James Elston

Published April 9, 2024

A low loss header makes sure different heating systems and components of a single system work together without getting in one another’s way.

It manages the speed of the water flow in a central heating system, and allows only the water required for a secondary circuit to flow through. In other words, it acts as a sort of “braking system”.

It can also connect different heat sources to a single system. So your underfloor heating, your boiler, and your radiator can all be connected to one heat source – the boiler.

In this article we will discuss the importance of a low loss of header, how it works, and the benefits and drawbacks that come with it.

How Does a Low Loss Header Work?

The main job of a low loss header is to separate the flow of the primary heat source from the secondary circuit through hydraulic separation.

Hydraulic separation is a term for when two (or more) circuits flow at the same time within the same piping system, without interfering with one another. Put simply, when there are two pumps in the system, each working independently, hydraulic separation allows the water to flow at each pump’s specific flow rate in the circuit.

This is achieved by installing primary and secondary circuits to separate flows. And, as we’ve mentioned, that separation is done by none other than low loss headers.

When there’s one system of pipes but more than one water circuit, things can get complicated, causing an unbalanced system or reverse circulation.

A low loss header has a large chamber. When water flows into this chamber, it loses its pumped velocity, and slows down (which is why the low loss header is considered a “brake”). Only water required for the secondary circuit continues to flow through, while the rest flows into the return pipework of the primary circuit.

So, if a boiler has an internal pump, and that pump pumps to a low loss header, then almost all the water will flow back to the boiler. Almost no water will continue into the heating system.

In that way, a second pump can be connected to the other side of the low loss header without interfering with the boiler system.

Low loss headers can be used for more than just a system with two pumps. They can also be used in systems where there are multiple heat sources or boilers.

A low loss header can be used:

  • In systems with multiple boilers
  • When there are multiple heating circuits
  • When there are multiple heat sources (like renewable and traditional energy sources)

Common boiler issues that low loss headers help with

Low loss headers can solve some common issues that are often prevalent with boilers, including:

  • Temperature fluctuations: Low loss headers can maintain consistent temperature by managing the flow rate. 
  • Inefficient circulation: If two circuits have different flow requirements, a low loss header ensures that each functions at its optimal flow rate. 
  • Removing impurities: When air or dirt becomes trapped in a boiler, it can cause damage (and strange noises). The low loss header has air and debris separation capabilities, removing these impurities and allowing the system to operate without issues. 
  • Pressure fluctuations: By separating (and balancing) the primary and secondary circuits, a low loss header helps to stabilise system pressure. 

Pump size considerations

Low loss headers are usually found in systems with two or more circulating pumps. 

If the pump is too big, you risk overloading the low loss header. This can lead to:

  • A high flow rate
  • Erosion of pipes
  • Problems with heat transfers

If the pump is too small, it will also impact how your low loss header works. Apart from affecting your heating system, it can lead to:

  • Insufficient flow rate
  • Issues with heat distribution
  • Reduced boiler output
  • Issues with the primary circuit return temperature
  • Energy wastage from the low loss header

Low loss headers must be sized correctly for the boiler flow rate and the flow rate of water throughout the system.

Types of Low Loss Headers

Below are the common types of low loss headers.

Horizontal low loss headers

A horizontal low loss header is turned on its side.

This can save room and the units often come with commercial boiler installation packs. If you have limited space at the office building, then a horizontal low loss header is ideal for you.

Horizontal low loss headers are best for well-insulated buildings and those with small, intricate pipework.

Although they’re functional, we’d recommend avoiding horizontal low loss headers if possible. They’re not capable of venting out the air and provide no dirt separation, meaning you’d also have to install air and dirt separators. They also need frequent inspections.

Vertical low loss headers

As the name suggests, a vertical low loss header is installed upright or vertically, and allows air to rise to the top of the header, where it’s released.

Air is trapped when the water’s velocity drops. As the water slows down, the pressure drops, releasing air. The air then rises in the header and can be easily released through the automatic air vent.

These kinds of low loss headers also trap debris at the bottom of the header, which can be cleaned through a drain valve.

Vertical low loss headers are mainly found in residential heating systems with multiple circuits and variable flow rates.

