What is a low loss header
Why should I fit a Low Loss Header?
In some cases the flow rate through the system circuit will exceed the recomended maximum flow rate through the boiler, or it may be that the system flow rates are simply unknown. In other cases the reverse is true, where the boiler flow rate exceeds the maximum system flow rate (particularly true in some multi boiler systems). Fitting a Low Loss Header allows the creation of a primary circuit, within which water velocity can be maintained at the required constant, regardless of changes or requirements in the secondary circuits.
To sum this up in plain English: The amount of water going around the system (radiators, cylinder etc) might be to much or to little for the boiler.
difference between the flow and return is to great, it puts a huge strain; through thermal expansion and contraction, on the heat exchanger. Also the temperature of the water passing through the heat exchanger is important, particularly with condensing boilers, these have there own specific requirements to operate at maximum efficiency. For a boiler to go into "condensing mode" the return temperature should not be higher than about 55'C. So in some cases temperature sensors are fitted on the header to allow control over the primary circuit temperature.
Again in plain English: The temperature of the water coming from the system, might be to hot or to cold for the boiler.
To sum this up, the primary cicuit is acting like a circulating reservoir of hot
water into which the secondary circuits can dip as needed.
Secondary circuits are therefore normally pumped.Source: www.vaheating.co.uk