CIRCULATOR PUMP SIZING
- Chart of What is Pump Head Range
Once you have selected the features of the circulator pump you are contemplating as per the discussion in the previous article, your next effort will be focused on finding a sizing chart and gaining an understanding of what pump's head range is most appropriate to your needs. So first lets introduce the concept of "HEAD".
HEAD = the amount of pressure a circulator pump must develop in order to create a desired water flow (as in GPM, gallons per minute) within the system or out the other end, as in a fire pump. The pressure is measured in feet of water.
In discussions about what is an appropriate pump head range you must consider both "Static Head" and "Dynamic Head".
Static Head: = this refers to the pressure required to lift a certain amount of water vertically a set distance, so it reflects the weight of the column of fluid in your system. The greater a pump must lift the fluid against the effects of gravity the greater the static head it must develop to overcome the resistance. Think of pumping water up hill.
Now here's the key! In closed loop systems like those present in outdoor wood boilers, gravity is not a factor because the water pressure is equal on both sides of the pump, effectively creating zero static head.
Dynamic Head: This is a calculation that considers the cumulative resistance of various components of your plumbing circuit
- length of pipe
- pipe diameter
- number and configuration of all the fittings
- viscosity of the fluid
- flow rate
Manufacturers produce reference tables to give you the friction loss associated with each plumbing component. Then you would layout the necessary configuration of your system, add up the resistance of each fitting according to these tables and that which is appropriate for the diameter and length of your piping, and finally adjust this number for your flow rate and fluid viscosity to ultimately get a figure for your Dynamic Head. It is measured in feet as well.
So what is pump head range? In hydronic heating systems we suggested static head is equal to zero and we only had to consider Dynamic Head. So a circulator pump sizing chart would show you over what range of Dynamic head that this pump will operate at and what flow rate you can expect with that specific head calculation.
|Pump Head Range||HEAD (in feet)|
|High||over 20 feet|
So a "pump curve" is a graphical representation where the horizontal axis gives you a pump's flow rate potential (gpm) for a given pressure ( in feet of head). Since hydronic heating systems have virtually zero static head and only dynamic head, most systems can get away with a circulator pump with what would be considered a "low" pump head range.
You'd then have to calculate the necessary flow rate to create enough heat for your particular application. This will depend on the exact type heat distribution system you are using, the heat requirements, R-values etc. But once you have the gpm and the head you can reference the pump curve data for a particular circulator pump's sizing chart to confirm that you are making an appropriate selection.
Often I have found that installers prefer three speed pumps to allow after-the-fact adjustment of flow rates making the Grundfos circulator pumps an appealing purchase.
If you're still with me, the next article in our series is a list of the top 10 tips to successful use and installation of a small hot water circulator pump.