Sections Include:

• Overview
• Simple Calculation Methods
• Impeller Design affect on Head Pressure
• Affect of Electromagnet design on Pump Head Pressure
• Head Pressure Improvement Suggestions

Updated 3/27/13

Overview

An aspect of any water pump (not air pump), including the water pump aspect of a filter such as an aquarium canister filter that many aquarium and pond keepers over look, yet can be a significant factor in choosing the correct pump is head pressure.
I will attempt to describe in basic and more advanced terms this important aspect of "mating a correct water pump to your aquarium or pond application including the addition of a UV Sterilizer.

The most simple description of head pressure is that this is the force (or resistance) placed on the "head" (outlet) of the pump by gravity, weight of the water column past this point, water features, and devices in-line past this point that often constrict or impede flow (such as UV Sterilizer, pond "spitter", fountain, waterfall feature, or Fluidized Filter).
In other words the ability to LIFT water while maintaining current (think torque in a truck that allows the "lifting" or pulling of a load up a hill), not simply move water on a level plain as in the current/flow directly out of the water pump with no pressure placed on the pumps flow.
Another way to think of this is to take a 5 gallon bucket full of water; to simply tip and pour out the water takes very little energy or "lift" (head pressure for our example), however if you were to climb a ladder and pour this same bucket would take more energy or lift (again head pressure in our example).

The majority of aquarium and pond water pumps list their maximum head pressure, so this can be used for determining your end water flow (after head pressure is applied), which may be essential for not only determining the correct pump to purchase, but the correct UV Sterilizer (if desired) to be purchased since flow rate is a major factor in UVC Sterilization effectiveness.

However the listed "GPH or "LPH" is almost always the flow rate directly out of the pump (head) without ANY pressure from tubing, devices, filter media, or even water or gravity applied.
An example/analogy would be a car/truck engine: the rated horsepower is what the engine produces at the crank shaft, not the wheels after transmission, vehicle weight, etc is applied. So as a rough analogy; a 200 hp engine is going to work much better in a 2000 lb vehicle than a 10,000 vehicle. This analogy applies to a pumps rated flow rate before head pressure is applied.

"Real World" Canister Filter Flow Rates:
This aspect is sometimes confusing, especially when one product is more factual with what a pump filter will deliver in the "real world" applications.
An example here would be a Sunsun 303B Aquarium Canister Filter which like most related filters provides only the gph (370 gph) immediately at the "head" of the pump, while the Rena Filstar XP3 provides both the immediate flow rate out of the pump head (350 gph) and the ACTUAL flow rate (187 gph) out of the filter itself after hoses, filter media, etc are applied. Based on questions, many will think that the SunSun and related filter is substantially stronger when in reality these filter are nearly equal once head pressure is applied.

The implications here are important for mating an effective UV Sterilizer as noted previously few canister filters rate their "true" flow rate such as the Fluval FX5 with a 0 head pressure rating 925 gph, when in reality the typical head pressure flow rate is 600 gph or less.
While far from an exacting formula, a typical canister filter flow rate with under tank placement is about 50% to 60% of the published 0 head flow rate.
This is the number you should use for mating your UV Sterilizer.

It is noteworthy that you can increase or decrease the head pressure via the placement of your canister filter and other equipment run off the filter, such as a UV Sterilizer or Fluidized Sand Bed Filter.
For example you can have an aquarium where the stand or table it is placed on allows for the filter and other equipment to be placed alongside at the same level as the base of the aquarium. This would provide for considerably more head pressure than an installation that has the canister filter and other equipment under the stand/cabinet at say 24 inches.

Water Pump Designs;
Most aquarium water pumps are open impeller designs that are greatly affected by head pressure. Propeller pumps, just by their design can handle almost no head pressure and thus are only intended for under water applications with no lifting of water out or into an aquarium.

A few pumps, in particular those designed for ponds or large aquarium systems such as the Rio HF Pump series have closed designs that can handle much more head pressure and are thus better suited for lifting water through multiple devices, water features in ponds, or deep sumps.

The bottom line is regardless of the pump design you choose, know its maximum head pressure so as to calculate what the actual flow will be with the devices you might add in-line.
For instance, do NOT expect a 1000 gph pump with a maximum head pressure of 10 feet to pump any more than 700 gallons per hours lifting water 3 feet from a pond to a waterfall (other factors including water in-line likely will reduce this even more). This also applies to aquarium applications too!

Simple Calculation Methods:

This section will provide simple and reasonably (but not 100% accurate) methods to calculate head pressure on a pump or filter.

(1) Vertical:
This one is the most obvious; Add 1 foot of head pressure for every 1 foot of vertical tubing past the pump or filter to the discharge point in the aquarium or pond (do not count tubing coming from an aquarium to a canister filter as this is gravity assisted and does not add to this calculation).
This major factor of head pressure is a "one way" factor, meaning that if your pump/filter is pumping water to a UV Sterilizer 24 inches under the aquarium water level, you do NOT count the distance down, then back up as "down" is a given based on gravity/siphon. The head pressure is added on the trip back up to the tank/aquarium.

