When replacing a fountain pump or choosing a new one, first there are some key terms to keep in mind:

“Head”: This is the maximum vertical lift of the pump. For example, a 6′ head means the pump is rated to pump water up to 6 feet high. Note, however, that at 6 feet the pump would be providing very little water, with gallons per hour around zero. So if you need to pump, say, 200 gph at 72″, you will probably need about a 300-600 gallon per hour pump to do the job.

“GPH” : Gallons per hour, usually rated at different heights

“GPM” : Gallons per minute, usually rated at different heights

“Pump Curve” : The amount of water volume “curved” according to various heights. A 500 gallon per hour pump, for instance, might pump 500 gallons per hour at 0″ lift, 350 gallons per hour at 24″ of lift, and so forth. When buying a pump for the first time or when seeking a replacement pump, it is essential that you know how many gallons per hour you want to pump and at what height (head).

“Water Volume” The total volume that you will be pumping is controlled by a few factors. One factor is the size of the pump, as covered above. But you also must consider how wide your tubing will be. Tubing is measured in two ways: inside diameter (i.d.) and outside diameter (o.d.). Very skinny i.d. tubing will greatly reduce water flow. Many customers are shocked when they find that, after hooking up their 500 gallon per hour pump to 1/2″ inside diameter tubing, they are only getting what they consider a trickle.
We had an engineer do some calculations for us to illustrate the problem. Using a 300 gph pump with 1/2″ tubing is going to restrict your flow to 253 gallons per hour. By increasing the pump to 450 gallons per hour, but still using 1/2″ tubing, you will increase volume only slightly, to 264 gallons per hour! The lesson is this: When purchasing a pump, find out what size of tubing is supposed to go with it. Another problem is running the tubing too far. Long lengths of tubing create resistance. If your pump calls for 1/2″ i.d. tubing, for instance, but you are running the tubing twenty feet from the pump, it is
a good idea to use 3/4″ tubing instead so as not to cut down too much on flow.

How much water do I need? What size of pump? This question is answered in part by whether you want a “trickle” or a roar. When you buy a fountain, you will usually find a recommended flow. For waterfalls, use this as a rule of thumb: for every inch of stream width or waterfall “sheet,” you will need to deliver 100 gallons per hour at the height you’re pumping. So if you are building a 12″ wide waterfall that is three feet tall, you need to buy a pump that will be pumping 1200 gallons per hour at three feet of height. For small ponds, whenever possible, it is a good idea to recirculate the water once
an hour, more often if possible. Thus, if your pond is 500 gallons, try to buy
a pump that will recirculate water at a rate of 500 gallons per hour. For really
large ponds, this is not necessary and is far too expensive.

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