DN125 (5") low loss header — hydraulic separation for 1,230 kW systems
What is a low loss header and why specify the UKGP-LLH-11?
A low loss header — sometimes called a hydraulic separator, hydraulic header, common header or buffer bottle — decouples the primary boiler circuit from the secondary heating circuits so that primary and secondary pumps no longer fight each other. The UKGP-LLH-11 DN125 (5") low loss header is sized for up to 1,230 kW at a recommended terminal ΔT of 11 K, with the cross-sectional area calculated to keep internal velocity below 0.1 m/s. That low velocity is what makes a low loss header work: it lets air rise and be vented, lets sludge settle and be drained, and most importantly stops one circulator from pulling flow through another circulator in reverse.
DN125 (5") low loss header sizing rules
The classic sizing rule is 3D — the body diameter is at least three times the largest branch connection — and the UKGP-LLH-11 comfortably meets that on the duty bracket shown. To check it against your job: take the total primary flow in m³/h, divide by the rated flow figure on the spec table, and confirm the answer is less than 1.0. If you are over 1.0 step up to the next body size. For staged or modulating multi-boiler installations, size on the maximum simultaneous primary flow, not the headline kW; condensing boilers running at low fire produce far higher flow per kW than the nameplate suggests.
Threaded vs flanged connections
The DN125 (5") low loss header ships with the connection type shown on the specification panel. Threaded BSP variants suit small commercial plant rooms where copper or press-fit primary pipework lands directly into the header. Flanged PN16 variants are the right choice for steel-pipe boiler rooms, district-heating sub-stations and any installation where the header may need to be unbolted for service. Flange drilling is to EN 1092-1, so the UKGP-LLH-11 mates one-for-one with standard butterfly valves, strainers and DPCV control valves without bespoke adapters.
Air vent, dirt drain and insulation jacket
Every UKGP low loss header is supplied with an air-elimination tapping at the top and a sludge-blow-down tapping at the base. We recommend fitting a Flamco-style auto air vent at the top so dissolved air liberated by the velocity drop is removed continuously, and a full-bore drain ball valve at the base so magnetite and commissioning debris can be flushed without taking the system off-line. A pre-formed 50 mm EPP insulation jacket is available as an option and is strongly recommended on chilled water duty to prevent condensation on the steel body.
Typical applications for the UKGP-LLH-11
DN125 (5") low loss headers are specified on cascaded condensing boiler plants, heat-pump primary circuits where the pump curves of the heat pump and the buffer cylinder do not match, mixed-temperature systems with separate UFH and radiator loops, and any retrofit where an old single-pump system is being expanded with secondary circulators. Pair the UKGP-LLH-11 with our matching air and dirt separator to take both air and sludge out in a single plant-room footprint, and with our chemical dosing pot to add inhibitor cleanly after commissioning — both linked below.
Price and dispatch for the UKGP-LLH-11
The UKGP-LLH-11 DN125 (5") low loss header is £1,845 ex VAT and is held in UK stock for next-working-day delivery on orders placed before 3 pm. Specials — larger DNs, PN25 flanging, insulated jacket, branded for the M&E contractor — are quoted inside one working day. See our complete guide to low loss header sizing and our DN-by-DN spec table further down this page for full technical data.