calorifiers or underfloor heating. Its job is to hydraulically decouple the two sides so that the primary and secondary pumps cannot fight one another, regardless of how many circuits are running at any given moment.

The header is sized so that fluid velocity inside it falls to roughly 0.1 m/s. At that velocity, the pressure drop across the header is effectively zero, while entrained air rises to the top for venting and heavier dirt and magnetite settle at the base for blowdown.

Why hydraulic separation matters

In modern multi-boiler and variable-flow systems, each circuit has its own pump with its own duty curve. Without separation, the highest-pressure pump dominates: weaker secondary pumps are overpowered, modulating boilers see unstable return temperatures, and condensing efficiency collapses. A correctly sized low loss header gives every circuit an independent zero-pressure reference, so each pump operates at its true design duty.

How to size and install correctly

• Match primary and secondary flow rates

  Ideally the secondary flow should equal or slightly exceed the primary. If secondary flow is higher, hot return water mixes back into the secondary flow — boilers stay condensing and ΔT is preserved.

• Size for ~0.1 m/s internal velocity

  This keeps the pressure drop across the header negligible. Our calculator below applies this rule along with the standard 3× DN rule (header diameter ≈ 3× connection DN) used across UK plant rooms.

• Allow 3× DN between tappings

  Vertical pitch between primary flow, primary return, secondary flow and secondary return tappings should be at least three connection diameters apart to prevent short-circuiting and ensure clean separation.

• Vent, drain and insulate

  Always specify an automatic air vent at the top, a drain at the bottom and full insulation. A magnetic dirt pocket or downstream side-stream filter further protects boilers and plate exchangers.

• Commission with pump speed control

  On variable-speed primary pumps, control off return temperature or ΔT — not differential pressure across the header (which is near zero by design). Set secondary pumps to deliver the design flow at the index circuit.

Hydraulic balancing checklist

A low loss header only delivers its full benefit when the surrounding circuits are themselves balanced. Before commissioning:

• Calculate design flow for every secondary circuit from kW ÷ (1.16 × ΔT).
• Select each pump on the index circuit duty and verify the operating point sits left of the curve knee.
• Fit a double-regulating valve or PICV on every branch and proportionally balance to within ±10% of design flow.
• Confirm primary flow ≥ sum of running secondary flows under worst-case load.
• Record return temperatures at the header — primary return should track the lowest secondary return when boilers are condensing correctly.

Frequently asked questions