Common faults of heating rods and heaters – why they don’t heat or trip the circuit breaker
Common faults of heating rods and heaters – why they don’t heat or the circuit breaker trips
The heating rod has stopped heating, the water heater is not heating the water, or the circuit breaker keeps tripping – these are situations we encounter with customers quite regularly. Most faults have clear, easily identifiable causes that a technician can detect without special equipment. The problem is that people often misdiagnose the fault, replace the entire unit when replacing a single component would suffice – or conversely, replace only components when the whole unit is past its service life. This article will guide you through the most common faults step by step, explain what causes them, how to diagnose them, and when it is still reasonable to repair and when it is better to replace.
We will focus on both basic types of equipment: heating rods for bathroom radiators (electric heating inserts with a typical power of 150–600 W) and heaters for water heaters and storage tanks (classic flanged or immersion heating elements with flanged mounting). Their faults differ somewhat, but many diagnostic principles are the same.
Why the heating rod in the radiator does not heat – 7 most common causes
When a customer calls saying that the "rod does not heat", it is actually a whole range of different situations. The bathroom radiator does not show any heat, or it heats only weakly, or the outages are intermittent. Let’s break it down systematically.
1. Burned-out heating element (short circuit or broken resistance wire)
This is by far the most common fault. The heating element of the heating rod consists of a resistance wire embedded in magnesium oxide insulation (MgO) inside a steel sheath. The wire can either break (open circuit – the rod does not heat at all, the circuit breaker does not trip) or short to the sheath (leakage current – the circuit breaker trips immediately when turned on).
Breakage usually occurs after long-term operation, when the resistance wire oxidizes and mechanically weakens. A short is more typical in situations where the rod operates in aggressive conditions – in hard water, when limescale deposits settle inside the radiator and change the thermal regime of the element.
Diagnosis: Disconnect the rod from the electrical network, remove the terminal cover, and measure the resistance between phase and neutral with an ohmmeter. For a 300 W rod at 230 V, the resistance should be approximately 176 Ω (R = U²/P = 230²/300). If you measure infinity (break) or a few ohms (short), the element is faulty. Also measure the resistance between any conductor and the sheath – it should be in the megaohm range, ideally over 2 MΩ. If you measure a few kiloohms or less, the element is damaged.
2. Tripped (discharged) thermal fuse
Most heating rods have a built-in one-time thermal fuse that activates in case of overheating. This typically happens when the rod operates in air – that is, when the radiator is not filled with water or liquid. The fuse burns out once and is not resettable. The rod then does not heat at all and the resistance is infinite, even though the resistance wire itself may be in good condition.
We encounter this situation after renovations, when someone turns on the bathroom before the system is filled with water. The thermal fuse is usually built into the element or the head – some types allow the fuse to be replaced, but most cheaper products require the entire rod to be replaced.
3. Faulty thermostat or temperature switch
Heating rods with a thermostat (such as heating rod with thermostat for radiator (300 W, white)) have a built-in bimetallic or electronic thermostat that regulates the temperature of the liquid in the radiator. If the thermostat gets stuck in the "off" position, the rod will stop heating despite the heating element being in good condition.
The thermostat can jam due to excessively high settings, mechanical damage, or long-term operation under extreme conditions. Diagnosis is simple: try setting the thermostat to maximum and observe whether the rod starts heating. If it does, the thermostat is likely working, but it may have been incorrectly set. If it does not, measure the voltage at the thermostat’s output terminals at maximum setting – it should be 230 V. If it is not, the thermostat is faulty.
4. Improper installation or leakage at the thread
The heating rod must be sufficiently immersed in the liquid to transfer heat properly. If the radiator is only partially filled, or if the rod does not extend deep enough into the radiator (for example, due to incorrect length), local overheating can occur. This also leads to faster degradation of the element.
Leakage at the threaded connection is not a direct cause of the rod not heating, but it can cause the liquid level to drop and gradual overheating. If you are interested in the installation procedure, see the article Installation of a heating rod into a radiator – step by step in this Knowledge Center.
5. Problems with the power cable or socket
It may seem trivial, but in practice it is surprisingly common: the rod is functional, but the problem is in the power circuit. Bathroom sockets are protected by a residual current device (RCD) and fuses, which can be in various boxes. The customer assumes the rod is faulty, but it is enough to reset the RCD in the distribution board.
It also happens that an extension cord or connector does not have sufficient cross-section for a long-term load of 300 W, which leads to overheating of the connections and contact breakage. Always connect heating rods to a fixed installation or to a bathroom socket that meets the IP44 standard.
