Why Dishwashers and Washing Machines Fail Early in London’s Hard Water, and the Cleaning Routine That Helps

Every kitchen I work in south of the river has the same tell. Pull the lower rack out of a dishwasher on Haydons Road and run a fingernail along the underside of the spray arm – chalky grit packed into the jets like old grout. Machines here rarely die of old age. They die of accumulation, and what accumulates is calcium carbonate that dissolved out of the chalk under the North Downs long before it reached anybody’s mains. I’ve cleaned houses around SW19 for a good while now, and I can guess a client’s appliance repair history from the state of their kettle before I’ve opened a cupboard.

What is London’s water doing inside your machines?

Hardness across Greater London sits somewhere between 250 and 350 milligrams per litre expressed as calcium carbonate. Sutton and Croydon come in at the harder end, because SES Water draws heavily from chalk boreholes. Affinity’s supply up into Harrow is much the same story. Thames Water covers most of the rest and the figures barely move. There is no soft-water borough worth the name. If a friend in Walthamstow tells you their water is different, ask to see their kettle.

Calcium carbonate has one property that makes it a specific menace to appliances rather than a general nuisance: it gets less soluble as water gets hotter. Dissolved calcium sits quietly in solution at 15°C. Push that same water past roughly 60°C against a metal surface and it comes out of solution and bonds to the metal as hard scale. The hottest surface inside your dishwasher or washing machine is the heating element. Scale finds it first, every time.

Everything else follows from that. Spray arm jets narrow. The pressure chamber tube on a washing machine and the turbidity sensor on a dishwasher silt up and start reporting nonsense to the control board, which then runs cycles that make no sense to anybody – a rinse that fills three times over, a drain phase that never completes. Door seals stiffen and stop compressing. Drain pump impellers catch flakes of scale sheared off the tub wall and jam solid. No single one of those is fatal. Together they are the chalk tax, and it gets paid either in monthly maintenance or in a replacement machine four years early.

Why the heating element always goes first

Limescale is a poor conductor of heat. A layer a millimetre thick on an element does two things at once: it stops heat reaching the water, and it traps heat inside the element. The thermostat reads cool water, so it keeps the element energised, so the element cooks itself. You notice it as a cycle running half an hour longer than the display promised. Then one morning it’s a machine that trips the RCD the moment it starts to heat. Two clients in Southfields lost machines that way within a year of each other, same estate, same explanation from the same engineer.

Why does the salt compartment matter more than the tablet you buy?

Your dishwasher contains a water softener. Most people I work for have no idea it’s in there. It’s a small column of ion exchange resin that swaps calcium and magnesium ions for sodium ones, and it can only keep doing that if it’s flushed periodically with brine. That is the entire purpose of the screw cap in the floor of the machine. Coarse dishwasher salt only – table salt carries anti-caking agents that clog the resin bed.

All-in-one tablets are sold with the strong implication that you can stop bothering with any of this. The softening component in them is a chelating agent, usually MGDA or GLDA since phosphates were withdrawn from domestic dishwasher detergent in 2017. Chelators bind calcium in the wash water perfectly well for an average load in an average town. In water at 300 mg/l they exhaust partway through the cycle, which is exactly why the glasses at the back come out cloudy while the plates look fine. Fill the salt. It’s about two pounds a kilogram and a working London kitchen gets through a kilogram every six to eight weeks.

There is a genuine counter-argument, and it deserves stating: over-softened water combined with a hot alkaline cycle etches glassware, and that damage is permanent. Cloudiness that doesn’t wipe off with vinegar is etching, not scale, and no amount of salt will fix it. Which is what the hardness dial is for.

Where the hardness dial hides

It’s buried in the settings menu, usually shown as H0 through H7, and it almost always ships set to H3 or H4 – calibrated for European average water and close to useless here. Look up your postcode on your supplier’s site, divide the milligrams per litre by 17.8 to get German degrees, and set the dial accordingly. Most London kitchens want it near the top of its range. The job takes ninety seconds. I have never once walked into a house where somebody had already done it.

What actually kills a washing machine here – the scale or the way we wash?

