Descaling a hot tap before it fails

Instant hot water taps fail for one reason far more often than any other, and it is not a manufacturing fault. It is limescale on the heating element, laid down slowly by hard water, insulating the element from the water it is trying to heat until it overheats and cuts out for good.

The failure looks sudden. It never is. Twenty minutes of maintenance twice a year prevents almost all of it.

Why scale is worse here than in a kettle

A kettle boils and then stops, and you can see inside it. An instant hot tap holds water at temperature continuously in a sealed tank you never look into, which is close to ideal conditions for calcium carbonate to come out of solution and stick.

Because the deposit insulates, the element has to run hotter and longer to reach the same output temperature. That accelerates further deposition, which makes it run hotter still. It is a loop that ends with a thermal cut-out, and by the time the flow visibly drops, the element has usually been working too hard for months.

How hard is your water

Your water supplier publishes this and it is the number that decides how often you need to do anything. Roughly:

Much of southern and eastern Spain, the Paris basin, southern England, and large parts of central and eastern Europe sit in the top two rows. If you have never checked, assume hard until you know otherwise.

The signs, in the order they appear

• White crust around the spout and the aerator. Cosmetic, but it tells you what is happening inside.
• Flow drops slightly. Usually the aerator rather than the tank, and worth cleaning before you assume the worst.
• Water arrives cooler than the setting, or takes noticeably longer to get hot. This is the one that matters. It means the element is insulated.
• Ticking, knocking or a gravelly sound from the unit as it heats. Steam forming under a scale layer and breaking free.
• Cutting out and resetting itself. The thermal protection is doing its job. Act now, because the next stage is permanent.

Doing it

Read your own manual first, because tank designs differ and a few models have a dedicated descaling mode that does most of this for you. Where they do, use it. What follows is the general shape of the job.

1. Isolate it. Switch the unit off at the socket and close the supply valve under the sink. Give it an hour to cool if it has been on. Descaling solution on a hot element is both ineffective and a good way to get a faceful of steam.
2. Drain the tank by the method your manual specifies. Most have a drain point or a procedure for emptying through the spout.
3. Use the right solution. A proprietary descaler, or citric acid at roughly one tablespoon per litre of warm water. Citric acid is cheap, food-safe and effective. Do not use hydrochloric or sulphamic drain products, which attack stainless steel and seals.
4. Fill, and leave it. Twenty to thirty minutes is normally enough. Longer does no harm on a heavy deposit. You should hear it fizzing quietly as it works.
5. Drain and rinse properly. At least three full tanks of clean water. Any acid left behind will taste unmistakably of it, and the first cup will tell you if you have cut this short.
6. Clean the aerator separately. Unscrew it, soak it in the same solution, brush it with an old toothbrush. This alone restores flow on a surprising number of taps.
7. Refill, then power up. Water first, always. Switching on a dry element is one of the few ways to destroy one of these instantly, and it is not a warranty matter.

Vinegar, and why we do not recommend it

Household vinegar does dissolve scale and plenty of people use it. It is weaker than citric acid at a sensible concentration, so it needs longer contact time, and the smell lingers in a sealed tank through more rinses than you will want to do. It is also harder on rubber seals over repeated use. Citric acid costs very little and does the job better.

Reducing how often you need to bother

An inline filter cartridge on the cold feed is the effective answer in a hard water area, and most will noticeably extend the interval between descales. Change it on schedule, because an exhausted cartridge does nothing while looking exactly like a working one.

Beyond that: run the tap for a few seconds each day rather than letting it stand for a fortnight, and if you are going away for a long period, switch it off at the socket rather than holding a tank at temperature for nobody.

One warranty note

Scale damage is not a manufacturing fault and is excluded from conformity cover, here and effectively everywhere else. That is not a technicality invented to avoid claims: a descaled tap lasts many years, and a neglected one in hard water can fail inside two. The returns policy sets out what is and is not covered.