PV diverter immersion control

A PV immersion diverter controls an electric immersion heater so that available solar surplus heats stored water instead of being exported. It does not move electricity through a separate storage device. The hot-water cylinder stores the result as heat.

The system needs a cylinder with a suitable immersion heater. It has no useful output in a combi-boiler home with no stored hot water.

Measuring surplus

A diverter normally measures net power near the grid connection using a current transformer or compatible meter. When the property is exporting, the controller can increase immersion power. When another household load starts, it reduces immersion power to avoid importing for the diverter.

The critical measurement is grid flow. A separate solar CT may improve monitoring but is not always required for diversion because the controller can act on import and export at the boundary.

Commissioning should prove direction with known loads. A reversed or misplaced CT can make the immersion run during import or stay off during genuine surplus.

Proportional control

A fixed immersion element has a rated full power, but a purpose-designed diverter can control the average power delivered to it. Implementations include controlled bursts or other approved power-electronic methods.

The controller and element must be compatible. Electronic thermostats, contactors, smart fused spurs and other controls can behave differently when supplied by a modulated waveform.

Do not use an ordinary relay as if it were a proportional diverter. A relay only turns a load fully on or off. If the solar surplus is below the element rating, an on/off system can import electricity whenever it closes unless additional logic and a suitable threshold prevent it.

Immersion circuit and safety controls

The immersion remains a fixed high-power heating load. Its circuit needs correct cable, overcurrent protection, isolation and terminations.

The cylinder thermostat and independent over-temperature cut-out must remain effective. A diverter must not bypass them or become the only temperature control.

The IET notes that a 3 kW immersion can exceed 13 A at the upper end of the permitted supply-voltage range. Circuit and connection design should therefore follow the appliance instructions and BS 7671 rather than assuming that a generic 13 A fused connection is always suitable.

Electrical work should be designed, installed, inspected and tested by a competent person. Access to intake conductors for the CT can involve hazards even though the clamp itself is non-invasive.

Cylinder capacity and thermostat position

Diversion stops when the cylinder thermostat is satisfied, even if solar surplus remains. The useful thermal capacity depends on:

cylinder volume
starting and stopping temperatures
immersion position
hot-water draw during the day
standing loss
mixing and stratification

An upper immersion may heat only the top portion. A lower element can heat more of the cylinder but must be part of the approved cylinder arrangement.

The diverter does not establish a complete bacterial-growth control. Normal hot-water targets, any pasteurisation process and scald protection remain part of the cylinder and system design.

Multiple outputs and priorities

Some diverters can control two immersion elements or another resistive load. The priority can be sequential, such as heating the upper cylinder element before the lower one.

Adding a second output does not create more surplus. It only decides where available energy goes. Both circuits and control states need commissioning, including what happens when the first thermostat opens.

Loads with motors, compressors or complex electronic power supplies should not be connected unless the diverter manufacturer explicitly approves them.

Coordination with a battery and EV charger

A solar battery, EV charger and immersion diverter can all respond to the same export. Without a clear priority, their controls can hunt or compete.

Possible strategies include:

fill the battery before heating water
maintain a minimum battery charge rate, then divert the remainder
heat water first when the cylinder would otherwise use expensive fuel
charge the EV before the immersion when departure energy is required

The preferred order depends on storage losses, service needs and the household’s import and export contract. Record it in the control design rather than letting independent devices infer it from the same CT.

Export payment changes the value

Diverted solar is not free if the household gives up a paid export. Compare the value of one unit of exported electricity with the cost of one unit of useful heat displaced.

For an electric immersion, the conversion at the element is close to one unit of heat per unit of electricity, although cylinder losses still occur. For gas or a heat pump, compare delivered heat after the appliance efficiency or COP.

A simple decision test is:

value of diverted heat = avoided delivered-heat cost - export income forgone

The household’s dated tariff and alternative water-heating cost are the relevant financial inputs.

Failure and failsafe behaviour

If the CT, meter or communications link fails, the product should enter its declared safe state. It should not default to running an immersion from grid power without an intentional boost command.

The controller should also retain the cylinder thermostat as the final demand and stop when the safety cut-out opens.

At handover, demonstrate loss of surplus, thermostat satisfaction, manual boost, priority change and any communications alarm.

Electrical load and control data

Compare:

number and type of controlled outputs
maximum load and approved element types
grid-sensing method
single or three-phase operation
response threshold and stability
local controls without cloud access
metering and data export
failsafe behaviour
cylinder-sensor options
warranty and support provider