This makes it easier to calculate the size of your expansion tank - but you must also include the fluid volume contained in your boiler, heat exchanger, and coils. This data must be obtained by the manufacturer of these items. Knowing the total fluid volume also makes it easier to estimate the amount of additives for water conditioning. Note that this does not include the other water quantities in the boiler, and coils. The pump for this system must produce rated flow at the specified head loss.
Most engineers specify pumps based on anticipated head losses.
Rarely do we get the chance to build our system, measure the losses, then specify the pump. So, we must consider the impact that a deviation could have when our piping systems are built. While we may specify a certain number of elbows, in actual practice, this number may increase. If your system has a lot of valves and fittings, the impact of this variation could lead to some unpleasant surprises - if your pump doesn't have enough capacity. It would be appropriate to apply a safety factor when sizing your pump.