Heating and Chilled water - Higher than expected flow rates

 Product(s):Hevacomp Mechanical Designer
 Subarea:Pipe Sizing


When considering pipe sizing of LPHW and CHW systems, mains losses must be taken into account in your rule of thumb or hand calculation for your results to match those calculated by Netsys.


An iterative process is used to carry out pipe sizing in the Netsys LPHW and CHW programs. Initially pipe sizes are estimated using the following formula:

This gives an initial flow rate based on flow and return temperatures at the boiler. The program then works these figures back through the system to give the total flow rate at the boiler. Pipe sizes are then estimated on these initial flow rates.

Pipe work emissions are then calculated and used to re-compute the flow rates through the system. These are worked back to produce a new total flow rate at the boiler. This flow rate is then proportioned through the system by connected load.

Pipes are then resized on the new flow rates and heat emissions for pipe sections are computed again. This process continues until no pipe sizes change.

Users often look at parts of their system in isolation and use the equation at the top of the page to check flow rates to an emitter using the flow and return temperature at the boiler (set in your System Marker). However the flow and return temperatures at the emitters will not be the same as at the boiler due to mains losses. This makes this part calculation inaccurate.

The example below illustrates how the use of the boiler flow and return temperatures can differ from what the actual flow and return temperature may be.

 With the flow rate equation from earlier, the user would expect the following for the pipe feeding Emitter 3:

However, due to mains losses the flow and return temperature at the emitter will not match the cylinder. In our example we need to consider 85°C flow temperature and 80°C return temperature. This would result in:

This demonstrates that the flow rate at the emitter could be more than double the figure calculated using the boiler flow and return temperatures.