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Why is my Pump under-performing?

Pondkeeper Hints and tips on what to do if you think your pond pump flow is not working correctly

26 September 2020

There are lots of reasons for lower than expected flowrates, other than a faulty pond pump.  The most common cause of a low flowrate is the way in which it has been installed. Before you decide that it is the pond pump itself that is underperforming consider some other possible reasons.

We will always try to identify if any of the following are causing problems with your pumps flow rate:

  • Are there any blockages?
  • Are there any T-pieces/Y-pieces which could cause problems?
  • Is the impeller faulty?
  • How high is the pump pushing?
  • What size hose are you using?


Begin with the straightforward checks.  Is the pump clogged up, has one of the flexible pipes been blocked or squashed? First begin with the pond filter cage, are the holes in the cage blocked in any way. It is always worth giving a pond pump cage a regular clean so debris is less likely to create blockage. Next remove the pond filter cage to gain access to the pond pump itself – you should be able to locate the pond pump impellor – can you rotate it simply with your finger? Sometimes grit or debris may stop the impellor spinning freely.

Checking the basics is always the best way to start.  Don’t forget to have a good look around your pond filter too. If there is a spray bar fitted top the pond filter can water flow freely. Is the filter blocked with much preventing the water pushing through?

You may also want to check any T-Pieces or Y-Pieces that you have fitted inside the unit. 

Faulty Impeller

The most common cause in a pump decreasing water is the Impeller. This is a small piece inside the motor of the Pump.

In short, if this stops spinning, then the pump's flow rate will be reduced. The impeller can be damaged due to the unit being blocked or from loose debris being pushed through the pump.

These are perishable parts, so you will need to replace the impellers over time. We would recommend replacing these every 2 years for maximum efficiency; although your impeller may last longer with good maintenance.

An easy way to check if the impeller is faulty or not is to open your pump and spin the impeller.

  • If the impeller spins with resistance, then the impeller is ok. 
  • If the impeller spins freely, then it will need to be replaced.

Pond Head Height

Pond head height also needs to be considered. The pond head height is simply distance between the surface of the pond and the highest point that you are pumping to. Ie, the distance from the water surface to where the water flows out (your pond filter exit point if a gravity filter or the top of your waterfall).

Due to the force of gravity the higher you pump the water, the llower the flowrate will be. Always base any flowrate figures on your pond pump on the flow at the head height you will be using (these will be available on the pond pump box or manufacturer literature). Remember that if your system contains a filter and UV you should allow about 2ft (0.6m) additional lift for the back pressure that these units produce

Pond Hose Size

Top tip is to always use the largest bore (diameter) pond hose that will connect to your pond pump. This is because a smaller dameter pond hose will constrict the flow of water so increasing friction which will negatively affect flow rate. Most pond pumps will come with a hosetail (typically 20mm-40mm) which means different sizes of pond hose can be connected. If possible select the highest size hose (eg, 40mm). Make sure you cut down the hosetail to the size you require. Before choosing pond hose size check the size of the pond filter inlet to ensure it is able to accommodate the same size of pond hose. One further check is to ensure the pond hose has a smooth internal surface so to reduce friction – a ribbed feel on the outside is fine and helps to protect the hose from damage. Try to keep runs of pond hose as short and straight as possible to reduce friction.

To help with understanding when maintenance should be undertaken on your pump, we have created a schedule below which may help. We find that pumps that are actively maintained often last much longer than a filter that is hardly maintained.

  Every 2 Weeks Every Month Annually Every 2 Years


  Clean, Inspect and Secure    Replace
O-Rings   Clean, Inspect and Lubricate  Replace  
Strainer Cage Clean and Inspect      
Impeller Chamber Clean and Inspect      
Impeller Clean and Inspect     Replace

If you have any queries, please contact us.

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