6. Air or insufficient filling in the radiator
The towel radiator must be filled with liquid (usually a mixture of water and antifreeze, or plain water). If there is an air pocket in the area of the heating element, the element operates partially in air, overheats quickly, and the thermal cut-out is activated. A characteristic symptom is that the radiator feels cold or only slightly warm at the bottom, where the heating element is located.
Draining air is done using the air vent at the top of the radiator – try to open it and check if air is escaping. If the air vent is blocked by limescale deposits, it requires cleaning or replacement.
7. Fault caused by hard water and limescale deposits
In areas with hard water (hardness above 30 °dH), calcium carbonate deposits form on the surface of the heating element. A layer as thin as 1 mm reduces heat transfer and increases the surface temperature of the element by approximately 10–15 °C – at a thickness of 3–4 mm, the degradation is very significant. The element operates at higher temperatures, wears out faster, and its lifespan is reduced. In extreme cases, overheating and activation of the thermal cut-out may occur.
This issue is less typical for towel radiator heating rods (closed system with a permanent fill) and more pronounced for immersion heaters in water heaters, where fresh water with a high calcium content is constantly heated.
Why the immersion heater in the water heater is not heating – specific causes for storage tanks
An immersion heater in a storage tank is structurally different from a heating rod for a towel radiator. It has a flanged mounting (most commonly 5/4" thread), is larger, and operates in a different temperature and chemical environment. Therefore, the faults have a slightly different character.
Limescale deposits and "clogging" of the element
This is the most common cause of immersion heater failures in water heaters in Slovak and Czech households, where water hardness reaches values of 20–40 °dH. A thick layer of limescale forms on the surface of the element, acting as a thermal insulator. The water heater heats more slowly, electricity consumption increases, and eventually overheating of the heating element occurs.
Zinc-coated heaters, such as GRBT immersion heaters for small and medium tanks with a brass flange and zinc-coated surface (1400 W), are resistant, but even they will eventually fail under very hard water and long-term operation without descaling. If the water heater was working normally and the heating has gradually slowed down, the first thing to check is the degree of limescale buildup on the element.
We recommend reading the article Maintenance of immersion heaters – cleaning, descaling, and anode inspection, where the descaling procedure is described in detail.
Worn magnesium anode
Most water heaters have a magnesium anode that protects the tank and the heater from corrosion. When the anode is consumed (a normal process, the anode sacrifices itself for the tank), corrosion begins on the tank or heater body. Corrosion can cause micro-cracks in the heater casing, through which water enters and causes a short circuit.
The anode should be inspected every 2–3 years and replaced according to water hardness. If the anode is completely consumed and has not been replaced for several years, the heater may be damaged by corrosion from the outside, not just by deposits from the inside.
Thermostat or thermostatic sheath failure
Water heaters typically have a thermostat and a thermal cut-out separated from the heating element – they are placed in a separate capsule attached to the tank wall. The thermostat is usually adjustable in the range of 40–75 °C. If the thermostat fails in the "off" position, the water heater will not heat the water to the desired temperature or will not heat at all.
The thermal cut-out (usually at 90 °C) is a protection against overheating. If it is activated, a manual reset is required – some types must be reset using a button accessible through the service cover. Customers are often unaware of this and think the water heater is broken, while in fact, it only needs the reset button to be pressed.
Stainless steel versus zinc-coated heaters – different susceptibility to faults
When choosing a replacement heater, it is important to consider the material. GRBTN immersion heaters for small and medium tanks made of stainless steel AISI316L (1400 W) are significantly more resistant to corrosive environments, but they are also not immune to mechanical damage or overheating due to limescale buildup. AISI316L stainless steel is a specially alloyed steel with added molybdenum, which can withstand aggressive water with a higher chlorine content. We discuss this topic in more detail in the article Immersion heater for water heater: brass, zinc-coated, or stainless steel AISI316L?
Why the circuit breaker trips – causes and solutions
Tripping of the circuit breaker or residual current device (RCD) with a heating rod or immersion heater is a symptom that should always be taken seriously – it is a protective device that reacts to a real problem and should never be simply "reconnected without diagnosis."
Short circuit in the heating element – leakage current to the chassis
This is the most common cause of RCD tripping (sensitivity 30 mA). When the insulation between the resistance wire and the metal casing of the element is damaged (as a result of overheating, corrosion, or mechanical damage), leakage current to the chassis begins. The RCD detects this and disconnects the circuit.
Identification: Disconnect the heating rod from the power supply. If the residual current device (RCD) holds, the cause was the rod. Measure the insulation resistance between the phase conductor and the sheath – if it is below 1 MΩ, the insulation is damaged and the element must be replaced.
