How Do Garden Pond, Fountain & Waterfall Pumps Work?
Probably all pond pumps whether used for fountains, waterfalls or filters are
what are generally called centrifugal pumps. All centrifugal pumps behave in the
same kind of way ...
Electric power to the pump motor turns an
impellor at high speed
The pump impellor has a series of small
chambers separated by vanes and the vanes are designed in such a way that
they almost scrape the circumferential pump body in which they are turning.
On each revolution of the impellor water
contained in each small chamber is discharged by centrifugal force through a
"gap" in the body of the pump and a pipe is attached to this gap so that the
water can be directed to the falls, filter or fountain arrangement.
Because the vanes are rotating at high speed
the water flow through the "gap" appears continuous
Water flows into the small chambers through a
central aperture called the pump suction .... this central point must be
kept clear at all times for the pump to perform efficiently and effectively.
To prevent blockages the suction is almost always inside a type of slotted
cage or body and in many cases there is a foam sponge inside the body cage
which prevents even small particles getting into the vanes of the rapidly
circulating pump impellor to prevent possibly breaking the impellors.
What Makes The Pump Impellor Turn?
Electricity flows through wires totally enclosed in a block of resin to
insulate the wires totally from any possible contact with water and thus make
the pump safe for immersion in the pond water .... safety is of critical
importance and is why a complete waterproof design is essential. This need for
absolute safety means that if the pump electrics fail then the pump cannot be
Electric current that flows in wires surrounding a magnet creates a turning
force so the body of most fountain or waterfall pump impellors consist of a
cylindrical magnet through which a ceramic or stainless steel shaft runs. As
soon as the power is switched on the magnet turns around the shaft (which
remains stationery) and the impellor which is permanently attached to the
rotating magnet then also turns at the same speed as the magnet.
The vanes of the pump impellor in small fountain pumps tend to be straight
while larger pumps have curved vanes ... the curvature increases the efficiency
of the pump meaning more water can be delivered to
a greater height using less or equal power.
If you remove the impellor always be very careful especially if the shaft is
ceramic ... a pump shaft made from ceramic material is very hard and allows the
life of a pump to be extended greatly BUT the shaft is brittle so handle with
care. Most shafts sit inside rubber or plastic bushes to hold the shaft rigid
and in a central position. These bushes are small and can be easily lost.
While holding the impellor gently turn the vanes and generally you'll find
they only turn in one direction (especially if vanes are curved) and trying to
turn them in the opposite direction the vanes come up against a stop. The reason
for this is that electric current is oscillating and in practice therefore a
rotor could go clockwise or anti-clockwise depending upon phase position. If the
vanes initially start to rotate in the wrong direction the "stop" forces them to
turn back in the opposite direction.
What Can Go Wrong With My Garden Pond Pumps?
Modern UK pond pumps from companies like Hozelock, FishMate, Blagdon, and
Oase are almost perfect. These pumps run day in and day out if looked after and
will last for many years ... most pumps these days have 3 years guarantee and
this is indicative of reliability. However there are occasional problems that
can be summarised as follows ...
The impellor shaft breaks during handling ...
replace impellor shaft only.
The vanes become detached from the magnet ...
buy new impellor (the test here is if the impellor rotates in both
directions fully and pump flow is greatly reduced or zero).
Suction becomes blocked ... remove
restriction. If pump has sponge this blocks with algae and after a while it
blocks every day. At this stage replace sponge. Our suggestion is always get
a modern pump without sponge or one that will probably run happily without
the sponge in place.
If the pump does not deliver any water flow
and the impellor is fine and correctly seated in the bushes then the motor
is probably burned out and as such the whole pump would have to be replaced.
You can safely close a valve on the delivery
side of a pond pump but always make sure the suction remains completely open
and free of restrictions. Any restriction on the delivery side will reduce
pump flow ... e.g. kinked pipes, bends, too many connectors and too small a
diameter tubing or pipe will all restrict water flow. Always minimise the
length of tubing or pipe used between the pump and the filter or waterfall.
Excessive pipe length will lower flow rate (measured in gph or gallons or
litres per hour, lph).
How to choose the right pond pump
An important article has been written in this regard. In essence you decide
the right pond pump by considering 5 things ... We suggest you read the
pump comparison article here and also remember
using 2 pumps is often cheaper and always better
than running 1 if you have fish especially
Determine the optimum pump
Optimize pump head
(pressure) by removing any obstructions to flow and minimising actual height
to which you want to pump water
Purchase price of the selected pump
Running cost of the selected pump which is
mainly the cost of electric power
Guarantee period and value of the pump
guarantee (read the instructions)