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Full in-house design & manufacture to EN / ASME / PED.

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FlexEJ has an accredited Environmental Management System.

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 Alert

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 expansion joint 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

Frequently Asked Questions

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.

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.

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.

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.

A. We have an online low loss header calculator we’ve built to help you – or do  please get in touch with the details of your system and we will be pleased to help.

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.

A. Yes – strictly speaking hydraulic separation between circuits is achieved using 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.

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.

Looking to upgrade the hot water system?

Don’t miss FlexEJ’s new range of space-saving, energy-efficient PhexPak Packaged Plate Heat Exchangers. Read more now.
Or call us for advice on 01384 881188.