Tripping of a standard circuit breaker – overload
A standard (overload) circuit breaker reacts to a current higher than its nominal value. A 300 W rod at 230 V draws only 1.3 A, so an overload will not occur. The problem arises when the circuit is overloaded by multiple appliances, or if the circuit breaker is too small (for example, a 6 A circuit with old wiring is sufficient, but a 1 A circuit breaker would be problematic).
If the circuit breaker repeatedly trips only when the heating rod or heater is in use, and all other appliances are disconnected, it is likely that a thermal short has occurred in the element – the resistance of the element has dropped and the current is higher than what corresponds to the rated power. This can happen due to a partial short in the MgO insulation.
Tripping on switching on – inrush current
When a cold resistive element is switched on for the first time, a short-term inrush current can occur, as the resistance of the cold conductor is slightly lower than when it is warm. In the case of standard heaters and heating rods, this effect is negligible – it is more typical for motors and transformers. If the circuit breaker trips only when switching on and then holds, check the contacts in the socket or terminal block – it may be a poor contact, which can cause a spark when switching on.
Moisture in the terminal block
In a bathroom environment, it can happen that moisture (vapor, splashing water) enters the terminal block cover. Wet insulation has a lower resistance and can cause a leakage current. The solution is to dry it out – remove the cover, let the components dry thoroughly, or use technical alcohol. If the terminal block is dry and the RCD holds, the problem was moisture. In the long term, it is advisable to secure the cover so that vapor cannot condense inside.
Failures specific to heaters in water heaters – practical examples
From practice, we know that customers with water heater issues usually face one of three situations: the heater does not heat water at all, heats it to a lower temperature than set, or heats it but the RCD occasionally trips. Here are typical scenarios:
Scenario 1: Water heater in a new building after 4 years – slow heating times
The customer has an 80-liter electric water heater in an area with water hardness of 35 °dH. After four years of operation (without any maintenance), it takes twice as long to heat water from 15 °C to 60 °C as before. After removing the heater, a 3–4 mm thick layer of limescale is found on its surface. The element is functional, the insulation is intact, but the performance is significantly reduced. Solution: chemical descaling using citric or acetic acid (solution, not concentrate), followed by preventive replacement of the anode.
Scenario 2: Water heater stops heating after a power outage
After a storm with a power outage, the customer finds that the water heater is not heating. The most common cause is an activated thermal fuse (resettable at 90 °C), which tripped due to unstable voltage or a surge in the grid. It is sufficient to open the service cover on the front of the water heater, find a small red or black button (thermal fuse), and press it. The water heater should then run normally again.
Scenario 3: New heater stops heating after two months in a mountain cabin
The customer replaced the heater in a cabin where it is not used for several months of the year. The new heater was galvanized, but it immediately stopped heating after the first resumption of operation. Upon inspection, it turned out that the heater was installed without proper sealing, and moisture entered the thread, which froze during the winter and cracked the threaded flange. Solution: replace the element, this time with a stainless steel variant (e.g., GRBTN stainless steel heater AISI316L) with proper sealing and a recommendation to drain the water heater before the seasonal shutdown.
How to prevent failures – preventive maintenance
Most failures of heating rods and heaters are not sudden – they are long-term processes that can be slowed or stopped with proper maintenance.
- Annually: Visual inspection of the heating rod in the panel radiator – check whether any water traces are dripping from the thread area, whether the terminal block cover is undamaged and dry.
- Every 2 years: For heaters in water heaters – descaling of the element, check of the magnesium anode. In areas with water hardness above 25 °dH annually.
- Every 3–5 years: Replacement of the magnesium anode even if it is not completely consumed, if you are in an area with aggressive water.
- At each replacement: Check the seals, replace the sealing rings (O-rings) – even if they look good, it is reasonable to install new ones after removing the heater from the tank to prevent leaks.
- Regularly: Monitor heating times. If heating takes significantly longer than before, it is the first sign of scaling.
For more information on selecting the correct power, see the article What power of heating rod do I need – 150, 300 or 600 W?, where we also address the impact of deposits on the nominal power.
When to replace and when to repair?
This is a practical question to which there is no single answer, but in practice we follow several criteria:
Heating rod in a panel radiator: It is always cost-effective to replace the entire rod, as repairing the heating element is not economically viable. The price of a new rod is low, and installation is simple. If the thermostat trips, some types allow replacement of only the electronics, but for cheaper products, complete replacement is simpler.