Looking at Vertical Head Pressure another way we can use this accurate formula:
1 vertical foot = 0.433 pounds per square inch (psi)
or conversely
1 psi = 2.31 vertical feet

As an example; A pump with a rated Head Pressure of 7 feet, would have 3.03 psi (7 divided by 2.31)

(2) Horizontal:
There are not 100% accurate formulas that I have found for measuring horizontal head pressure (other than the very complicated Bernoulli's Equation).
However both in my observations and other reading it is safe to say that horizontal head pressure equals a vertical run of .25 to .4 times its length.

(3) Add approximately 1 foot of head pressure for every 90 degree turn in tubing/pipe under 3/4" ID (inside dimensions). Add approximately 1/2 foot of head pressure for larger dimensions for 90 degree turns

(4) Although I do not have firm calculations for tubing/pipe sizes, I can state that smaller diameter tubing or pipes add to head pressure, but this can have variables within itself in calculations including pump outlet size and pump impeller design.
The pump outlet size is a major determining factor that you can calculate; for instance if you have a pump with an outlet designed for 1 inch tubing and you use a reducing part (often supplied with many pumps) for say 1/2 inch, you are literally going to reduce your total head pressure by half.
However if your pump is designed for only 1/2 ID tubing, using this size tubing/pipe is not going to subtract from calculation.
I also should point out that not all pumps are designed with the best outlet sizes for their design (which includes head pressure, impeller design, and more), so these calculations can be off if for instance a pump is designed with a 1 inch outlet when in reality its design is better suited for 3/4 inch outlets.

As a side note, in tests I have conducted with several pumps and 1/2 ID tubing with one foot of head added I have not been able to achieve flow rates beyond 350 gph regardless of open impeller pump sizes, wattages, stated flow rates, etc.

The bottom line is this is an aspect I have often seen where clients/customer of mine over the years have over looked. An example was a person who had a 3800 gph pond pump connected to 1 inch tubing, even though the pump was designed for 1.5 tubing/pipe. This reduced the flow immediately by 1/3!!

(5) This next calculation is not one where I can state it is exact, but it still should not be over looked and that is devices added inline to your pump. This can vary from device to device.

Generally speaking a UV Sterilizer will add 2 feet to head pressure, but this can vary considerably by UV size and design. As an example a small well designed 8 watt UV would add as little as a foot to head pressure, however some, such as the Coralife 9 Watt Turbo Twist can double this for a similar size UV. A large UV such as the TMC 110 Watt PRO Pond /Aquarium UV will add as much as 4 feet of head pressure (although less if 2" ID inch pipe is used).

Devices such a Fluidized Filters can add anywhere from 2 to 4 feet of head pressure (more or less). Again design is a factor as well as the sand size (smaller requires less head pressure), but again as a generalization consider a minimum 1 foot for every 6 inches of sand fluidized plus the added elbows in the device add to head pressure as noted earlier in section #2.

(6) If all else fails in calculations or you have already purchased a pump and simply desire to know the end flow rate, this can be simply calculated quite exactly;

Simply place a container under the outlet of your pump as it empties into the aquarium or pond after passing through all devices and water feature. Then time how long it takes to fill the container in exact measurements.
As an example if a 1 gallon container fills in 10 seconds, that is 6 gallons per minute or 360 gph (this works for metric using liters too).

Admittedly this can be rather difficult for very large flow rate pond pumps that pump say 10 times the previous examples flow rate (3600 gph versus 360 gph), as this would fill 1 gallon in a second. This would required a 5 gallon bucket that might be difficult to position 100% in the water flow discharge; the end result would be 5 gallons in 5 seconds to produce 3600 gph.

Affect of Impeller design on Pump Head Pressure

The design a of a water pumps impeller has a major impact on whether it will produce higher flow rates through a higher flow rate curve.

Here are three common designs and their affect on water head pressure (Please Click on the pictures to enlarge);

The "Propeller" pump impeller design has almost no head pressure and is not intended for any real lifting of water.
However it also is very efficient at moving reasonably high volumes of water with a very smooth rotating current and requires very low electrical wattages to do so. These are popular in pumps such as the Premium Seio Propeller pumps for under water circulation devices, especially in reef tanks.
Another advantage of this impeller design is that it also has very low start up resistance making it the best design for wave makers that automatically turn pumps on and off to simulate waves.

The standard open impeller is the most common design for aquarium and light duty pond water pumps. While not as good for wave makers, these tend to be the most versatile design which use reasonably low electrical wattage.
Even with these impellers there is a very wide variance in design quality with some having lighter magnets, thin blades or even slight hybrid propeller designs (the Fluval Pumps have impeller designs that are not well suited for adding much head pressure). As pure water pumps/ power heads go, the newer upgraded Rio Plus Pumps are the superior design in this class from what I have used and seen.

