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Pool & Spa Cartridge Filters - Frequently Asked Questions

Posted by Julie R. Thank you to our Supplier for this Information Magnum Pool Cartridges (NZ) on

Frequently Asked Questions

When do I need to clean my cartridge filter?

Because a filter is designed to collect unwanted particles, debris, oils, soaps etc, it will need to be cleaned periodically. Basically the more you use your spa or pool, the more often you have to clean the cartridge filter. From very high usage (every 2-3 weeks) to low usage (every 2 months).

A blocked cartridge can put undue pressure on your pump and motor and decrease the life of these parts.

How do I clean my cartridge filter?

Please see "Spa and Swimming Pool Filter Maintenance Blog" for further advice on cleaning.

When do I need to replace my cartridge filter?

Magnum cartridge filters (our supplier) do not have a defined termination point, but over a period of time your cartridge will gradually block up and will need replacing.

There are three factors in determining whether you cartridge needs replacing:
  1. The cartridge gets blocked quickly, and you have to clean your cartridge more often.
  2. Restricted water flow and high differential pressure (differential pressure is the difference between the pressure coming in and the pressure going out, caused by a blocked cartridge).
  3. A cracked or damaged cartridge (this is very rare, and is caused by high differential pressure).

In a spa or pool with very high usage this can be within a year, but normally between 18 months and 2 years.

Why is the endcap cracked?

If the cartridge filter is very old and dirty, it needs to be replaced (although it is rare for the endcap to crack because of being blocked).

If the cartridge filter is new and fairy clean, then the filter housing or skimmer box could be plumbed backwards. 

Simply put, the inlet and outlet on the filter housing were reversed during installation, which means the water flow is going through the cartridge filter ‘inside out’ rather than ‘outside in’.

The pleats also appear to be rounded and puffy, rather than sharp.

Our advice is to contact us with a photo or bring the cartridge back to us for confirmation.


In the dimension chart, there are two cartridges with the same dimensions, which one do I need?

Some cartridges are the same size but the difference is the amount of filter material (number of pleats). Manufacturers of filter housings do this so they can use the same housing for different size pools or spas. If the filter housing does not stipulate the type of cartridge (e.g. 50sq ft or 75sq ft) then please contact the manufacturer or installer of the filter housing. If this can not be done, it is better to over spec than under spec, (e.g. 75 instead of 50). If it has more square footage than it needs, it will normally last longer.