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Low Loss Headers

FlexEJ's range of Low Loss Headers, also known as Hydraulic Separators, covers popular sizes from DN32/70kW to DN150/1,480kW - with larger sizes and multiple circuit designs to order. These robust and cost effective units are manufactured in our UK factory and the following standard sizes are available at direct-buy prices from our web shop on a 24hr despatch basis.


Low Loss Header - Hydraulic Separator

What is a Low Loss Header?

Low Loss Headers decouple the boilers from the system...

  • Materials
    Carbon steel with BSP threaded connections or PN flanges.
  • Application
    Heating water services, boiler arrays.
  • Rating
    HP Range - 10 Barg @ 110°C.
    LP Range - 6 Barg @ 110°C.
  • Web Shop
    Available online with next day delivery from our web shop.

Hydraulic Separation in HVAC systems

1 Hydraulic Separation2 Do I need a low loss header?3 Boiler cascades4 Flow rate & velocity5 Need something different?6 Let us help

HVAC systems helpIf a system has more than one pump operating there is the possibility of the pumps interacting with one another which results in instability. To avoid this a hydraulic separator – a low loss header – is introduced into the system.

Low Loss Header temperature variationsNot always, it depends on the system design and components. Low Loss Headers do introduce mixing of flow and return which results in temperature variations, this should be taken into consideration in the overall system design and control.

LLH Boiler CascadingA very common application of Low Loss Headers is with boiler cascades, the hydraulic separation decouples the boiler cascade from the system. This should allow, with proper system design, the boilers/boiler circuit flow to optimise at best efficiency for any system conditions.

Low loss header flow rate velocityThe flow rate and design flow velocity normally determine the size of the Low Loss Header body. Commonly the total boiler power and temperature difference are used as indicative of flow rate but secondary circuit flow rates may be higher and should be considered.

Custom-designed buffer vesselFlexEJ can design and build to order large, multi-circuit low loss headers, manifolds, buffer vessels and piping spools. Please let us know what you need.

Let FlexEJ help with low loss header design Call our HVAC specialists on +44 (0) 1384 881188 or email us at sales@flexej.co.uk or use LIVE CHAT on this web site - we are here to help, please get in touch.

Can't see what you want? Need help?

Call us. Or use Live Chat.

For instant advice. We can custom-design and engineer almost any low loss header configuration you need for delivery in the UK or worldwide. Call our expert team on +44 (0) 1384 881188. Or email us sales@flexej.co.uk

FlexEJ's Low Loss Headers

Robust and great value for money, FlexEJ's HVAC Low Loss Headers — also known as Hydraulic Separators — are manufactured by us in the UK to our own design. DN32 to DN150 are standard stock and available on a 24hr despatch basis directly from our web shop. Other configurations are available. FlexEJ can also manufacture to meet specific client requirements, please get in touch.

Low Loss Header Applications

The hydraulic separation provided by a low loss header allows multiple pumped circuits to work independently from one another - which is particularly important in achieving the most efficient operation and control of condensing boilers and boiler cascades.

The use of a low loss header is recommended to minimise interaction between primary circuits and the secondary circuits, the primary typically working at a constant flow rate and the secondary at varying flow rate and water temperature. For a full discussion of low loss header design please see our what is a low loss header page.

LP Low Loss Header Range

Designed for simpler, lower power systems these low loss headers have threaded connections from DN32 up to DN50 and PN6 flanges on the DN65. All include a single thermopocket. Vent and drain connections are supplied plugged, there is an optional valve kit.

