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Water softeners: what they do and what installing one involves
Hard water in UK homes, what a domestic ion-exchange water softener does, salt and the bypass, why you should keep an unsoftened drinking tap, and the water regulations that cover installing one on the rising main.
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If your kettle furs up quickly, soap does not lather well and there are white marks round the taps, you probably live in a hard water area. A water softener is one answer. It is also a plumbed-in appliance on your main water supply, so it is worth understanding what it does and what installing one involves before you buy.
What hard water is
The Drinking Water Inspectorate (DWI) explains that water hardness is caused by dissolved calcium and magnesium, and that it varies with local geology. It classifies hardness, measured as mg/l of calcium carbonate, like this:
| Description | mg/l as calcium carbonate |
|---|---|
| Soft | up to 100 |
| Slightly hard | 100 to 150 |
| Moderately hard | 150 to 200 |
| Hard | 200 to 300 |
| Very hard | more than 300 |
The DWI page links to a map of typical hardness in England and Wales. It says that for an accurate figure you should ask your water company, because there is local variation. DWI also notes that water companies often publish a water quality summary on their website.
The everyday effects are familiar. DWI says hard water causes scaling in hot water systems, kettles, irons and appliances, and leaves tide marks on basins, baths and toilets. It also produces less lather from soap and washing products. Our limescale guide covers cleaning it off.
What a domestic softener does
Most home softeners use ion exchange. DWI describes it as a process that replaces the calcium and magnesium (the hardness) with sodium. Water passes through a bed of resin that holds on to the hardness. Every so often the softener regenerates: it rinses the resin with brine made from salt in the unit, and flushes the hardness away to drain.
Points that follow from how they work:
- Salt. You top up the softener with salt made for the purpose, usually tablets or blocks. Running out of salt means the water stops being softened. Use the type the maker specifies. Dishwasher salt is a different product (see dishwasher plumbing).
- A drain. Regeneration produces waste water, so the softener needs a suitable drain connection.
- Power. Many softeners have an electronic control head that plugs into a nearby socket.
- A bypass. Installations usually include a bypass, so water can still reach the house if the softener is being serviced or has a fault. Ask your installer to show you how it works.
- Maintenance. DWI says that for softeners it can be very important to backwash them regularly and replenish salt. If the property has been empty for a while, the unit will need attention when you return. Follow the maker's instructions.
Keep an unsoftened tap for drinking
This is the most important point. DWI advises that you keep an unsoftened supply to the kitchen tap for drinking and cooking. It says some softeners can add significantly to the sodium content of drinking water. It also says that artificially softened water should not be used to make up baby feeds.
Water Regs UK, which publishes guidance on the water fittings regulations, says in its FAQ on softeners and conditioners that, ideally, a drinking water tap supplying untreated water should be kept. It says whole systems may be treated, but only on condition that the treated water remains wholesome.
Installing one: the rules
A softener goes on the rising main, usually just after the stopcock, so installing one is plumbing work on your drinking water supply. It has to comply with the Water Supply (Water Fittings) Regulations 1999 in England and Wales. Scotland and Northern Ireland have their own equivalent rules.
Two points from the official sources:
- Notification. Regulation 5 of the regulations as made lists installations that need notice to the water company and its consent before work begins. The list includes "a water treatment unit which produces a waste water discharge or which requires the use of water for regeneration or cleaning". Check the current rules and your water company's guidance before work starts. Water Regs UK's notification guidance is a good place to start.
- A qualified installer. DWI says to always use a qualified plumber to install a water treatment device, and to check in advance with your water company that the device meets the water fittings regulations. It also warns against buying from door-to-door sellers who offer to test your water.
Before buying, DWI suggests asking your water company for its water quality report for your address. That will also tell you how hard your water actually is. See water quality and hard water areas for other questions worth asking.
Softeners vs "conditioners"
Softeners are not the same as the in-line scale "conditioners" or inhibitors sold for single appliances or pipes. DWI says magnetic devices "do not soften the water". It recommends buying one only on a sale-or-return basis, and asking for performance data in a home setting. See our limescale guide.
Softener and salt at Wickes
We haven't tested this. Figures are as the maker states them (checked 13 November 2026).
BWT WS355 standard water softener. Wickes says it has an electronic control system and a low voltage transformer that plugs into a nearby socket, a low salt alarm, and a 15mm fitting kit as standard. Wickes lists it as WRAS approved and built and tested to BS EN 14745, with 2 years on-site labour and 5 years parts warranty. Have it installed by a qualified plumber, with an unsoftened kitchen drinking tap.
BWT water softener salt tablets, 25kg. Wickes describes it as tablet salt that complies with BS EN 973:2009 for water treatment, and says it is suitable for all water softeners. Check your own softener's manual for the salt it needs.
Is softened water safe to drink?
DWI advises keeping an unsoftened supply to the kitchen tap for drinking and cooking, because some softeners add significantly to the sodium in the water. It also says softened water should not be used for baby feeds.
Can I fit a water softener myself?
It is plumbing work on the rising main, it has to comply with the water fittings regulations, and the water company may need notice before work starts. DWI advises using a qualified plumber and checking with your water company first.
How do I know if my water is hard?
Ask your water company, or look for the water quality summary on its website. The DWI hardness page also has a map of typical hardness in England and Wales.