Storage Water Heater No Hot Water: 8 Key Reasons Why
Storage Water Heater No Hot Water:
8 Key Reasons Why
Introduction
It is a common frustration for a user to turn on the heater switch, see the power indicator light up, wait for 10 minutes, and step into the shower expecting a nice hot shower.
Despite waiting, the water temperature remains at room level. Even after waiting another 15 minutes inside the bathroom, the hot water still does not seem to come through.
This is extremely frustrating, and the question that immediately comes to mind is: what caused it?
Technical Background
1. Faulty Heater Switch (Isolator Switch)
The isolator switch, typically mounted on the wall, acts as the primary control for power delivery to the heater. While the indicator light may suggest the switch is operational, internal contacts can degrade due to arcing (sparking during switching) or oxidation. This creates high electrical resistance, preventing current from reaching the heater.
Prevention: Always replace the heater switch whenever the heater is replaced no matter how good the condition it may appear.. The life span of the switch is similar to that of the heater. Very often, the heater switch would fail before the heater.
2. Burnt Heater Supply Cable
The cable connecting the heater to the electrical supply must withstand the appliance’s high current draw (e.g., 13A for a 3kW heater).
Substandard cables, the most common being used are those with BS2004 or labelled 3C 70/00076 or no label at all. BS2004, based on imperial unit, has been withdrawn since 1995. These are not legal to use in the high current circuit like heater.
Loose terminations can cause localised overheating, leading to insulation melting, copper oxidation, and eventual open-circuit failure.
According to Table 4D2A of Code of Practice for Electrical installation SS 638, the cable size used for a heater circuit with 20A MCB should be 2.5mm2 or larger in accordance with BS6004 :2012 specification.
Prevention: The technician connecting the cable to the heater has to be supervised by an EMA LEW. He is to use cable with BS6004 and 2.5mm2 printed on the PVC jacket and ensure all connections are securely tightened. Communicate with the LEW if have any doubt.
3. Degraded Connection Unit (Terminal Block)
The terminal block inside the heater’s electrical compartment consolidates live, neutral, and earth connections. Over time, vibration, moisture ingress, or thermal cycling can loosen terminal screws, creating poor contact.
Loose connections increase resistance, generating excessive heat (via Joule heating: ) that carbonises terminals and wires. Inspect for blackened terminals, melted plastic, or brittle insulation.
Prevention: Replace the terminal block each time when there is a heater replacement to avoid having to regularly check and tighten electrical terminals to prevent overheating, and replace corroded terminal blocks.
4. Internal Electrical Compartment Faults
Within the heater’s electrical housing, components such as relays, fuses, or wiring harnesses can fail. A corroded neutral busbar or a fractured live wire disrupts the circuit.
This is very much due to manufacturing defects. It is not common but it happens.
For Joven JH and JVA series, leaking water from the corroded heating element mounting is common wetting the electrical components.
Prevention: Look out for water leaking sign from the electrical especially the heater brand is Joven.
5. Failed Heating Element
Heating elements, typically made of nickel-chromium alloy, convert electrical energy into heat via resistance. Avoid heater of less known brands that often come with the cheapest parts like heating element. Stick to reputable brands like Rheem, Ariston or Joven.
Prevention: Avoid heater of less known brands that often come with the cheapest parts like heating element. Stick to reputable brands like Rheem, Ariston or Joven.
6. Malfunctioning Thermostat
The thermostat regulates tank temperature by interrupting power to the element once the set temperature (e.g., 60°C) is reached. A faulty thermostat may fail to close its contacts (no power to the element) or stick in the open position (permanent cutoff).
Prevention: Test the thermostat periodically to ensure it responds correctly to temperature changes. Replace it if it shows signs of malfunctioning.
7. Tripped Thermal Cut-Out (Overheat Protection)
Most heaters include a non-resettable or resettable thermal cut-out (often a red button) that disconnects power if the tank exceeds safe temperatures (e.g., 85°C). This can occur due to a stuck thermostat, element short-circuit, or blocked venting.
Ariston, Rheem and Joven model have this feature.
Repeated tripping indicates a persistent fault, such as a grounded element or insufficient water flow.
Prevention: If the thermal cut-out trips frequently, investigate and fix the root cause. Ensure proper water flow and check the heating element and thermostat for faults. Also avoid lesser known brands.
8. Overloaded Circuit Tripping the MCB
Storage heaters demand significant current (e.g., 13A for 3kW). If the heater shares a circuit with other high-load appliances (e.g., washing machines), cumulative demand may exceed the MCB’s rating (e.g., 20A), causing it to trip.
It is common to connect a socket outlet to this circuit. Using a hair dryer rated at 1600w and run the washing machine with dryer 1800w will result in total draw of 4,700w (20.43A), tripping the MCB, cutting the power to the circuit.
Prevention: Ensure the heater is on a dedicated radial circuit with an appropriate breaker (e.g., 20A MCB). Avoid running high-powered appliances on the same circuit.
Safety and Maintenance Recommendations
- Electrical Safety: Always isolate the heater at the consumer unit before inspection. Use a voltage tester to confirm circuits are de-energised. Consult a LEW if unsure.
- Preventative Measures: Schedule annual inspections of terminals, elements, and anodes.
- Professional Intervention: Faults involving MCBs, rewiring, or internal heater components should be handled by a LEW or electrician under supervision by a LEW.
Conclusion
A lack of hot water in a storage heater is rarely random—it typically points to a specific electrical or mechanical failure. Systematic troubleshooting, starting from the power source (switch, cables) and progressing inward (thermostat, elements), can help identify the root cause.
However, given the risks of high-voltage systems and pressurised tanks, expert assistance is strongly advised for complex repairs. Regular maintenance prolongs component life and ensures reliable operation.
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