Low Loss Header Calculator

The FlexEJ low loss header calculator will allow you to size a low loss header based on the flow or power in your primary and/or secondary circuits. If you have any questions or require guidance on using this calculator please contact us.

The FlexEJ LLH Calculator

Please rotate your screen. Our LLH calculator requires just a little more horizontal space than we can squeeze into your current rotation.

Design Values

Primary dT Deg C Primary dT
Used only when Power (kW) selected as unit below to calculate mass flow. If your system dT is not in this list please convert your power to a flow rate and input that below.
Body Design Velocity m/s Body Design Velocity
Design maximum velocity in LLH body. Decreasing the velocity will increase body size. You can change this value at any time allowing different solutions to be modelled.
Nozzle Design Velocity m/s Nozzle Design Velocity
Design maximum velocity in the connection to the LLH body. Decreasing the velocity will increase connection size. You can change this value at any time allowing different solutions to be modelled.

Calculation Result

Flow l/s Body Flow
The greater of Primary and Secondary total flow is used to calculate the body.
Body Size Body Size
The minimum nominal pipe size for the body giving a flow velocity less than the Body Design Velocity set above.
Velocity m/s Body Velocity
The actual velocity in the body based on the calculated body size.

calculator notes

Low Loss Headers can either have multiple connections to the body - 1 pair per circuit - or these can be combined before/after the LLH in horizontal headers (our preference) which results in just one pair of primary connections and one pair of secondary connections.

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.

Standard Products and Enquiry Form

As you use the calculator, you will see below any available FlexEJ standard low loss headers that match your application and an enquiry contact form.

FlexEJ standard low loss headers are designed for use with horizontal headers.

We offer short lead times on manufacture of bespoke low loss headers for use without horizontal headers - individual circuit connections to the body - and larger sizes above the standard range.

Please also visit our main Low Loss Header product page and for a full technical guide the What Is A Low Loss Header page. Our standard low loss headers are available at factory-direct prices and ship same-day from stock from our Web Shop.

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


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?

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.

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.

Got a quick question?

Use Live Chat. Or call us.

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