Why can’t I use just any pump for my pond?    © 2001 By David A. Dec  

Pump head or total dynamic head is the most misunderstood topic among pond owners. Most pond owners think their pump head is between 5 and 7 feet. They are shocked when they find out that it is actually between 20 and 40 feet. Matching a pump to the pump head is vital, but seldom done.

If you buy a pump that is way too small it may only move a trickle of water, or possibly none at all. One that is a bit larger can still be too small to give good aeration, filtration and surface skimming. Overloading a pump that is too small can result in a shorter pump life, and more repairs. Often people who buy too small a pump will buy 1 or more of the same pump, so they wind up running several pumps with higher operating costs than 1 properly sized pump.

On the other hand, choosing a pump that is too large will not only waste a lot of money to run it, but can actually result in damage to the plumbing and equipment.

In order to pick out the correct pump there are 5 steps you need to go through:

1. Determine the volume of your pond;
2. Determine the flow you want based on the pond’s volume;
3. Determine the correct pipe size to move the flow you want;
4. Determine the water pressure needed to move the desired flow rate through your system;
5. Determine the proper pump that will give you the desired flow rate at the required pressure.

I. Determining the volume of your pond

The first thing you need to do is determine the volume of you pond. If you have not done that yet it is the length (ft) x width (ft) x depth (ft) x 7.48 gallons / cubic foot = U.S. Gallons.

If your pond has a very irregular shape you may want to back-calculate its volume by measuring changes in the salinity when you add salt. There are some very good salinity test kits that are extremely accurate.

The procedure is to:

1. First measure the salinity of your pond. 

2. Then add an amount of salt (Morton's Purex) that you think will raise the salinity by 0.1%. For instance, 1,000 gallons weighs 8,330 pounds, and 0.1% salt would be 8.3 pounds (1 gallon of water weighs 8.33 lbs). Make sure you do not add any new water until this test is over . 

3. Measure the salinity level again after allowing 24-72 hours of water circulation to even out the salt concentration.

4. Subtract the original lower salinity number from the newer higher salinity number.

5. Take the increased % salinity and use the following equation:

   1. Divide the number of pounds of pure salt you added by the % salinity change times 8.33 to get the gallons of your pond.

   2. For example, if you added 66 pounds of salt to get a 0.1% salinity change:

      66 / (.001 x 8.33) = 7,923 gallons

II. Determining the flow you want

If you have a pond that is under a few thousand gallons you may want to turn it over 2 to 3 times per hours. This is similar to large marine aquarium owners who have learned to turn over their aquariums' water a minimum of 3 times per hour. If it is a larger pond you may want to turn it over only once every 2 hours.

Peter Waddington, in his book “Koi Kichi”, says the real volume of water a fish lives in is determined by multiplying the pump's flow per hour times 24 hours per day. For instance, 3,333 gallons/hour yields a "real volume" of 80,000 gallons that the Koi actually live in, regardless of the actual size of the pond. This says that the water pump’s output is more important than the size of the pond. So people with smaller ponds will want to turn them over more often than those with larger ponds.

So let’s say you have a 5,000-gallon pond, and you want to turn it over every 1-½ hours. We simply divide the size of your pond by the number of hours you want for a complete turnover to get your flow rate. So for our example the flow needs to be 5,000 / 1.5 = 3,333 gallons per hour (GPH) or 3,333 / 60 = 55.5 gallons per minute (GPM).

The flow rate is very important and determines the size of you piping and pump.

III. Determining the correct pipe size for your pond

The Plastic Pipe and Fittings Association (PPFA) says PVC pipe should be designed for a maximum flow-rate velocity of 5 to 8 feet per second (fps) through the pipe. They say 8 fps is ok for pipe sizes less than 1” in diameter, but it should be less than 5 fps for pipe sizes of 1 ¼ “ or larger. Higher velocities can actually cause pipe failure and rupture, as well as astronomically large resistance to water flow, which necessitates higher horsepower requirements, and higher operating costs.

