Solar hot water diversion

Operating a solar immersion diverter, setting priorities and diagnosing cylinder or metering limits.

Solar hot-water diversion uses surplus PV generation to run an immersion heater. In operation, the main questions are whether the controller is measuring grid flow correctly, whether the cylinder can accept more heat and whether another flexible load has priority.

The goal is controlled self-consumption, not simply maximising the diverter’s energy total.

Normal operating sequence

A typical sequence is:

Solar generation exceeds current household demand.
The grid sensor detects export above the controller threshold.
The diverter increases immersion power.
It trims power as clouds or household loads change.
It stops when surplus disappears or the cylinder thermostat opens.

A small steady export can be intentional to prevent the controller oscillating around zero. The appropriate margin is product-specific.

Decide the priority

Where a home also has a battery or solar-aware EV charger, define which load should take surplus first.

The decision depends on:

hot-water demand and the cylinder’s alternative heat source
battery state of charge and expected evening load
EV departure requirement
import and export prices
conversion and storage losses
available solar forecast

There is no universal best order. Heating water that would otherwise be heated by a costly direct-electric tariff can be valuable. Diverting solar that could earn a higher export payment than the heat it replaces can lose money.

Change one controller’s priority rather than creating several independent export targets that cause devices to chase one another.

Check value without a live tariff table

For each unit diverted, compare:

avoided cost of useful heat - export income forgone

If the immersion replaces direct electric heating, the avoided energy is straightforward but cylinder standing loss still applies. If it replaces gas, account for boiler efficiency and any standing cost already incurred. If it replaces heat-pump hot water, compare against electricity divided by the operating COP.

The calculation uses the household’s dated contract rates rather than a generic annual saving.

Manual boost

A boost intentionally runs the immersion when solar is insufficient. Depending on the product, it may import from the grid.

The owner should know:

whether boost runs at full power
how long it lasts
whether it stops at the cylinder thermostat
whether a schedule repeats
how it interacts with battery discharge

A forgotten repeating boost can make a diverter appear to import unexpectedly. Use a one-off boost for an exceptional need and verify that it expires.

Cylinder reaches temperature early

Once the thermostat is satisfied, the diverter has nowhere to send further surplus. This is normal.

If more thermal storage is useful, check the cylinder and plumbing design before raising a setpoint or changing an element. Sensor position, immersion height, mixing valve and scald risk all matter.

Do not raise temperature merely to consume more solar. The hot-water hygiene strategy and safety controls take priority over an app’s self-consumption percentage.

The diverter does not run in sunshine

Operation depends on:

surplus remaining after household loads and battery charging
the controller reading grid flow in the correct direction
the cylinder thermostat calling for heat
an energised immersion circuit with a healthy safety cut-out
the intended output and priority being selected
a working CT or meter connection

Bright conditions do not guarantee surplus. A battery, EV, heat pump or household appliance may be using all generation.

Avoid bypassing the thermostat or safety cut-out as a diagnostic test.

The diverter imports from the grid

A brief change can occur when a large load starts between sensor updates. Persistent import needs investigation.

Possible causes include:

reversed or misplaced CT
wrong phase assignment
response margin set incorrectly
manual or scheduled boost
battery and diverter controls competing
loss of meter communications
a controller configured for generation rather than net-grid measurement

Test with a known resistive load while observing the grid meter and diverter. The import or export register is stronger evidence than an animated app arrow.

Three-phase systems

The controller must assess the connection in the way the site and supplier meter treat phase flows. A single-phase immersion can consume surplus on one phase while another still imports.

Use a product and metering arrangement designed for the supply. Confirm phase mapping and any netting assumption during commissioning. Do not infer three-phase capability from the total kW rating alone.

Seasonal behaviour

Summer can provide more surplus than the cylinder can accept. Winter may provide too little for meaningful diversion, so the main hot-water system remains necessary.

Judge performance over appropriate seasons. A low winter total is not a fault if there was little export. A useful monitoring comparison includes:

solar generation
grid export before and after diversion
immersion energy
manual-boost energy
cylinder service and main heat-source energy

The diverter total should not be counted as a cash saving without subtracting export income and other heat that would have been used.

Hygiene and scalding

A diverter follows available surplus and the cylinder thermostat. It does not prove that the stored-water bacterial-control process has completed.

Keep the system’s defined hygiene control, any periodic pasteurisation and thermostatic mixing-valve maintenance. Do not disable another heat source solely because the diverter warmed part of the cylinder.

Maintenance and handover

The owner can check displayed grid direction, accumulated diversion and alarm state. Electrical covers, intake conductors and immersion terminals are not user-serviceable.

Keep a record of:

CT or meter position and direction
output circuit and element rating
priority order
import margin or threshold
boost and schedule settings
cylinder thermostat arrangement
local and cloud account access

After a meter exchange, consumer-unit alteration, battery installation or router change, verify operation again. Each can change sensing or communications without damaging the diverter itself.