Before you buy a Whole Home Rainwater Filtration System, have this conversation first.

This is one of the most important questions a buyer can ask, and the question that almost no one thinks to ask before they purchase.

Many rainwater filtration systems are sold in large enclosures. The outer dimensions can be substantial. But the actual filter cartridge inside the enclosure varies significantly between systems. Some use a full 20-inch x 4.5-inch cartridge. Others use a 10-inch, a 15-inch, or even a non-standard size such as 19.5-inch or 20 x 2.5-inch. The internal filter size is sometimes not stated on product brochures or marketing materials.

This matters for two reasons. First, the size of the filter cartridge directly affects performance. A larger cartridge has more contact surface with the water, which generally means longer filter life, better maintained water pressure, and less frequent replacement. Second, non-standard sizes often mean you can only buy replacement filters from the original supplier, at the price they set, for as long as you own the system.

Before you buy any rainwater filtration system, ask two specific questions: 'What is the exact size of the filter cartridge inside, in inches?' and 'Can I buy replacement filters from anywhere, or do I have to buy them from you?' Ask for both answers in writing. They are the single most useful pieces of information you can have when comparing systems on long-term value, not just upfront price.

Warranty length is the easiest thing to compare and the least useful. The structure and the conditions are usually what matter.

It is worth asking whether the warranty covers both parts and labour, or only parts. A parts-only warranty can leave you paying significant labour costs even when a faulty unit is replaced free.

It is also worth asking how the warranty is split over its full term. Some warranties advertise a long headline number but break the coverage down differently underneath, with parts covered for a shorter period and labour for the remainder.

And it is worth asking what conditions apply. Most warranties have some, which is normal and reasonable. What matters is knowing exactly what they are before you commit. As one example, some warranties may not be honoured if you relocate the system to a new property.

Ask for the full warranty terms in writing before you buy. The terms tell you more than the headline number ever will.

Many filtration systems are sold on marketing claims about what they remove and how well they perform. Few are backed by independent laboratory testing from a recognised Australian institution.

Without independent testing, there is no way to verify what a system actually removes or how much of it. Ask any supplier whether their system has been tested by an Australian university or independent laboratory, and whether they can show you the results.

It is the difference between being told a system works, and knowing it does.