How do you determine the velocity of the flow rate in feet per second? The equation is:

Velocity in fps = .4085 x GPM / d²

Where GPM = gallons per minute, and d = inside diameter of the pipe in inches.

The following table shows the results of these fps calculations for various pipe diameters (d) and flow rates in GPH and GPM:

Table One

So we need to pick a velocity that is less than 5 fps from the above table. So looking at the above table for our example, we want to look down the 3,600 GPH column (since we want a flow of 3,333) until we find an fps that is less than 5. When we do that we see 4.10 fps corresponds to a 2-½ “ pipe.

One 2” pipe would be pushing the envelope, but we could use 2-2” pipes; like one 2” pipe from the bottom drain, and another 2” pipe from the skimmer. Both pipes could terminate in the ends of a Tee fitting, with valves for each, with the center branch feeding the pump. By the way, 2-2” pipes have about the same area as 1-3” pipe.

IV. Determining the water pressure needed from the Total Dynamic Head (TDH), or the sum of all the sources of pump head Ph, for your design

Head is best defined as “resistance to flow”. A higher head means you need more pressure to overcome it. The term “head” is further modified by whether the resistance is encountered on the suction side of the pump (suction head (H_S) from the pond to the pump) or the discharge side (discharge head (H_D) from the pump to the pond); whether it is caused by the standing height of the water (static head h_sh = height of the waterfall or fountain above the water’s surface) or by the movement of water through the system (dynamic head = h_d); whether the resistance is caused by simple friction due to fittings and pipe sizing (friction head = h_f ) or by the equipment resistance (h_e).

TDH = H_S + H_D = (h_sh + h_d + h_f + h_e)_S + (h_sh + h_d + h_f + h_e)_D

In order to determine the total dynamic head (TDH) we need to consider all of these sources:

1. When water flows through pipe there is a pipe friction or resistance at the inside surface of the pipe that needs to be overcome. That friction is a function of the diameter and length of the pipe.
2. When water flows through fittings like elbows, Tee’s, valves, check valves, etc. there is turbulence that also causes resistance to water flow. This resistance is a function of the total number of each type of fitting, and is expressed in feet, as an equivalent length of pipe, not as pump head.
3. When water flows through a leaf-basket / strainer, skimmer, drain, etc., there is more resistance to flow, depending on the open area of that component, as well as how plugged up the holes are with algae, leaves, etc.
4. When water flows through a filter, the resistance to the flow depends on the valve, filter media, size of the filter, the internal plumbing, the flow rate, how dirty it is, etc.
5. When water flows through an Ultra Violet sterilizer the center UV tube increases the resistance of that section of pipe to water flow by decreasing the area for water flow, i.e., the effective diameter.
6. A heater also will increase resistance to water flow as it squeezes the flow down into a smaller 1” tube, and makes a “U” turn in the heat exchanger, and adds more pipe length and fittings to the design.
7. Another source of TDH or Ph is the static lift in the pond design. An example of this is the height of a fountain, statue, or waterfall above the surface of the pond water.

This TDH or Ph is the most difficult calculation for everyone, because it is very complicated. Here is a table of the resistance in feet of pump head for every 10-foot length of pipe as a function of water flow:

Table Two

Where do these values come from? The PPFA says to use the Hazen-Williams Equation.

The equation is:

Ph = 104.4 / C^1.852 x (GPM)^1.852 / d^4.8655

where Ph is the pump head in feet per 10 feet of pipe, GPM is gallons per minute, d is the inside diameter of the pipe in inches, C is a pipe smoothness coefficient that is 150 for new PVC; 140 for smooth walled copper, brass, etc.; 100 for ordinary iron pipe; and 80 for old iron pipe.

Lasco’s PVC fittings website also uses this equation to show the friction losses. However, they convert their results to Pounds per square inch (PSI) per 100 feet of pipe length.

So according to the above table, if we have 30 feet of 1.25" pipe, and a flow of 3,333 GPH, the pump head due to the pipe alone, without any fittings, would be 4.22 * 3 = 12.66 feet of pump head for 1 ¼ “ pipe; similarly it would be 1.