"This must be the best value, longest-life phosphor-dosing scale inhibitor on the market. The unit contains a large quantity of dosing crystals which can last for years: Even when they're fitted on the cold supply to the whole house (except the kitchen sink) most of the mass of crystals is still going from ones I've fitted 5 years ago. When fitted to e.g. an electrically-heated shower they should last practically forever.
Fitting is slightly fiddly: it can be hard to get good, secure fixings for the 4 closely-spaced screw holes into less than perfect walls (e.g. old crumbly bricks) and the water connections, involving 3/4" BSP male x male adapters which screw into the housing, are a bit clunky and add to the size of the unit which can be an issue when trying to fit it into confined spaces. (Don't try to use a fitting with a 3/4" male thread in place of the plastic adapters: it is surprisingly easy to cross-thread them and strip the thread in the adapter housing!) Flexible bath tap connectors (with 3/4" threads) can be used.
Incidentally the check valve supplied with the kit is for compliance with water regs: since the inhibitor adds a minute amount of chemical to the water, and even though the chemical is food grade, the water is considered to be no longer fluid category 1 - wholesome water - and must be prevented - by the check valve - from getting back into the water mains. If there is another check valve upstream of the inhibitor (e.g. at the incoming main) that will do instead."