It is important that you select the correct installation kit to suit your pipework and hot water system in your property. This avoids any chance of the incorrect installation kit from restricting the flow.
If you have conventional gravity fed, or indirect, system which has a cold water tank in the loft space and hot water cylinder, then the standard 15 mm kit would be suitable.
If you are still on 15 mm pipework but have a combination boiler, or direct type system, then you are better to go with the 15mm full flow installation kit. This has the benefit of using uprated fittings and connection hoses and ensures that you get a minimum of 15 mm bore through everything and therefore get no restriction on flow.
If you are unsure, then please call us on 0845 10 80 685 or email for clarification.