Correctly sizing your low loss header is essential to achieve maximum efficiency and benefit from the system. 


The low loss header should be sized based on the maximum flow rate of the system. This sizing is carried out looking at a number of factors; output of the boiler(s), system load, design, and potential temperature differential.

The header needs to be sized so that the pumped pressure resistance through the header at full flow is as small as possible. 
It is important that the volume of water circulating through the boiler loop is correct for the installed boiler minimum flow rates, output, and design differential temperature. This ensures good boiler control, especially when using high efficiency, low water content boilers.


What size low loss header do I need?


Low loss headers are sized in cubic metres per hour. NB: It is better to oversize your low less header than undersize. 

There is a rule of thumb for sizing low loss headers based on the principles that:

  • The higher the output of the boiler, the larger the header diameter and longer the header needed.
  • The larger the system design temperature drop and the lower the flow rate, the smaller the header diameter and shorter the header needed.

Determining the minimum diameter of your low loss header should be based on the maximum output of your boiler(s) and system delta T (taking into consideration any future demand, and remembering that you never undersize, but can oversize your low loss header, as it is much less detrimental than under-sizing.) 

The length of your low loss header should then be a simple calculation based on 4 x diameter. This is the minimum separation between your flow and return and, again, you can size up. 

The maximum flow in your low loss header should be less than 0.5m/s

Hamworthy can provide more detailed sizing guidance for low loss headers as well as corresponding low loss headers specifically designed to match the performance of our boilers. Contact your local Hamworthy expert or our in-house technical team

Other things to consider when designing and installing a low loss header.


All secondary circuit flows, and the flow from the boilers, should be at the top (if it is vertical) or one end (if horizontal) of the header pipe. All secondary circuit returns and the return to the boilers should be at the bottom, or opposite end, of the header. This will help to ensure correct entry temperature into the secondary circuits by avoiding temperature dilution of the flow temperature along the low loss header.

In your overall heating system design, flow in the primary circuit should be equal to or greater than that in the secondary circuit. If this is not the case some flow reversal from the secondary system can take place in the low loss header, mixing with hot primary flow and reducing temperature. 

If the boiler doesn’t have an integral pump, then additional pumps will be needed. Their size will be relative to the resistance and flow rate of the boiler(s).

You should consider the location and type of pumps, for example, single boiler shunt pumps or a single primary circuit pump. This is especially important if using a single primary pump and multiple boilers. In this case the boiler connections should be configured for a “reverse return” so the single primary pump will give even flow across all boilers. Each choice has its benefits, and the choice will depend on your installation, system design, control philosophy and budget.


Consider what is installed on the secondary circuit. Mixed temperature heat emitters such as underfloor heating (low temperature) and radiators (high temperature) will introduce more variables in the system. 

Consider the number of boilers on the primary circuit.

  • What are the manufacturers stated minimum flow rates for the installed boilers? (always refer to the technical manuals)
  • What are the temperature differentials on the circuits?


The building
There are numerous considerations when designing your commercial heating system which will be dependent on your building, project and system requirements. 

You can get more detailed guidance on low loss headers from our CIBSE-accredited CPD seminar, your local Hamworthy expert or our in-house technical team