Selecting a circulator pump for a hydronic heating system is a straightforward procedure of matching the water pumping capability of a pump to the flow resistance of the piping. In a closed hydronic system, i.e., one not open to atmospheric pressure there is no need to consider the height of the water in the boiler, radiators or piping. That’s because when the pump starts all the water in the closed loop starts moving. There is no “static head”. The only thing the pump needs to do is overcome the resistance to water flow. Small sized fractional horsepower pumps are more than capable of doing the job.
You must know several items to make an intelligent selection, the desired flow rate, the piping or equivalent piping resistance and the pump curve. The pump curve plots pump capability (feet of head) vs. flow (gallons/min). These pump performance curves are available from all pump manufacturers. The Taco brand pumps curves are shown in Figure 1.
FIGURE 1
Notice the wide selection and the fact that the slopes of the curves are all negative, i.e., more flow means less head. Later I’ll use the Taco 007 pump, a very common pump in residential applications, in an example. The equation of this pump is:
H (circulator) = 10.88 – 0.206(f) – 0.00971 (f)^2
H is head in “feet of head”; f is flow in gallons per minute.
The equation is derived from the manufactures published pump curves using simple polynomial curve fitting techniques.
Fluid flow in piping is described by the equation
H (pipe resistance) = acL(f)^1.75
H is head in “head of feet”, a (alpha) the fluid flow properties, c is a constant based on the size of the pipe, L is the length of the pipe and f is the flow in gallons per minute.
Notice the flow is raised to the 1.75 power. Observation of the equation shows the obvious, the longer the pipe (L) the more the resistance (H) and the larger the flow (f) the greater the resistance. In fact the resistance increases much more as the flow increases because of the 1.75 power.
The equation is a special case of a widely used Darcy-Weisbach equation for fluid flow and the Moody friction factor for turbulent flow in smooth piping. Don’t be overwhelmed it’s fairly easy to use. Derivation of the equation is way beyond any need here.
The fluid properties factor a (pronounced alpha) is based on the fluid’s density and viscosity. c is a constant base on the size of the pipe and is read from a chart based on the pipe type and size. L is the length of the pipe in feet. f is flow rate in feet per second.
For any piping-pumping system an operating point balance is created where the pump curve and the piping resistance curve intersect. It’s called the operating point. See Figure 2. The piping resistance is fixed and the pump operates with exactly enough output (Head) to overcome the piping resistance. The water flow rate is determined by reading from the graph the flow rate (gal/min) along the horizontal axis. In this example the flow rate is 2.42 gallons per minute. Without making piping changes you cannot change this flow rate. To increase the flow rate you must lower the piping resistance by making the pipes larger or select a more capable pump. This is an important concept to understand.
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FIGURE 2