How To Choose a Garden Pond-Pump for Pressurized Pond Filters
Pond filters serve 2 purposes and the more important of these 2 by far is the water purification or cleaning stage. The second purpose is to assist in removing suspended solids (although no filter will remove green suspended algae particles because they are so very tiny). Without good pond filtration your pond will sooner or later become a death trap for your fish. Learn about pond-filters here.
Pond filters fall into 2 main categories ... pressurized filters and gravity flow filters. The traditional filter for most garden ponds until fairly recent times was the gravity filter ... essentially a box containing biomedia on which bacteria could grow. Water was pumped into the box and after passing over the biomedia overflowed through a hole in the side by gravity back into the pond or onto the waterfall. In more recent times small neat and compact pressurized pond filters have taken the market by storm.
These pressurized filters (example of Fish-Mate shown left) are placed between the pump and the waterfall or pond and the water is forced through the unit and out onto the waterfall or back into the pond directly. The big advantage of this kind of filter is that it is small and can be more easily hidden or built into the landscape.
The New Pressurized Filters Created a Need for More Sturdy Filter Pumps
The new pressurized filters normally contain in an enclosed cylindrical chamber a mixture of foams and different biomedia like Alfagrog (best low cost biomedia of them all) used in pressurized UV Pond Filter Combo from Fish-Mate.
The foam and beds of biomedia actually restrict the flow of water and this is why more powerful pumps had to be developed. In addition filter cleaning mechanisms had to be designed to prevent too frequent clogging by algae build up in the foams.
The pumps so developed have generally been classified as filter-pumps although they are of course not restricted to this application. They do not have foams in the suction which is a true blessing. I personally class the development of these solids handling filter pumps (so called because small solids can flow through the pump without damaging the impellor) one of the major improvements in pondkeeping over the last 10 years.
Solids handling robust filter pumps have reduced the amount of maintenance and impellor failure by a dramatic amount ... and it has always been the hassle and unreliability of equipment that held back pond keeping as one of the most wonderful part of gardening.
Add the reliability of the modern filter pump to the ease of cleaning
pressurized filters and the result is a virtually maintenance-free pond.
Most suppliers of filter pumps also make pressurized pond filters combined with UVC (for control of green suspended algae) and to make the correct choice of filter and suitable pump easier they package the pump, UVC and filter together. You can see what I mean in the picture opposite. This shows a Hozelock Bioforce Filter with a Hozelock Titan filter pump. Together the are called Hozelock Ecoclear system.
Pipe Friction Losses, Pressure Drop Across the Pressurized Filter & Final Choice
As explained previously energy from the pump motor is divided between pumping water to a height (expressed as pressure head) and volume flow rate of water and some energy also goes into pushing the water against the sides of the pipe connecting the pump to the waterfall. It was explained that the longer the pipe and the lower the diameter the more energy is needed to overcome what are called PIPE FRICTION LOSSES. These losses are very complex to calculate so add 20% to the head as an approximation of the pressure loss due to friction in the pipe.
Well the fact that the pressurized filter is fill of foam and biomedia this becomes another resistance to flow that takes up part of the pump motor energy. What this means to you is that in addition to measuring the head distance from pond surface to waterfall inlet you must also add 20% for pipe friction loss and another 30% for pressure drop in the pressurized filter.
In the example of a pump for waterfall used previously we then need a pump that will deliver at least 1,800 litres per hour NOT at 1.2 metres (note that originally this was 1 metre before pipe friction losses were taken into account) but at 1.5 metres head. This is the information you need to pick the right pump ... but only after you understand the pump running costs and how to calculate or compare different makes and models. There can be highly significant differences in running costs between pumps that do the same or similar duty.
Retailers will normally not provide you with this information so take care.
I have created 12 different pond calculators and these are available free at my other site Perfect Pond Keeping One of the calculators allows you to accurately specify any pond pump even taking pipe friction losses into account. I would suggest you get these Excel calculators and play around with them.
Pond Keeping Units
Now we've started to talk litres you might find this table below useful. It helps to translate one set of units to another quite easily.
| Unit | Imp Galls/min | cu ft/min | Litres/min | cu m/min |
|---|---|---|---|---|
| Imp. Gallons | 1 | 0.16 | 4.55 | 0.005 |
| Cu.Feet | 6.23 | 1 | 28.32 | 0.028 |
| Litres | 0.22 | .04 | 1 | .001 |
| Cu. Metres | 220 | 35.32 | 1,000 | 1 |
Example ... 1 cu.m per minute is the same as 1,000 litres per minute or 35.32 cu.ft per min or 220 imperial gallons per minute.
Factors to be aware of with all pumps. This is a good page to read before you buy any pump.