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What does Head Height mean, and why should I care?

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Pumps produce both flow and pressure; flow is pretty easy to see, especially over a waterfall, but pressure is hard to visualize. The concept of Head Height is an easy way to visualize the pressure a pump produces. One Pound per Square Inch of pressure will lift a column of water 2.31 feet in the air, so a pump that has a shutoff height of 23.1 feet produces 10 psi of pressure. This matters because the higher a pump has to lift water, the less water it can deliver. It makes sense that a given pump will deliver more water to a lower waterfall than a higher one. Head Height charts or graphs tell you exactly how much volume a pump will deliver to any given height, as in "2400 GPH @ 5' of Head". What you'll need to know, then, is the head of your own system. In a perfect world, the head of a system would equal the highest point above pond's surface, so a four foot high waterfall would give you 4 feet of head, but nothing's perfect. The water from the pump is carried to the waterfall via tubing, and the size of the plumbing really matters. The friction from restrictive plumbing can force the pump to work as hard as if it were lifting water three or four times higher than the waterfall, effectively tripling or quadrupling the head, which means a lot less water flow than you're paying for. To keep this 'Friction Head' to a minimum, always increase the plumbing 1-2 sizes larger than the outlet of the pump. If the outlet is 3/4", use 1" or larger plumbing. If it's 1-1/2", go up to 2" tubing. With upsized plumbing, you can use the following rule of thumb to find the Total Dynamic Head (THD):

For every 10' of Tubing, add 1 foot of Head
For every Check Valve, add 1' of Head
For every fitting, add ½' of Head

So, if your waterfall is 4 feet tall, with 10 feet of tubing, two elbows and a check valve, your system would have a THD of 4 + 1 + 1 + 1 = 7'. Now you can check the charts to see which pump gives the flow you need at 7' Head. If there's a choice, look for the pump that uses the least Wattage; that'll cost you the least to run.