The fault in the heating element has been established, the new part is ready and we can begin. If you like repairing things around the house yourself, this job is a good chance to get to know what hides under the cover of a boiler. You need the right tools, a knowledge of safe work with electrical appliances and a little patience, especially while the water is running out of the tank.
We will go through the replacement with photographs of one job. We will also explain why we carry out certain steps in exactly that order, because the work is much easier once you understand what happens inside the boiler.
What kind of boiler we are repairing
In front of us is a vertical pressurised storage boiler with the water connections underneath. Its heating element is immersed directly in the water and fixed to the flange, the metal plate that closes the service opening of the tank.
On the flange there are also the magnesium anode, which helps protect the tank from corrosion, and the sleeve for the thermostat probes. We will take that assembly out, take it apart and put it back with a new heating element and a new anode. On boilers of a different design, especially those with a dry element, the procedure is not the same.
Before you begin
This job means working on electrical connections and opening the tank. Do it on your own only if you are trained for such work and have the right instruments. If you cannot reliably check that there is no voltage and that the appliance is safe after the repair, leave that part of the work to a qualified person. Where the manufacturer requires an authorised service, keep to that requirement.
Get everything within reach
It is no fun looking for the right spanner while water is dripping from a connection you have just opened. So before you start, clear the space under the boiler and get ready:
- spanners and screwdrivers that fit the connections and the flange;
- instruments for checking the absence of voltage and for the final electrical tests;
- a hose for draining, a basin for water and limescale, cloths and gloves;
- the right new heating element, a magnesium anode and gaskets.
Choose the heating element by the model of the boiler. Besides the power rating and the voltage, its shape, its dimensions and the way it is fixed have to match as well.
First switch off the supply to the boiler at the distribution board and make sure nobody switches it back on while you work. A dark lamp on the bathroom switch is no proof that there is no voltage.
If the water in the boiler is hot, now is the moment to replace it with cold water. While the supply is still open, run the hot tap and wait for cold water to come. That way boiling water will not meet you during the draining.
Close the supply and check that the valve holds
Leave one tap open on the hot side, then close the valve that brings cold water into the boiler. Once the remaining pressure drops, the water at the tap should stop running.
This is a simple but useful check before you take anything apart. If the water keeps running without stopping, the supply is not reliably closed. Sort that out first, and only then pick up the spanner.
Once you have confirmed that the supply valve holds, close the tap as well. The other taps connected to the boiler should stay closed too. That way you stop air entering through the hot line and let the vacuum hold the water back while you take off the safety valve. Even so, keep the basin and the hose ready, because some water can run out.
You will free the hot water connection only once the drain hose is in place.
Take off the cover and clear the access
When you take off the bottom cover you will see the thermostats, the wiring and the terminals of the heating element. Before you touch anything, check that there is no voltage with a two pole tester. The tester itself is checked for correct operation before and after that check.
Photograph the terminals and note the markings of the conductors. That photograph will be a useful reminder later on, together with the wiring diagram of the boiler.
If the thermostats are in the way or could get wet during the draining, take them out carefully and put them aside where it is dry. When you pull the probes out, mind the thin capillary tubes: do not bend them sharply, twist them or pull them by force.
Why we let the water out at the cold connection
Both connections are at the bottom of the boiler, so it seems logical that the water could run out through either of them. Inside the tank, however, they look different.
The cold water pipe ends near the bottom, while the hot water pipe is extended almost to the top of the tank. That is where the hottest water is taken from, while new cold water enters from below and pushes it towards the tap.
Once we close the supply, that pushing is gone. This is why an open hot tap will not empty the boiler.
We let the water out through the lower, cold connection, and through the hot one we let air into the tank. The air takes the place of the water that leaves. Without it a vacuum would build up and the draining would slow down or stop, much like liquid that struggles to leave an upturned bottle until air gets into it.