Water heater element: The heating element itself is replaced as a whole unit (not repaired). If the fault is only in the thermostat or temperature fuse, these are available as spare parts. If the fault is in the element itself (short circuit, broken wire, insulation failure), the entire element must be replaced. Replacement is relatively simple and is described in the article Replacing a water heater element – when and how to do it.
Rule of thumb: if the appliance is older than 8–10 years and has its first major failure, it is usually worth replacing the whole unit, not just repairing it. After ten years, the sealing is degraded, the flange may have limescale in the thread, and the anode is consumed. Repairing one component will likely lead to another problem in a year or two.
Most frequently asked questions (FAQ)
Why does the residual current device trip immediately after turning on the heating rod?
Immediate tripping of the RCD upon turning on indicates a leakage current to the chassis – that is, an insulation fault in the heating element. The resistance wire is in contact with the metal casing and current flows back via the PE conductor. This cannot be repaired – the element must be replaced. Before replacement, measure the insulation resistance (between phase and chassis) to confirm the fault and not just assume it.
Heating rod in the towel radiator is heating, but the radiator is cold – what is wrong?
If the rod is heating (you can confirm this by touching the body after about 5 minutes – it will be hot), but the radiator is cold, the problem is in the circulation of the liquid inside the radiator, or the liquid level is too low – an air pocket is preventing heat transfer. Check the liquid level via the bleed valve at the top of the radiator. If the towel radiator is in a closed system, add liquid and bleed the system. If the towel radiator is filled separately, add liquid (distilled water or a mixture with antifreeze according to the manufacturer's instructions).
Can the cause of the heating failure be an incorrect phase – that is, incorrect wiring?
Yes, although this is relatively rare. If the rod was connected without a neutral (zero) conductor, or if the conductors are swapped at the terminal block, the element will either not heat at all or will heat with reduced power in a three-phase connection (this practically never happens in bathroom installations). In any case, if you have any doubts about the wiring, call an electrician – work in the bathroom falls under increased safety requirements (zones 1–3 according to STN 33 2000-7-701).
How can I determine if the water heater element is scaled without removing it?
An indirect method is to measure the heating time: determine how many minutes it takes for the tank to heat from 15 °C to 55 °C (you can measure the temperature of the discharged water). If you know the heating time of a new water heater (or estimate it based on power – for example, a 2 kW element heats 80 liters by 40 °C in about 93 minutes), compare it with the current one. A significant increase (by 30 % or more) indicates significant scaling. Another indicator is increased electricity consumption in winter months compared to the same period a few years ago.
Is it safe to replace the heating rod yourself, or must an electrician do it?
Replacing the heating rod in a towel radiator (the mechanical part – unscrewing and screwing the rod into the radiator thread) is a common installation task that a skilled person can handle. The electrical part – disconnecting and connecting the cable to the terminals – is usually better left to an electrician in the case of a fixed installation. Stricter standards apply in bathrooms. If you are unsure, it is always safer to leave the electrical installation to a qualified professional. A detailed procedure is available in the article Mounting a heating rod in a towel radiator – step-by-step guide.
What is the average lifespan of a heating rod and a water heater element?
A heating rod in a towel radiator in a closed system with clean liquid has a lifespan of 10–15 years under normal load. A water heater element, which is in constant contact with fresh water containing minerals, has a shorter lifespan – usually 5–10 years with regular maintenance (descaling, anode replacement), or 3–5 years without any maintenance in areas with hard water. An AISI316L stainless steel element has a longer lifespan than galvanized ones even in aggressive environments.
Conclusion – diagnosis in the right order saves time and money
Most faults in heating rods and water heater elements are predictable and to a large extent preventable. When something stops working, do not proceed to replacement without diagnosis – you will save time, money, and frustration from repeated faults. Always start with the simplest (power supply, circuit breaker, RCD, thermostat) and proceed to more complex causes (element, insulation, anode).
If you are unsure during diagnosis, or if the circuit breaker continues to trip after replacing the rod, do not attempt further tests – contact an electrician. In a bathroom environment, safety is the top priority and electrical faults in a wet environment can have serious consequences.
When choosing a replacement product, keep in mind the correct power, thread type, and body material. For towel radiators, we recommend products with a thermostat (such as heating rod with thermostat 300 W), which prevent overheating and save energy. For water heaters in areas with hard or aggressive water, consider a stainless steel variant (GRBTN stainless steel heater AISI316L 1400 W), which has a longer lifespan and lower susceptibility to corrosion and scaling. Further advice on selection can be found in the articles How to choose a heating rod for a bathroom radiator – what to focus on and What thread and flange diameter do I need for a heater in a tank.
Do you have a question about this topic?
Not sure or dealing with a specific situation in your household? Write to us – we are happy to help.