When a storage water heater stops heating the water, the fault can often be localised quite logically by following the electrical circuit.
The basic principle is simple:
we follow how far the electrical energy gets and at which point normal operation stops.
For that it is useful to understand the electrical diagram of a typical storage water heater as well.
In a simplified form it looks like this:
the mains → the water heater switch → the safety thermostat → the working thermostat → the heating element
On a large number of water heaters the indicator lamp on the heater itself is connected so that it lights when the element is under voltage, that is, in parallel with it.
So two ordinary indicator lamps, one on the bathroom switch and the other on the water heater, sometimes already give quite good information about which part of the circuit the fault should be looked for in.
Of course, the actual electrical diagrams differ and the position of the indicator lamps is not the same on every model. The following algorithm refers to the usual diagram in which the indicator on the switch shows that the supply to the water heater is on, and the lamp on the heater shows the supply to the element.
The indicator on the bathroom switch does not light
If the switch is on but its indicator lamp does not light, the first question is:
does the supply reach the water heater at all?
The possible cause can be:
- a circuit breaker that is off or has tripped;
- an RCD that has tripped;
- a faulty switch;
- an interruption of the supply;
- a faulty contact or conductor.
At this stage there is still no reason to suspect the thermostat or the element straight away.
First it has to be established whether there is a proper supply at the appliance itself.
Why did the breaker trip?
If the protective device has reacted, it is not enough just to switch it back on.
It has to be established why it reacted.
One of the common faults of a heating element is damage to its electrical insulation.
The resistive conductor of the element has to be electrically insulated from its metal sheath and from the tank.
If that insulation fails, a fault current can arise towards the protective conductor and the metal casing of the appliance.
If the installation has a working RCD, it can react even at a relatively small leakage current.
So the combination:
we switch the water heater on → the RCD trips immediately or after some time
often justifies the suspicion of a breakdown of the element to earth.
That is not established by assumption but by measuring the insulation resistance.
A circuit breaker and an RCD are not the same.
A circuit breaker reacts to excessive current and to a short circuit, while an RCD reacts to the difference of currents between the live conductors, that is, to a current that is leaving by a path it should not.
So a fault of the element to earth need not necessarily make the circuit breaker react, but it can activate the RCD protection.
The indicator on the switch lights, but the lamp on the water heater does not
Now we already know something more.
If the particular installation has the diagram we described, the supply has reached the water heater but the element circuit is not closed.
The next question is:
what is between the incoming supply and the element?
Most often these are:
the safety thermostat → the working thermostat → the element.
If the water is cold, the working thermostat is calling for heat and the element lamp still does not light, one of the first things to check is the safety thermostat.
The safety thermostat has interrupted the circuit
The safety thermostat has a simple task: if the temperature exceeds the permitted value, it interrupts the supply to the element independently of the working thermostat.
If we find the safety thermostat tripped, it is not enough just to press its reset.
The following question has to be asked:
why did the safety protection react at all?
One of the logical causes is a faulty working thermostat.
If it did not interrupt the supply when the water reached the set temperature, the element kept working until the independent thermal protection reacted.
In such a situation:
a tripped safety protection can be the consequence of a fault of the working thermostat, and not a fault of the safety protection itself.
So resetting it without testing the working thermostat is not a complete repair.
And what if the working thermostat is sound?
Then the safety protection itself has to be tested.
It too is a mechanical and electrical device that can fail.
On some bimetallic thermal protections the characteristics of the mechanism can change over time, so the protection starts to react at a lower temperature than intended or becomes unreliable.
We can then have the situation:
the water has not reached a dangerous temperature → the working thermostat is sound → the safety thermostat nevertheless interrupts the circuit.
In that case the cause of the problem can be the safety protection itself.
The lamp on the water heater lights, but the water stays cold
In the typical diagram in which the indicator lamp is connected in parallel with the element, this is a very useful symptom.
If the lamp lights, it means that the circuit is under voltage up to that point.
If the water is still not heating, it is logical to suspect the element itself.