Dual purpose low loss headers

Dual purpose low loss headers can multitask, acting as a low loss header, and air and dirt separator.

This unit is the perfect fit if you’re looking for a lower-maintenance low loss header. It cuts down the need to clean the device regularly, reduces wear and tear on secondary circuits, and is good for the lifespan of the primary circuit.

The Advantages Of A Low Loss Header

Now that we’ve discussed how a low loss header works, let’s take a look at some of its advantages.

Improves efficiency and performance

A low loss header improves the efficiency and performance of a heating system by maintaining consistent flow rates, which helps save energy.

Condensing boilers benefit a lot from low loss headers, as they help to maintain the return water temperature. 

Extend boiler lifespan

A low loss header can regulate the flow in your primary and secondary circuits. That means your boiler functions properly for longer since it’s not exposed to irregular flows.

That reduces stress on the boiler’s parts. It also helps to maintain consistent flow temperatures and prevents overheating – all of which will help ensure a longer boiler lifespan.

Constant flow rate

Low loss headers are ideal for maintaining a constant flow rate through the process of hydraulic separation.

However, while the flow rate in the primary circuit remains constant, it can fluctuate in the secondary circuit. Now, this might sound bad, but it avoids any interaction between the different circuits, ensuring the primary circuit’s temperature represents the maximum flow rate and load of the entire system.

The Disadvantages Of A Low Loss Header

While low loss headers are great, they do come with some minor downsides.

Space requirements 

Low loss headers can take up a significant amount of space if your home or office building is on the smaller side. This is due to the additional pipework needed.  


Low loss headers aren’t cheap. If you have to alter existing systems, the financial outlay might actually outweigh the benefits. You could stand to pay upwards of £400 – without installation fees.

Ask yourself whether it is really necessary before making the investment.

Heat loss risk

The more pumps and heat sources you have, the more surface area there is in the entire heating system. And, larger surface areas mean more risk of heat loss.

Adding a low loss header to the mix means you’ll need to insulate everything properly, and often boilers have to heat the water to higher temperatures to compensate for additional heat loss (in the case of poor insulation).


Distortion occurs when a higher temperature is required at the boiler to get the heat to the radiators or underfloor heating (called the “emitter”) to the desired temperature. This distortion occurs at the low loss header, as flow rates (and therefore temperature) differ on either side of the chamber.

When Not to Use a Low Loss Header?

Let’s discuss when it is best to not use a low loss header in your heating system

When you have a simple system configuration

In a system where the primary circuit directly serves a secondary circuit without any complexity, the hydraulic separation offered by a low loss header is unnecessary. 

Systems with a single boiler and a consistent flow rate won’t benefit from a low loss header.

Limited demand variation

Systems where there is almost always the same demand, that doesn’t experience fluctuation in flow rates or temperature differentials won’t need the balancing abilities of a low loss header. 

Space and budget constraints

A low loss header not only takes up extra space but also comes at an additional cost. If you’ve got a tight budget or smaller property, then a low loss header isn’t justifiable. Always remember that you want to get a return on investment in terms of system efficiency. 

It might be best to investigate other solutions. 

If your system already has hydraulic separation

Some boilers are designed to incorporate hydraulic separation, so adding a low loss header to the system will be redundant.


What is the flow rate of a low loss header?

A low loss header doesn’t have a fixed flow rate. It varies depending on the specific requirements and system.

Is a hydraulic separator the same as a low loss header?

Yes, it is! While the terms are sometimes used in a slightly different context, a hydraulic separator and a low loss header refer to the same component in a heating system.


A low loss header is a key component of almost every modern heating system. It allows different parts of the system to work together, acting as a brake to regulate water flow rates between the primary and secondary circuits.

A properly installed low loss header will improve the efficiency and longevity of a building’s heating system. You should consider upgrading to these devices if you want to stabilise water flow and manage the amount of air and debris in your system.

James Elston

Boiler Expert

James Elston is the top boiler replacement and heating expert at Eco Happy. He has over 20 years of experience in the industry, focusing on Gas Safe boiler installations and offering home-heating and energy-saving solutions to homeowners across the UK. From sourcing the most energy-efficient combi boiler to providing specialist heating advice, James ensures that Eco Happy maintains the highest standards and best customer service.

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