The "closed impeller" design is generally the most heavy duty water pump impeller.
This design can handle many more in line devices and its head pressure falls off much more slowly towards its maximum head pressure (where it obviously falls off to 0). In fact this is an aspect of pumps that utilize these closed impellers that is difficult to measure in the previous sections calculations (which I made many generalizations).

This design is best for deep sumps, multiple water features/devices, multiple aquarium systems, and simply larger ponds or aquariums.
A good example of a well made closed impeller pump is the Rio HF Pump

Effect of Electromagnet design on Pump Head Pressure

This is rather straight forward, but still occasionally missed by aquarium or especially pond keepers, and that is the design of the electromagnet of most typical "Mag-Drive" pumps used for aquariums and ponds.

One way to think of this in terms of car/truck engines is horse power versus torque.
Many simple pumps (or in the case of high flow pond pumps; "cheap") have the horse power to move a lot of water, but almost no torque to lift water if there are any devices in-line or debris in the water.

The electromagnet that drives a propeller pump is generally very "simple", meaning it does not need to be a very heavy winding that uses much current, thus this design is generally quite efficient as for electrical usage. As an example a Seio 530 Pump uses only 7.5 watts yet moves 530 gph. This is an excellent pump for what it is designed for, but it has absolutely no torque and therefore should be used for nothing more than underwater current, not running a UV Sterilizer or similar. With most standard impeller design and especially closed (or partially closed) impeller design pumps the electromagnet is much heavier and will require more electrical current (wattage). This will obviously increase depending upon the load and flow it is designed for. Unfortunately not all pumps marketed for applications such as ponds in particular have the heavy duty electromagnet to provide the torque to provide the "lift" necessary for head pressure. So even if the flow is good or even the impeller design is excellent, often the electromagnet is not, so the end result is often poor head pressure or worse; a shorter life span due to an electromagnet burn out from attempting to run a large pump and impeller design (often in harsh pond conditions) with an inadequate electromagnet design.
A good example is the Via Aqua 4900 pump (although I like many Via Aqua pumps as good, albeit more economy pumps, this is one to be avoided). Another is the entire Laguna Max-Flo pump line.

Often a dead give away is a pump of what may seem "too low of wattage" for a high flow pump. This may be fine a for a propeller pump, but not a pump used in sumps or ponds to lift water, especially with more debris in the water column or devices in-line.

Head Pressure Improvement Suggestions

This most basic suggestion is to adjust the level of lift if head pressure (slow water flow) is a problem.

As an example, say you are using an Internal Filter or Power Head Pump to run a UV Sterilizer (often utilizing intake and return adapters) such as the Vecton 8 Watt UV; I would recommend hanging the UV Sterilizer just below the rim of the aquarium with minimal tubing between the UV Sterilizer and the intake and return connections. Such a short drop will reduce head pressure considerably versus placing the UV Sterilizer at the base of the aquarium or even lower such as at the base of an aquarium cabinet (which may be too much head pressure for many small Internal Filters and Power Head Pumps).

The above suggestions work for many similar applications, and that is moving any device closer to the aquarium (or pond) thus reducing vertical lift.
Horizontal lift is also and issue, but not nearly as much so as vertical lift; that said reducing any unnecessary horizontal tubing/piping can improve head pressure as well.

Removing/Reducing turns and twist in the piping/tubing can help improve head pressure too.

If the problem is a waterfall on pond, this may obviously be more difficult to adjust (other than simply changing pumps to one with higher head pressure), but if this can be done or if changes in piping routing can be accomplished, this can help improve flow rates.

References, Additional Reading:

*Pipe Pressure Loss Calculator
This is excellent reading for those with a more technical engineering aptitude.

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For other articles to help readers make well informed decisions about their aquariums or ponds, please consider reading these:

*Aquarium Lighting; the most in depth & researched article anywhere on the internet.

*UV Sterilization; Sterilizer Information; as with the Aquarium Lighting, this article is a must read for aquarium or pond keepers.

*Pond Care Information; Basic but complete information about pond care with links/resources to more in depth pond care help/information.

Other Recommended Reference Sites

-A useful source for current Aquarium Information and Resources (Pond too). Basic and in depth articles from Aquarium Lighting; Help & Information, Filtration, Fish Nutrition, UV Sterilization Information, Ich, Pond Care, Nitrogen Cycle, and much more. Well researched and up to date aquarium and pond articles, answers, help, and links. Based on 33 years Professional experience & research in Los Angeles and now in Oregon. This Aquarium and Pond Information resource is a must read for any aquarist serious about current aquatic information and articles

For a friendly, Knowledgeable, aquarium forum with in a family atmosphere, Aquarium Forum; Everything Aquatic & Board is an excellent place to go for information, help or simply to share your love of the aquarium and pond hobby and help others. A superior place for information over such places as Yahoo Answers

FISH AS PETS; Articles and commentary of Interest to the Aquarium Hobby; Such as Parasite Retailers, Planaria & Detritus Worms in Aquarium, Melafix Dangers; is Fish Lore Correct?, & Celestial Pearl Danio, Galaxy Rasboras

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