Hydronic Balancing Part 4: How to Develop a System Curve

By Chad Edmondson

What is a system curve and how is it used to develop a balanced hydronic system? 

The “system curve” is a graphical representation of the head losses and gains of a particular piping system that result from changes in flow.  And it’s all based on what you already learned if you read our previous blog on Hydronic Balancing Part 3: How To Use The System Syzer:

As you double the flow through the piping the pressure drop increases by the square.  In other words, the pressure drop increases by four times what it was.

When used in combination with the pump curve, a design engineer can determine the system head and flow long before the system is installed and the pump is turned on.  It’s all based on the same math:

 Why System Curves Matter

Pump curves represent the energy that is put into a system; system curves represent what the system takes out. A system will operate at the point at which these two curves intersect, as long as nothing else changes in the system (such as a valve being closed or partially closed). 

Design engineers want the system and individual circuits to operate at specific flows to satisfy the space heating and cooling requirements while staying within the operating range of the components.  That’s why the system curve is important.  It let’s the engineer know where and how much design adjustment or “tweaking” of valves will be needed so that once the system is balanced and the pump is turned on it will operate in a correct and efficient manner.   It’s not a matter of crossing your fingers – it’s a matter of knowing exactly how much resistance is in a given piping system and matching it precisely with the flow characteristics of pump.

How to Plot a System Curve

A system curve is developed by using Scale 5 of the System Syzer, just as we discussed in the previous blog.

Let’s say we have determined the design flow and head for our system to be 2200 GPM at 100 feet of head.  (These values would be based on the critical circuit.) Knowing this, we choose a pump capable of generating this much head and flow and we take the following steps to develop our system curve and determine the operating point of our system:

Step 1 – Set the System Syzer Scale 5 for 2200 GPM at 100 feet of head.

  Step 1 – Set the System Syzer Scale 5 for 2200 GPM at 100 feet of head.

Step 1 – Set the System Syzer Scale 5 for 2200 GPM at 100 feet of head.

Step 2 – Without changing the position (or settings) of the System Syzer, read off and record the head at various other flows.

  Step 2  – Without changing the position (or settings) of the System Syzer, read off and record the head at various other flows.

Step 2 – Without changing the position (or settings) of the System Syzer, read off and record the head at various other flows.

Step 3 – Plot these values to develop the system curve  

  Step 3  – Plot these values to develop the system curve

Step 3 – Plot these values to develop the system curve

Step 4 – Overlay the system curve atop the pump curve for the selected impeller trim to see where the lines intersect.

  Step 4  – Overlay the system curve atop the pump curve for the selected impeller trim to see where the lines intersect.

Step 4 – Overlay the system curve atop the pump curve for the selected impeller trim to see where the lines intersect.

Wherever the system curve intersects with the pump curve is where that pump will operate under full load conditions, when all valves are open and the system is at full flow design conditions. Remember, the “system” is everything from the pump discharge flange to the pump suction flange.  Here it will stay unless the resistance within the pipe system changes – i.e. a valve changing position.  As two-way valves open and close, the system curve will change accordingly and thus where it intersects with the pump curve.  Ideally the pump will have been selected to weather the demand range and safely ride the pump curve as demand changes. 

By understanding how to plot the system curve we can correctly balance a pump at system started-up!