What is a Low Loss Header?

A Low Loss Header (LLH) is a Hydraulic Separator – it is a device that allows one part of a hydraulic system to work independently from another part, particularly necessary with multiple, pumped circuits operating together.

The function of the low loss header is to separate the two systems: primary heat source(s) and secondary loads. It allows equipment to operate at a constant flow rate in the primary circuit whilst flow rates and temperatures may vary in the secondary circuit.

This avoids any interaction between the two circuits and enables the primary circuit return temperature to represent the load on the system. It also permits return temperature sequence control of multiple heat sources.

What is a Low Loss Header: pic

How is hydraulic separation achieved?

The hydraulic separation is achieved by providing a very low velocity – hence very low pressure differential – path between connections within the low loss header body. This allows any difference in flows between the circuits to bypass through the body without influencing the characteristics of either the primary or secondary.

What are the benefits of a Low Loss Header?

FlexEJ Low Loss Header and piping kit

Heating (or cooling) systems will typically contain multiple pumped circuits – multiple heat sources and multiple heating circuits. As any pump will naturally search for an operating point on its own curve, multiple pumps will tend to interact leading to unbalanced and unstable flows to each part of the system.

The benefit of this hydraulic separation is improved efficiency and performance of the heating system. Heat sources are safeguarded from irregular flows, ultimately increasing the life of the equipment. If installed vertically, the low flow velocity within the low loss header will allow dirt to separate to the bottom and air to the top of the body for easy removal.

Key design features of Low Loss Headers

The fundamental features of a Low Loss Header include a body section which has a much larger cross sectional area than the piping connected to it. The active length of the body is generally greater than three times the body diameter.

Both these features ensure a low pressure drop in the body and thus good hydraulic separation.

The connections to the LLH body are generally made at the same diameter as selected for the circuit piping. We recommend the use of horizontal headers as discussed below.

Vertical LLH are typically designed to include an AAV at the top and dirt valve at the bottom.

What Are Horizontal Headers?

Horizontal Headers refer to the preferred way the multiple primary and secondary circuits connect to the low loss header as shown below:

What is horizontal header: diagram

Alternatively each circuit may be connected to the LLH body independently - without horizontal headers – with one pair of connections for each circuit. The potential issue with this approach is unforeseen flow patterns in the LLH body. Given the purpose of the LLH is to provide a very low resistance path between connections some system conditions may give rise to undesirable flow between circuits.

Horizontal headers solve this problem by bringing together similar circuits into one connection to the LLH body. This ensures the expected flow pattern within the LLH regardless of variations in individual circuit flows.

All FlexEJ’s standard designs are intended for use with horizontal headers – we are happy to quote and build any required design.

How do you calculate the size of a Low Loss Header?

To calculate a low loss header all you need are the primary and secondary powers or flow rates – and the 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. The calculator works with either multiple connections to the body – 1 pair per circuit – or combined in horizontal headers.

The calculator covers both connection schemes by providing 4 separate primary and secondary inputs plus a total which represents the horizontal header implementation. Fixed standard values for the properties of water are assumed when converting from power to flow rate.

Are there temperature changes with a Low Loss Header?

Yes – depending on the flow rates. As the low loss header works by allowing mixing of flow and return there must be a change in temperature of the mixed flow. Referring to the diagrams below, in all cases the flows in and out of the primary circuit must be equal so F1=F3 and in all cases the flows in and out of the secondary circuit must be equal so F2=F4:

LLH Calculations when Primary=Secondary flow

Case 1: Primary = Secondary flow

As the system is balanced there is no vertical flow in the LLH body so F1=F2 and necessarily F3=F4.

As a result the Primary flow temperature is equal to the Secondary flow temperature so T1=T2 and T3=T4.

There is no mixing.

LLH Calculations when Primary > Secondary flow

Case 2: Primary > Secondary flow

The simplest version of this case is when there is no secondary flow at all - so all the primary flow has to go through the LLH body and into the primary return.

For all other conditions where F1>F4 a proportion of the flow equal to F1-F4 goes through the LLH body.

The mixing is only at the primary return so T3 will need to be considered:

LLH Calculations when Primary > Secondary flow

Case 3: Secondary > Primary flow

The simplest version of this case is when there is no primary flow at all - so all the secondary return has to go through the LLH body and into the secondary flow.

For all other conditions where F4>F1 a proportion of the flow equal to F4-F1 goes through the LLH body.

The mixing is only at the secondary flow so T2 will need to be considered:


Do FlexEJ have their own range of Low Loss Headers?

Yes. FlexEJ have a range of standard sizes - you can choose from to order direct from this website or over the phone – and we also manufacture to your specifications for one-off orders as well as OEM suppliers.

FlexEJ standard LLH are designed for use with horizontal headers. All products are available from our web shop and ship from stock.

Please follow the link to the product page for full details, prices and shipping.

Can FlexEJ custom manufacture Low Loss Headers?

FlexEJ Custom Low Loss header manufacturing

Yes we do. We offer short lead times on manufacture of bespoke low loss headers for use without horizontal headers – individual circuit connections to the body - and any additional features you require.

Please use the form below to contact us or call 01384 881188 to talk to our technical sales team.

Need a detailed quote?

Get A Quote. On us.

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


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.

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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 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

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    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.
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