FlexEJ LP Low Loss Headers

LP Features

All our LP low loss headers have:

  • Rating of 6 Barg at 110°C
  • Carbon steel body with BSP threaded connections except DN65 flanged PN6
  • Optional valve kit comprising Flamco AAV with integral shut off valve and DN25 drain valve
  • Single temperature probe port supplied with thermopocket and clip
  • DN32 to DN65 available from stock as standard
Connection Body Size (DN mm) Flow Rate (m3/Hr) Power (kW) Code
Buy Now
DN32-R1¼" 65mm 3.0 70kW FEJLLH032BSP
DN40-R1½" 80mm 4.7 110kW FEJLLH040BSP
DN50-R2" 100mm 6.9 160kW FEJLLH050BSP
DN65-2½" 125mm 12.0 280kW FEJLLH065PN6

HP Low Loss Header Range

The high power range is designed for larger, more complex commercial systems. All sizes flanged PN16 with a temperature probe ports on all connections as standard. Supplied with Flamco AAV and DN25 drain ball valve.

FlexEJ HP Low Loss Headers

HP Features

All our HP low loss headers have:

  • Rating of 10 Barg at 110°C
  • Carbon steel body with PN16 flanges
  • Flamco AAV with integral shut off valve
  • DN25 drain valve
  • Temperature probe ports on all connections
  • DN50 to DN150 available from stock as standard
  • Custom sizes, multiple connection adaptors and specifications
Connection Body Size (DN mm) Flow Rate (m3/Hr) Power (kW) Code
Buy Now
DN50-2" 100mm 8.6 200kW FEJLLH050
DN65-2½" 125mm 13.3 310kW FEJLLH065
DN80-3" 150mm 18.9 440kW FEJLLH080
DN100-4" 200mm 34.0 790kW FEJLLH100
DN125-5" 250mm 52.9 1,230kW FEJLLH125
DN150-6" 250mm 63.6 1,480kW FEJLLH150

Power rating and flow rate

The power rating quoted is a nominal value based on an assumed flow rate; the low loss header will perform above or below this figure.

The tables above show performance for the FlexEJ Low Loss Headers with a 20°C temperature difference at flow rates typical for the connection sizes. Please contact us for further application assistance.

All FlexEJ standard HP LLH are rated at 10 Barg at 110°C, standard LP LLH are rated at 6 Barg at 110°C, all calculated to PD5500 Category 3 and are SEP category under the PED.

Low Loss Header Calculation and Sizing

FlexEJ Low Loss Header Calculator

To calculate a low loss header all you need are the primary and secondary powers or flow rates – and the brand-new FlexEJ Low Loss Header Calculator.

This calculator will allow you to size a low loss header based on the flow or power in your primary and secondary circuits.

Manifold Connections, Vents and Drains

We suggest the use of distribution headers or manifold piping on all connections to the low loss header rather than making multiple connections to the LLH body itself. We will be pleased to quote manifolds on request.

The low flow velocity in the low loss header body promotes separation of air and dirt from the flow. FlexEJ provide a DN25 ball valve connection for dirt removal and a high quality automatic air vent valve with all HP low loss headers, the LP range have plugged connections and an optional valve kit.

Low Loss Header Design and Build

Low Loss Header Robot Welding

FlexEJ has full in-house pressure vessel and buffer vessel design and manufacturing capability. We will be pleased to quote any low loss header size, configuration or buffer vessel you may require.

If you need something even larger or more complex, we will be pleased to engineer and manufacture to your exact specifications - with connections for multiple circuits, integrated air and dirt sections, any diameter or length or any other technical requirements can usually be met, as required.

Custom built or delivered in 24hrs?

You are very welcome to call to talk through your requirements or any technicalities that are concerning you — on +44 (0) 1384 881188 — or please use the Get A Quote form below or order direct at UK factory prices from our low loss header shop pages with 24hr shipping as standard on all stock items.

Not sure what flange spec you need?

Use the FlexEJ Flange Finder to find out

FlexEJ stock a full range of specs and types for our bellows. Or call our expert team on +44 (0) 1384 881188.

