Immersion Heaters

How they Work

1. There is a jacketed heating element in the water tank.
2. Usually the element has a built in adjustable thermostat, but there are also some with no stat built in, and a separate external stat attached to the tank, usually at about 1/3 the way up the tank.
3. There is a timer, or less often just an on/off switch, on the supply to the heater.
4. Newer elements also have a secondary thermal cutout built in

2 Element Systems

This variant has a small upper element to give a relatively small volume of hot water which heats faster, as well as the main longer element that produces enough for a shower or bath.

Economy 7 Systems

Dual tariff systems typically heat the lower element at night, and use the smaller top element for top-up heat in the day.

Element Size

27" elements heat the whole tank.

11" elements are used for top heat only.

Faults

The majority of faults can be found by testing just 3 resistances:

• across the 2 heating element terminals
  • should be somewhere vaguely in the region of 20 ohms, but can vary a fair bit.
• across the 2 thermostat terminals
  • should be far below 1 ohm when cold.
• From heating element terminals to casing.
  • should be open circuit.

Insulation Breakdown

This common fault causes current to flow from live to earth in the element. This causes 3 problems:

• current flow is increased, which can sometimes cause fuse blowing, MCB tripping or burnt accessories
• A tripping MCB may prevent it starting.
• RCD will trip if its on an RCD protected power feed

This fault can be detected by measuring resistance between element connections and its casing.

Split Element

Elements often split open during the last phase of their life. Rapid resistance wire corrosion then occurs, breaking the element circuit. However the element continues to operate for a fair time by conducting through the water.

Despite what we were taught about electricity and water as a child, this condition does not causing any deaths or injuries in the UK, despite it being a widespread occurrence. In fact the principle of feeding mains direct through the water is standard practice in industry, albeit with a bit more precaution than is applied to domestic hot water. It is known as electrode heating.

In this phase of life,

• resistance testing from element terminals to casing shows very low resistance
• Heating may be faster or slower than usual due to less well controlled current
• Element may cycle on & off due to excess heating
• A heater on an RCDed feed will trip the RCD immediately.

Burnt thermostat

Burning of the thermostat connections is another common failure, and results in failure to function. Parts of the stat may be burnt to charcoal.

To test for this, resistance test across the thermostat connections. R should be a small fraction of an ohm with a cold element. If higher, the stat contacts are damaged.

If this occurs, the possible solutions are:

• new thermostat complete with built-in element
• wire across the built-in stat and add an external stat strapped to the tank.

Either way should clean out all traces of carbon deposits.

Burning or Fishy Smell

May be due to burning or cooking of the thermostat, the FCU or an end of the flexible cable. Inspection should show any discoloured and burnt material.

The solution is to replace the burnt part, and if the cable end is not clean & bright, cut it back several inches to where it is and reconnect.

Not enough hot water

This is caused by any of:

1. thermostat set too low. This can be a problem with new replacement elements, which often have stats that wont go as high as older ones, but the tank size requires the higher temp to have enough HW capacity.
2. replacement element too short
3. For curved element systems, replacement element is mounted the wrong way up, with the curved end pointing up instead of down
4. For 2 element systems, bottom element not working
5. With heat banks, a badly scaled exchanger reduces rate of heat throughput, but not capacity. The tell tale sign is that fast HW is not hot enough, but slow the flow down and the temperature is restored.
6. Insufficient HW system capacity for new appliances, eg power shower

Solutions:

1. Turn up stat, or if it wont go high enough, wire it out of circuit and replace with an external stat.
2. Replace the replacement element with the correct longer size
3. Refit element with curved end down
4. Troubleshoot lower element & its associated parts
5. Replace exchanger. Consider scale prevention before the exchanger.
6. Applying as many options in Increase Hot Water Capacity as possible may be enough. Turn down pumping rate. If these measures aren't enough, a larger HW tank will be needed.

See also Increase Hot Water Capacity for more options.

Recommendations

It is not recommended to put an immersion heater on a ring circuit.

It is not recommended to put an immersion heater on a 13A plug.

Elements that expand and shrink slightly will shed scale build-up, and last much better in hard water areas.

See Also

Increase Hot Water Capacity

Domestic Hot Water Systems

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