A low loss header – or common header - is a method of hydraulic separation between a boiler primary circuit and secondary system circuits.


When do you need a low loss header?

There are several reasons why you may need to install a low loss header in a heating system.


You may need a low loss header in a heating system for a large home or business premises especially with a low water content boiler. This low loss header will always need to be sized correctly depending on the flow rate of water around the building, the system Delta T and the boiler(s) minimum flow rate(s)


The secondary circuit pumps, in particular modulating pumps, can have an effect on the flow rate of the primary circuit  thus potentially lowering flow rates through the boiler(s) which is required to remain constant. 

In this situation a low loss header is particularly important as it allows multiple pumped secondary circuits to work independently from one another without also effecting the primary circuit flow rate.

With boilers connected directly to their heat loads variable flow rates may be experienced caused by different building demands or control valves such as VT valves, etc. This can lead to fluctuating  flow rates through the boiler(s), unless a hydraulic separation method – such as a low loss header - is installed in the system.


How does a low loss header work?


A low loss header enables hydraulic separation by providing a separation point between the primary and secondary circuit, or multiple circuits.


Low loss headers work on the principle that water will always take the path of least resistance. In a system with a low loss header the point of separation is known as the neutral point. In the simplest of terms a pipe or tube across which water flows, creating a shortcut across the flow and return pipework. When the flow of water reaches the large bore of the header, the water immediately loses most of its pumped velocity. The low loss header acts as a ‘hydraulic brake’ to manage the speed of the flow, and only the water/flow required for the secondary circuit(s) is carried through.   



The low loss header will take any excess capacity from the flow, and return it into the primary circuit. This allows boilers to operate at a constant flow rate in the primary circuit whilst flow and temperatures can vary in the secondary circuit, regulated according to need.

What are the different types of low loss headers?

Both vertical and horizontal low loss headers are available – the choice of which will primarily be determined by the space and the planned location of the header. Some low loss headers also have dual purpose as air and dirt separators

Vertical Low Loss Header

We recommend installing your low loss header vertically as – at this orientation - they can help collect sludge from the system. Due to the low flow velocity through the low loss header, the debris from the system has time to fall to the bottom of the header and is trapped for safe removal via a drain valve. This installation also allows released air to migrate to the top of the header to be vented.


Horizontal Low Loss Header

As this orientation does not bring the benefits of air venting and debris drop out we do not recommend this installation unless it cannot be avoided.  If space constraints dictate a horizontal low loss header installation, then you must ensure it has automatic air venting and is flanged at each end for full access and inspection.


Air and dirt separators

Whilst the vertical low loss header allows air and dirt to ‘drop out’ of suspension, some low loss headers are specifically designed as combined air and dirt separators. This saves the need to fit additional devices to the system and reduces both installation space and cost. Our Upton boiler pipework kits have the option of a combined low loss header and air and dirt separator.


What are the benefits of a low loss header?

Constant flow

By separating the system into primary and secondary circuits, the low loss header allows boilers to operate at constant flow rate in the primary circuit whilst flow rates and temperatures may vary in the secondary circuit. This avoids any interaction between circuits and enables the primary circuit return temperature to represent the overall/maximum load on the system.

Boiler life

By regulating the flow rate, a low loss header ensures that efficiency and performance of the system are improved, and the boilers are safeguarded from irregular flows - ultimately prolonging  the life of the boilers.

Cleaner systems

As discussed earlier, if installed vertically, the low loss header can help collect sludge from the system. We would however still advise that existing systems are cleaned and flushed to minimise debris in the water, and a dedicated air and dirt separator be installed on the system or that a low loss header with integrated air and dirt separation be used.

Air venting

In a vertical installation air is released as a result of low flow velocity – where water flow slows down and the change in pressure releases air – then rises in header and is easily removed via the automatic air vent.



Low loss headers – what should you consider

System suitability and design

Low loss headers do introduce mixing of flow and return which results in temperature variations and this should be taken into consideration in the overall system design and control.


A low loss header will not work correctly  without a suitably sized pump(s) installed in the system. If the secondary pump(s) are too small, the hot water won’t be able to reach the heat emitters  furthest from the boiler. If the secondary pump or pumps are oversized it may push hot water around the system too quickly,  waste energy, create noise and effect the primary flow rate by overcoming the neutral effect created by the header. At least two pumps will be required in a system with a low loss header - the primary pump from the boiler(s) to circulate the primary circuit through the low loss header and back to the boiler(s) and one or more pumps to circulate the secondary circuit(s) through the low loss header and around the secondary circuit(s).



The costs to alter an existing  heating system to include a low loss header may be prohibitively high versus the benefits achieved. For a single boiler system with a single secondary circuit it may not be necessary.


Space requirements

Space required for the installation of a low loss header and associated pipework may mean that it is not possible to accommodate the extra equipment.



Depending on the complexity of the system and the size of the building, the number of ports may make a low loss header challenging. This depends very much on how many heating circuits are required. However, extension kits are available to bolt onto most low loss header arrangements to help with this.