Frequently Asked Questions

Q. Does a Low Loss Header work as an air and dirt separator?
A. Yes, to some extent, as the flow velocity in the body is low. We provide a Flamco AAV and DN25 drain valve on all FlexEJ low loss headers. However, separation is not as efficient as using a dedicated air/dirt separator as the main low velocity section in the LLH is bounded by turbulent cross flows.
Q. Should a horizontal manifold be used to connect to the Low Loss Header?
A. Opinions vary but ideally all the primary circuit and secondary circuit flow and returns should be brought together in horizontal manifolds before connection to the low loss header.
Q. Where do the primary and secondary circuits connect to a Low Loss Header?
A. On a vertical low loss header the returns connect at the bottom and the flows at the top. If the connections are staggered side to side then the lower pair are normally the primary side.
Q. What size Low Loss Header do I need?
A. Sizing can be expressed in terms of flow rate or power. In heating systems, the total boiler power in the primary circuit is often taken as the sizing criteria. The power rating is directly proportional to the flow rate and temperature difference as given by m.Cp.Δt – Power (kW) = Mass flow rate (kg/s) * Specific Heat of Water (kJ/kg°K) * Temperature difference (°K). Our product table above shows both nominal flow rate and power ratings.
Q. Is there a Low Loss Header calculation?
A. We have a calculator set up, please get in touch with the details of your system and we will be pleased to help.
Q. How is a Low Loss Header designed?
A. Based on the flow rate through the low loss header the body is sized to achieve a very low flow velocity – typically less than 0.3m/s. This low velocity ensures a low pressure drop between flow and return.
Q. Is a Hydraulic Separator the same as a Low Loss Header?
A. Yes – strictly speaking hydraulic separation between circuits is achieved using a low loss header.
Q. What is a Low Loss Header?
A. A low loss header (LLH) provides hydraulic separation between multiple pumped circuits. This allows each circuit to operate independently and to be controlled for optimum efficiency. The header presents very low pressure drop bypass between the flow and return. FlexEJ low loss headers are provided with sensor ports on all four branches so whatever control system is being implemented you will have the port you need.
Q. How do you identify flanges?
A. Our Flange Finder contains details for many common flanges. Typically, to be able to make the identification you need to know the bore size or outside diameter and number of bolts. For technical assistance contact us or use our Live Chat facility.

Please visit our full FAQ page to see all our frequently asked questions.

Can't quite find the right bellow?

Try the FlexEJ Bellows Finder

Or call our expert team on +44 (0) 1384 881188


Who can you talk to?

Try our sales and customer service team on 01384 881188

They don't bite. And they know what they're talking about. Call the team any time from 8:30am to 3:30pm weekdays, 12:30pm Fridays or use LIVE CHAT if you would prefer a quick text answer. For more complex queries, send us any pro-forma documentation using the get a quote form below or email us sales@flexej.co.uk.

Call Us

Need a detailed quote?

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We will provide full technical details together with, pricing and shipping timescales. For more complex engineered quotes, preliminary sketches and calculations will be available.

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For instant advice use the Live Chat link to message one of the sales team. Or call them direct on +44 (0) 1384 881188

Get A Quote

Please complete the short form here and we'll get back to you within 24hrs Monday to Friday (quicker if marked urgent).

Alternatively, you can also call our expert team directly on +44 (0) 1384 881 118 from 8:30AM to 4:30PM weekdays (12:30PM Fridays), who will be only too pleased to talk through your requirements and technical queries over the phone.

Visitors are also welcome Monday to Friday although please get in touch beforehand to let us know you're coming.

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    Industry experience

    FLEXEJ Ltd was formed in 2002 by two professional engineers who brought together an already highly respected design capability with business and commercial skills gained in large multinational companies.
    • Our focus has always been on application and design – helping our customers find the best value solution.
    • We have a strong engineering and manufacturing team with experience in petrochemicals, refining, nuclear, desalination though to HVAC.
    • The team is also very familiar with meeting complex requirements for documentation, in process NDT and test on projects. We have supplied many large end-users such as EDF, Exxon, Shell, Reliance & QatarGas working with major contractors such as JGC, Foster Wheeler & Technip.
    FlexEJ Industry Experience