Scale is one of the most frequently mentioned problems with storage water heaters.
It is credited with slow heating, high electricity consumption, burnt out elements, noise, a reduced amount of hot water and almost every other fault that appears on an older water heater.
Part of those claims has a real physical explanation.
Part is exaggerated.
To understand what scale really does, one first has to understand how it forms and how it affects the transfer of heat from the element to the water.
Where does scale come from at all?
Water from the mains contains dissolved minerals.
The amount and composition of those minerals depend on the water source and on the local water network.
With hard water, calcium and magnesium compounds are especially significant.
When water is heated, its chemical equilibrium changes and part of the dissolved substances can separate out as solid deposits.
Over time they settle on:
- the surface of the element;
- the bottom of the tank;
- the walls of the tank;
- the thermostat sleeves;
- other parts that are in contact with the water.
The most intense deposition often occurs exactly near the element, because the surface temperature there is the highest.
Why does scale cling to the element?
An electric element develops heat in its resistive element.
That heat passes through the metal sheath of the element and is then given off to the water.
When the surface of the element is clean, the path of the heat looks roughly like this:
the element → the metal sheath → the water
When a layer of deposits forms on the surface:
the element → the metal sheath → scale → the water
Scale is a considerably poorer conductor of heat than metal.
So it represents an additional thermal resistance.
The element still produces heat, but gives it off to the surrounding water with more difficulty.
What then happens to the temperature of the element?
This is the most important consequence of scale.
If the element produces, for example, 2 kW of heat output, and an ever thicker layer of deposits forms between it and the water, a greater temperature difference is needed to transfer the same amount of heat.
So the temperature of the element itself rises.
In other words:
the water does not have to be excessively hot for an element under a thick layer of scale to work at a considerably higher temperature.
The thicker and less conductive the layer of deposits, the harder the working conditions of the element.
That increased thermal load is one of the reasons why elements in hard water can last for a shorter time.
How does the element fail because of that?
In a classic tubular element the resistive conductor is inside a metal sheath and is electrically insulated from it by a material that at the same time has to conduct heat well.
Excessive temperature can accelerate the degradation of those materials over time.
The consequences can be:
- a break in the resistive conductor;
- damage to the electrical insulation;
- a breakdown towards the metal sheath;
- the RCD protection reacting;
- a complete failure of the element.
So scale is most often not an electrical fault in itself.
It creates worse thermal conditions, and those can then lead to an electrical failure of the element.
Does scale increase electricity consumption?
This question requires a more precise answer than is usually given.
The claim often heard is:
"When the element gets covered with scale, the water heater uses much more electricity."
That is not quite a correct physical explanation.
An electric resistive element converts practically all the electrical energy it consumes into heat.
Scale cannot "eat" that energy.
If the element consumes 2 kWh of electrical energy, practically those 2 kWh end up as heat inside the system.
The problem is in how that heat is transferred from the element to the water and at what temperature the element has to work in order to give it off.
A layer of scale:
- increases the surface temperature of the element;
- worsens the local transfer of heat;
- increases thermal stress;
- can affect the circulation of water around the element;
- in more severe cases can lengthen the heating time.
Because of the longer heating, the heat losses through the insulation and the casing of the water heater can also rise.
But the main problem with scale is not a mysterious "disappearance of electricity".
The main problem is that the element works at a less favourable and higher temperature.
Does that mean scale has no effect on the electricity bill at all?
One should not go to the other extreme either.
Large deposits can change the normal working regime of a water heater and lead to somewhat greater actual losses.
However, with a classic electric storage water heater it is not correct to imagine scale as a layer that "traps" half of the produced heat and prevents it from ever reaching the water.
The heat that stays inside the tank in the end mostly ends up in the water, in the tank itself and in the surroundings.
So the story of scale is above all a story about:
the temperature of the element, the service life, the deposits and the working conditions of the water heater, and only then about energy efficiency.
Why does a water heater with a lot of scale sometimes crackle and "gurgle"?
Thick deposits can lead to local overheating of the water right next to the surface of the element.
