How to deal with a faulty direct hot water cylinder

How to deal with a faulty direct hot water cylinder

Sébastien, Rédacteur en chef

Guide written by:

Sébastien, Rédacteur en chef

351 guides

When your hot water cylinder, or storage water heater, breaks down the issue can usually be traced back to a faulty element, a corroded sacrificial anode rod, a thermostat that no longer regulates the temperature or a leaking T&P valve. Follow our guide to identify the issue and get your hot water cylinder up and running.

Important features

  • Changing a thermostat
  • Changing a T&P valve
  • Descaling a hot water cylinder
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Main reasons for water heater breakdowns


The first sign that your storage water heater has broken down is cold water. A direct hot water cylinder is an electrical unit that heats the water directly (without the need for a separate boiler). These cylinders are essentially a large white tanks composed of:

  • electrical immersion element(s): used to heat the water;
  • a thermostat which works with the element to control the temperature of the water;
  • an anode rod for anti-corrosion protection;

On the outside of the cylinder, you will find all the parts required to feed water into the heater:

  • Temperature and pressure relief valve: a safety device with several roles, including controlling the internal pressure of the cylinder (max. 7 bar);
  • Expansion valve: helps to relieve pressure and prevent the 'water hammer' effect;
  • Expansion vessel: helps to prevent overpressure by absorbing excess pressure;
  • Thermostatic mixing valve: used to limit water temperature at the outlet (max. temperature: 55 to 60°C);
  • Dielectric unions: fitted at the water inlet and/or outlet for anti-corrosion protection.

Some of these elements are mandatory and others simply recommended for optimal use. If your shower is running cold, the issue is usually with your electrics.

Detecting a water heater failure


After taking your first cold shower, it is best to work through a process of elimination in order to identify the issue. Start with the most basic solution.

Check that you're not simply just running your washing machine and dishwasher while you shower. If this is the case, you might just be asking too much from your water heater. If not, go to the fuse box and make sure that the heater hasn't tripped the fuse switch. Your fuse switch should be in the on position; if not, flip it and see if it stays up. If it does not stay up, your water heater has short circuited. There are several possible possible reasons for this:

  • Your electrical installation is faulty (two wires are touching, etc.);
  • Your cylinder is leaking;
  • An electrical component has short circuited (either the heating element or thermostat).

Detecting an electrical failure in 5 steps

Electrical connections
  1. Before you touch the water heater, turn off your power supply and check the heater with a voltmeter or multimeter in voltage mode to ensure the power is off.
  2. Open the access panel to expose the thermostat and heating element.
  3. Check that the wires are connected properly and that there are no leaks.
  4. If there are no apparent leaks, remove and test the thermostat by warming up the probe and observing the thermostat.
  5. If the thermostat is working, remove and test the element. Some types of immersion elements do not require you to drain the cylinder to gain access – but most do. It's critical to perform this test safely. If you do need to drain the cylinder, take advantage of this time to descale your water heater while you're at it!

Please note: this advice is intended to give you a better understanding of your hot water cylinder; any work with electrical appliances must be carried out by an authorised professional.

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How to change a storage water heater thermostat

While there may be any number of reasons causing storage water heater issues (leaks, faulty element, etc.), the most obvious place to start is with the thermostat.

  1. Switch off the power at the fuse box.
  2. Identify then disconnect the electrical wires from the thermostat before removing it.
  3. Before installing the new thermostat, set the temperature by turning the dial to the desired max. temperature (60°C).
  4. Fit and reconnect the thermostat
  5. Replace the access panel and turn on the power.
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How to change a T&P valve

If you are hoping to replace a leaking temperature and pressure relief valve, choose the right model depending on your unit and the type of water where you live. The role of this part is to regulate the amount of pressure inside the cylinder. This is essential as water expands as it heats, meaning it will occupy more space. A T&P works by letting out water through a discharge line to maintain a safe level of pressure (under 7 bar).