The simplest fault is a break in the resistive element.
The lamp can then still receive a supply and light, while current no longer passes through the broken element.
An element is checked by measuring its resistance.
For a 2 kW element at a voltage of 230 V the expected resistance is approximately 26 Ω.
If the instrument shows an open circuit, the element is faulty.
The lamp lights and the element is sound, how is that possible?
This is where a simple algorithm stops being enough.
An indicator lamp draws a very small current.
An element of several kilowatts draws a many times greater current.
So it is possible for the circuit to have a damaged, burnt or partly broken contact through which there is enough current for the lamp to light, but which cannot supply the element properly.
It is also possible for a conductor to be broken or burnt at a place that the position of the lamp in the particular diagram does not reveal.
So:
a lamp is a useful diagnostic indicator, but it is not a measuring instrument.
When the symptoms stop matching a simple algorithm, voltage, resistance and current have to be measured.
The element draws current, but the water still does not heat up
This is a much more interesting situation.
Let us assume that the following has been checked:
- the water heater has a supply;
- the thermostats are sound;
- the element has the correct resistance;
- a clamp meter shows that the expected current really passes through the element.
Electrically speaking, the water heater is working.
The element, therefore, is converting electrical energy into heat.
And yet the user says:
"The water heater runs all day and the water is never hot enough."
Then the fault may not be in the water heater at all.
A hidden leak on the hot water
A storage water heater can heat a limited amount of water, the amount that is in its tank.
If water is continuously escaping somewhere from the hot water system, new cold water is continuously entering the tank at the same time.
The element then works trying to heat the contents of the tank, but the system constantly takes the heated water away from it and replaces it with cold.
If the flow is large enough, the temperature will never reach the value at which the thermostat would switch the element off.
We have practically asked a storage water heater to work as an instantaneous one.
And it is not designed for that.
How can a leak stay unnoticed?
In a flat a hot water leak is usually noticed relatively quickly.
In houses and more complex buildings the situation can be different.
Part of the pipework can run:
- under the floor;
- through a wall;
- through a shaft;
- under the building;
- through the ground.
If a pipe bursts in such a section, the water can escape without a visible puddle in the room.
The consequences can be unusual:
the water heater runs all the time ↓ it consumes electricity ↓ the water meter records consumption
↓ the temperature in the tank does not reach the set value ↓ the user concludes that "the water heater does not heat".
And the water heater is in fact perfectly sound.
Electrical diagnostics is then no longer enough
If it is measured that the element really receives its rated power, the energy has to be going somewhere.
That follows from the law of conservation of energy.
If a 2 kW electric element works continuously, it continuously delivers approximately 2 kW of heat output.
If the temperature of the water in the tank nevertheless does not rise as it should, we have to look for where that heat is being lost.
One possibility is exactly the continuous replacement of heated water with new cold water because of a leak.
That is a good example of why diagnostics is not only a search for "the broken part".
Sometimes all the parts of the appliance are sound and the problem is in the system the appliance is connected to.
A simple diagnostic tree
For the typical installation we described, the logic can look like this:
The indicator on the switch does not light → check the supply and the protective devices.
The indicator on the switch lights, but the water heater lamp does not → check the safety and the working thermostat and the circuit up to the element.
The water heater lamp lights, but the water does not heat up → check the element and its conductors and contacts.
The element is sound and a normal current has been measured, but the water temperature does not rise → the problem should no longer be looked for only in the electrical part of the water heater; check the hydraulic system and a possible uncontrolled consumption or leak of hot water.
The essential point
Good diagnostics does not begin by replacing the most likely part.
It begins with the question:
"What can I already reliably conclude from this symptom?"
Every indicator lamp, every reaction of a protective device and every measurement result narrows the number of possible causes.
That is how a fault is traced step by step:
is there a supply → does the supply pass through the protections and the thermostats → does it reach the element → does the element draw the expected current → where is the produced heat going.
And it is exactly the last question that sometimes leads to the most interesting conclusion:
a water heater that "does not heat" need not be broken at all.