The washing machine has no softener in it. Nothing removes hardness from the water at any point in the cycle. What it does have is detergent, and in hard water we all use far too much, because the first thing calcium does is react with the surfactants to form insoluble calcium stearate. Soap scum. You get less foam, so you tip in more, so you make more scum. It doesn’t rinse away anywhere. It collects in the outer tub – the space between the drum you can see and the plastic shell you can’t – along with fabric conditioner residue and skin oils. Then it goes anaerobic and starts to smell.

I opened up a machine on Fortescue Road in Colliers Wood last spring because the client was convinced a mouse had died behind the units. It was the sump hose. Grey, greasy, near-solid, the texture of cold porridge, and the machine had been in the flat three years. She washed everything at 30 with liquid capsules and put conditioner in every single load. Textbook, and not her fault – that is precisely what she’d been told to do by the manual, the detergent bottle and a decade of environmental advice.

The advice is wrong in this city, and I’ll say it flatly rather than dress it up. Wash everything at 30°C in water this hard and you shorten the life of the machine enough to wipe out the saving several times over.

The thirty-degree habit

Liquid detergent contains no bleach and no oxygen release. Powder does – sodium percarbonate, which liberates hydrogen peroxide once the water clears roughly 40 to 60°C, plus zeolite builders that sequester calcium before it can reach the surfactants at all. A machine that never sees a temperature above 30 and never sees powder has no mechanism whatsoever for clearing what builds up inside it. Every load adds. Nothing subtracts. The energy saved on a cool wash is real and measurable; the carbon buried in a replacement machine at year five instead of year twelve is larger and nobody puts it on the label.

What’s the monthly routine that keeps them running?

Washing machine, once a month, empty drum: the hottest cotton cycle the machine offers – 90 if it has one, 60 if not – with 100g of soda crystals or a scoop of biological powder thrown straight into the drum rather than the drawer. Pick a fixed date so it happens, and treat it as the cheapest instalment of the chalk tax you’ll ever pay. That single cycle does more than any of the descaler sachets sold at the till in Sainsbury’s on the Broadway, most of which are dilute citric acid at eight times the price of citric acid.

Dishwasher, once a month, empty: 60g of citric acid powder in the base of the machine, hottest programme, nothing else in it. Rinse aid reservoir topped up while you’re there, because rinse aid lowers surface tension so water sheets off the glass instead of drying into spots.

Quarterly, both machines: the drain filter behind the kick panel on the washer, and the mesh filter in the dishwasher sump. Expect a hair grip and something you won’t be able to identify.

None of this is cleaner’s work, and I’d honestly rather you didn’t pay me to do it. It’s twenty minutes a month, it needs no skill, and if you’re booking me for four hours I’d sooner spend them on the oven.

The small jobs nobody does

Clear the dishwasher spray arm jets with a cocktail stick, both arms, all the way round – this is the single change that fixes most “it’s not cleaning properly” complaints. Pull the detergent drawer right out of the washing machine and soak it, because the siphon cap underneath is where conditioner turns to sludge. Wipe inside the fold of the door seal and check the drain holes at the bottom of it. Leave the door and the drawer open between washes.

Is a plumbed-in softener worth it, or is that overkill?

For an owned house, and if the mains stopcock is somewhere you can get at, yes. A block salt unit under the sink runs somewhere around £700 to £1,200 fitted, needs a hard water tap left unsoftened for drinking and for filling the kettle, and takes hardness out before it reaches anything. Kettles stay clean. Detergent doses drop by roughly a third.

What doesn’t work, and I’ve watched enough of them go on to be sure, is the clamp-on magnetic or electronic descaler. Clients fit them, believe in them for about four months, and the kettle furs up on precisely the schedule it always did. The chalk tax gets collected either way.

If you’re renting

You’re not installing anything, so the routine is the whole of your defence. Buy citric acid in a 500g tub rather than sachets, keep soda crystals under the sink, and write the monthly date on the calendar. When you move out, the machine you’ve been descaling belongs to the landlord, which is annoying and doesn’t change the arithmetic while you live there.

My own kettle on Kingston Road has been descaled somewhere north of forty times and has never been replaced.