Draining without flooding the bathroom
Put the basin under the connections and have the hose ready so you can fit it at once. With the taps closed, disconnect the cold water pipe, then take off the safety valve. The vacuum in the tank can hold the water back until you let air in, but be ready for some spillage while you take things apart.
Fit the hose to the cold connection and secure the free end well in the drain. Only then disconnect the hot water pipe from the boiler. Air now enters through the freed connection, and the water leaves through the hose into the drain.
Now you wait. How long the draining takes depends on the amount of water and on how clear the connection and the hose are. If the flow stops too early, check whether the hose has kinked or the passage is blocked with limescale. Do not conclude straight away that the tank is empty.
The old heating element is out
Once the tank is empty, put the basin under the flange. There is almost always some water and sediment left inside, and it will come out as you open it.
Disconnect the remaining electrical connections from the heating element and release the fixing of the flange. How it comes out depends on its shape: some are pulled straight out, while others have to be pushed inwards and turned to pass through the opening. Work slowly, especially if thick deposits have built up around the element.
On the element taken out in our example the damage is plain to see: the sheath has split and the resistance wire inside it is broken. What was hidden in the tank until now we can look at from close up.
Clean the flange and prepare the new assembly
Take the old heating element off the flange and, if anything is left of it, the magnesium anode as well. Clean the flange, especially around the seats of the gaskets. It is exactly those surfaces that decide whether the joint will stay dry after assembly.
Look the metal over before you go on. Deep corrosion or a deformed flange is not repaired by tightening harder.
Fit the new heating element and the anode onto the prepared flange, with the right new gaskets. Do not leave the anode out: it has its own role in protecting the tank, even though it takes no part in heating the water.
While the tank is open, remove the sediment that has gathered and the loose limescale. Work carefully, without knocking or scraping that would damage the inner surface. Clean the seat of the large gasket at the tank opening as well.
Now put the flange back with the new assembly. Check that the gasket is not twisted or pinched and tighten the fixing evenly, to the prescribed torque. Here patience pays off more than force.
Connect the water and fill the tank
Put the safety valve back on the cold connection the right way round, then connect the cold supply and the hot outlet. Use the right gaskets on the joints you opened. The outlet of the safety valve has to stay clear.
Next comes the filling, still with no electrical supply.
Open the hot tap, then open the supply valve gradually. Water will enter the tank and air will leave through the tap. At first you may hear hissing and get a broken jet, the tank is still venting.
When a steady jet comes without spitting air, close the tap. The tank and the joints are now under pressure and you can check your work.
Dry the joints and look carefully over the flange, the joint of the heating element, the safety valve and the connecting pipes. A dry paper tissue held against a joint will easily show even a small drop you did not notice by eye.
If something is leaking, close the supply and release the pressure before you take anything apart again. We carry on only when every joint is dry.
Put the thermostats back and connect the wiring
Put the thermostats and the other electrical components back in their places, and connect the conductors according to the wiring diagram of the boiler.
On the assembly shown in this example the probe of the working thermostat goes into the common sleeve first, deeper in, and the probe of the safety thermostat below it. It matters that both reach the position intended for them and that their places are not swapped. On a different model, check the arrangement intended for it.
Look over the terminals and the conductors, including the connection of the protective conductor. All the electrical components have to be completely dry.
Before the appliance is put back into service, electrical tests are needed, including the continuity of the protective conductor and the insulation resistance. That is the part of the job for which the right instruments and knowledge are necessary. Once the check has passed, put the protective cover back.
The moment to switch on
The tank is full, the joints are dry, the electrical part has been checked and the cover is in place. Now you can switch the supply on and set the temperature.
Do not rush to switch on before the filling: a new immersed heating element without water can overheat and be damaged quickly.
During the first heating, look over the joints again and check that the working thermostat switches the heating off once the set temperature is reached.
When you have checked all of that, the job is done. And along with hot water again there remains that good feeling of knowing what you have repaired with your own hands, and why it works now.