In tiny cavities and under the deposits, steam can form locally.
The bubbles form, separate and then condense again in the colder water.
That can produce sounds described as:
- crackling;
- hissing;
- gurgling;
- quiet knocks.
The sound alone is not enough to establish the amount of scale with certainty, but on an older water heater it can be one of the indicators of the condition inside.
Scale at the bottom of the tank
Not all the mineral material that separates out stays stuck to the element.
Part of it falls off and settles at the bottom of the tank.
In a water heater that has not been opened for a long time, significant quantities of solid sediment can accumulate.
It can:
- surround the lower part of the element;
- hinder the flow of water around it;
- retain a locally high temperature;
- create noise;
- make servicing and emptying the water heater more difficult.
The amount of deposits sometimes looks dramatic when a water heater is opened, but the amount of scale at the bottom alone is not enough to explain every fault the appliance has had.
Does scale reduce the volume of a water heater?
Technically, yes.
Every solid deposit inside the tank takes up a certain volume.
But the usual amount of deposits most often cannot explain the claim that an 80 litre water heater "has become a 30 litre one".
For a significant reduction in the amount of usable hot water, the following should also be checked:
- the temperature to which the water is heated;
- whether the thermostat is sound;
- the mixing of cold and hot water;
- the condition of the internal pipes;
- the actual consumption;
- a possible leak;
- other hydraulic problems.
Scale can reduce the useful volume, but every problem with the amount of hot water should not automatically be blamed on it.
Scale and corrosion are not the same
Scale is a mineral deposit.
Corrosion is a process of deterioration of a metal material.
Those are two different phenomena.
In steel enamelled tanks the steel sheet carries the mechanical load, while the enamel serves to protect the metal from contact with the water.
If the protective layer is damaged and the anodic protection is no longer sufficient, corrosion of the steel can over time lead to local thinning and perforation of the tank.
Deposits on the inside can in some cases cover a very small corrosion perforation and limit the passage of water through it.
So a tank can appear from the outside not to be leaking even though the metal is already seriously damaged.
Why does an old enamelled tank sometimes start leaking right after a service?
This is an important practical problem.
An older enamelled water heater can work for years without a visible leak, and after being emptied, opened, cleaned and refilled the tank starts to let water through.
That need not mean that a healthy tank has been damaged by the service.
It is possible that corrosion had already created a micro perforation or critically thinned the metal, while the mineral deposits and corrosion products were temporarily limiting the leak.
When the water heater is emptied and the deposits are removed, the already existing damage becomes visible.
After refilling, the tank is under pressure again and water passes through a place that was already damaged by corrosion.
In such a case the service revealed an existing fault, but for the user that is still an unfavourable outcome:
before the intervention the water heater was not visibly leaking, and after the intervention it has to be replaced.
That is exactly why preventively opening an old enamelled water heater is not always a harmless decision.
Do we recommend preventively cleaning scale out of a water heater?
Not as a universal service.
With a classic water heater with an immersed element, the mere presence of scale is not sufficient reason to empty and open a sound appliance.
The reason is not that scale has no effect on the element.
It does.
The problem is the economic justification of the intervention.
Emptying the water heater, removing the flange, cleaning, reassembling, filling and testing make up the greater part of the work that is done anyway when an element is replaced.
The classic immersed element itself is in many cases a relatively cheap part.
If we regularly paid for a complete service intervention only to extend the life of the element, the price of such maintenance would often be greater than the economic benefit we get by extending the life of the part itself.
So there is not much sense in paying for an expensive intervention in order to preserve preventively a relatively cheap consumable element.
What should be done when the element really fails?
When an immersed element fails and, in order to replace it, the water heater already has to be:
- switched off;
- emptied;
- opened;
- dismantled,
then the situation is different.
The cost of getting to the inside already exists.
Then it makes sense to remove the accessible deposits and inspect the condition inside within the same operation.
In other words:
scale is cleaned when there is already a technical reason to open the water heater, and not necessarily as a separate preventive service.
And why does it sometimes make sense to replace even a sound element?