Replacing a T&P valve   


Start by turning off the power to your hot water cylinder; do this the day before you start work to avoid burning yourself. Disconnect your water supply and turn off the incoming water to your heater. Place a bucket under the valve and have a mop at hand.

Turn on a hot water tap to relieve any remaining pressure and disconnect your discharge line. Remove the T&P valve with the bucket positioned directly beneath it. You do not have to drain your cylinder to change the T&P valve but it is recommended. Clean around the area where the valve was connected and discard the old valve. Apply some PTFE tape and screw in your new T&P valve. Attach it to your discharge line (reapplying PTFE tape here too).Turn your water supply back on and turn on a hot tap to get rid of any air. Wait for the tank to fill up and check your new valve for any leaks. Once the cylinder is full, open the T&P valve to test it out; you should hear it hiss as the water flows out. You should now have hot water again!

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How to descale a hot water cylinder

Why descale a storage water heater?

The benefits of descaling any appliance are obvious. More specifically, removing limescale deposits from your hot water cylinder and element, and checking the condition of your anode rod, will help ensure your water heater has a long service life. Limescale build up is common in water heaters as more sediment is released by hot water than cold. The amount of descaling you have to do will depend on the hardness of the water where you live; the harder the water, the more limescale build-up there will be.

Descaling is therefore an essential part of maintaining your hot water cylinder. You will have to start by draining your cylinder of water. Plan out this process in advance, turning off your power and water supply before everyone in your family has a shower. This will mean you will only have a small amount of water to drain by the time you get around to working on your water heater.

Descaling a hot water cylinder in 10 steps

Instantaneous water heaters: installation set-up
  • Turn off the power supply.
  • Remove the access panel.
  • Remove thermostat and electrical wires. 
  • Unscrew and remove the access panel where the anode rod and the element are fixed (if applicable)
  • There's no point in replacing an immersion element that still works. Instead, you can clean it up using a specialist anti-limescale product to avoid damaging the metal. You can also use a wire brush to remove deposits. If your storage water heater is equipped with a sacrificial anode rod that has worn out, you will have to replace it. If it is still in decent condition, there is no need to replace it. Other types of anode rod (imposed current or hybrid models) are not designed to corrode but they do need to be cleaned.
  • Remove any limestone deposits from the cylinder and replace the seal if it is worn.
  • Replace the access cover and tighten it. 
  • Reconnect the thermostat – remember the ground wire! 
  • Replace the access cover to the thermostat
  • Turn your mains water supply and electricity supply back on.

Please note that if your anode and immersion element(s) are in poor condition or appear to be faulty, they must be replaced.

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More information

For more information on heating water, check out the following guides:

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Guide written by:

Sébastien, Rédacteur en chef, 351 guides

Sébastien, Rédacteur en chef

A l’issue d’études dans le commerce, j’ai appris par opportunité la profession de charpentier. Noble mais dur métier, les hivers et la pluie m’ont incité à descendre des toits et à pousser les portes d’une quincaillerie où sans grande surprise, je me suis retrouvé quincaillier. Le métier m’a plu et au fil des années, j’ai accumulé les expériences dans plusieurs Grandes Surfaces de Bricolage. Entre deux magasins, je poursuivais mon apprentissage et travaillais en serrurerie, couverture, imprimerie ou dans le secteur industriel pour des sociétés de traitement de surface, de fabrication de vases d’expansion... L’envie de transmettre m’a ensuite conduit à me former au métier d'enseignant de Français Langue Étrangère, puis j’ai commencé d’écrire des articles de voyage, puis de bricolage pour Aujourd’hui Rédacteur en chef de la section Conseil technique de ManoMano, j’ai le plaisir de travailler avec une communauté de vrais experts dont un jardinier hors pairs, un soudeur émérite, un plombier passionné et encore beaucoup d’autres. Restituer un conseil impartial est ce que nous faisons de mieux, et ce que nous continuerons de faire pour vous.

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