AC filters are plumbed in undersink to supply one tap, within the scope of the skilled DIYer. They cost between £50 and £200, before any plumbing costs. Most cost between £100 and £200. Replacement cartridges cost between £10 and £60, and need replacing every six to twelve months. Some systems offer filters which have indicators of when the cartridge needs replacing. Cartridges are generally slightly cheaper than KDF filters and the cost of water correspondingly lower. Nitrate-reducing versions of undersink filters are available. Tap-flow is not affected, unlike RO systems.
Contaminants can flush back into the system from the filter with some systems. Compared to jug or end-of-tap filters, plumbed-in undersink filters are much more convenient to use and offer a higher level of purity. Depending on the model chosen, the cartridge costs are not necessarily more expensive than jug or tap filters, and the water can be comparable in cost.
Two makes of undersink filter, at the more expensive end of the range. These were the Everpure Citmart BW100 and the Ametek Fileder HM, priced at around £150-200. These systems did not flush back contaminants, and had indicators for cartridge change.
One smaller filter, the Berglen Tapmate AC 200, requiring more frequent cartridge replacement. Cartridges cost £12 and need replacing monthly.
One undersink filter, the Opella Castalia, can be adapted to filter a whole house system. The system is designed primarily for use with a sediment filter, not a carbon filter, but a carbon filter can be used. This means that all your water, including toilet, bath and laundry, can be filtered. The disadvantage is that cartridges will need replacing monthly or every six weeks. This is costly – about £3.50 per week -and inconvenient. Some plumbing advisors also discourage using carbon filters in this way, since bacteria can grow in the system when chlorine has been removed.