If the water heater has already been emptied and opened because of another intervention, the greater part of the cost of replacing the element has already been incurred.
With a classic and relatively cheap immersed element it can then be economically justified to fit a new part even if the old one is still electrically sound.
The reason is not that a sound element "has to" be replaced.
The reason is the ratio between the price of the part and the price of a repeated intervention.
If the old element fails a few months later, the following are paid for again:
- the call-out;
- emptying the water heater;
- dismantling;
- assembly;
- filling;
- testing.
So with a cheap consumable part it is sometimes more worthwhile to replace it while the system is already open than to save on the part and pay for the whole job again later.
That is an economic decision, not a universal technical rule.
Special caution with old enamelled tanks
With an older enamelled tank of unknown condition, the risk of the intervention can be greater than the benefit we would get from preventive cleaning.
If the tank has already been weakened by corrosion, opening it and removing the deposits can reveal a condition after which the water heater can no longer be put back into use.
So with such an appliance a practical question has to be asked:
is it justified to invest in servicing a tank whose real condition we cannot reliably know before opening it?
With a very old water heater it is sometimes more rational to plan the replacement of the whole appliance than to invest in a preventive intervention whose outcome is uncertain.
What about the magnesium anode?
In enamelled tanks the magnesium anode has a different role from the element.
It is not there to prevent the formation of scale, but to protect the exposed parts of the steel tank electrochemically from corrosion.
So its condition can matter for the life of the tank itself.
However, here too one has to distinguish the manufacturer's maintenance recommendation from an economic assessment of the particular old appliance.
With a newer water heater, for which the manufacturer prescribes checking and replacing the anode, such maintenance can have a clear justification.
With a very old enamelled tank that has not been opened for years, the decision to open it preventively has to take into account the risk that the intervention will show the tank to be already at the end of its service life.
Higher temperature and the formation of deposits
The higher the temperature to which the water is heated, the more favourable the conditions become for certain mineral deposits to separate out.
So constantly keeping a water heater at maximum temperature in hard water can accelerate the deposition of scale.
That, of course, is not the only criterion for choosing the temperature.
The following should also be taken into account:
- the hygiene of the water;
- the amount of hot water available;
- the risk of scalding;
- heat losses;
- the way the water heater is used.
So the working temperature should not be determined only by the amount of scale.
"Dry" and classic immersed elements
With a classic immersed element the metal sheath of the element is directly in contact with the water.
So scale is deposited directly on its surface.
With so-called dry elements the resistive element is not directly in the water but is placed in a separate sleeve.
That changes the working conditions and the way the element is serviced.
However, a water heater with a dry element is not free of scale either.
Deposits can still form on the surfaces that are in contact with the water.
So what is the real problem with scale?
If we reduce it all to physics, the most important mechanism is simple:
the element produces heat ↓ scale increases the thermal resistance between the element and the water ↓ a greater temperature difference is needed to transfer the same power ↓ the temperature of the element rises ↓ the element works in harder conditions ↓ the probability of its premature failure increases
Besides that, sediment, noise and other consequences can accumulate in the tank, depending on the design and the working conditions.
That, however, does not mean that every water heater should be opened periodically only because there is certainly some amount of scale in it.
The essential point
Scale is not harmless, but properties it does not have should not be attributed to it.
It:
does not "consume electricity" in itself; is not the same as corrosion; does not mean that every fault of a water heater is a consequence of deposits; is not automatically a reason to open a water heater preventively.
Its most important effect with a classic immersed element is a worsening of the heat transfer and an increase of the temperature at which the element has to work.
But a technical problem and an economically justified repair are not always the same.
If the element is relatively cheap and completely emptying and opening the water heater is expensive, there is not much sense in paying for a big preventive intervention just to extend the life of that element.
When a water heater is already being opened because of an actual fault, the deposits can be removed within the same job.
With old enamelled tanks one should be even more careful, because the deposits sometimes conceal already existing corrosion damage that becomes visible only after emptying and cleaning.
So our approach is not:
"there is scale, the water heater has to be cleaned".
But:
"an intervention should have a technical reason and economic sense for the customer".
