How do Economy 7 Immersion Heaters Work

Economy 7 immersion water heaters are designed to heat the water in the hot water tank during the off-peak (cheaper) rate electricity hours, the off-peak rate is normally cheaper than the peak rate and is generally from midnight through to 7am. Once the water has reached the desired temperature the immersion heater thermostat will switch off and stops heating the water.

Economy 7 on/off times

The off-peak electricity on/off times are generally between midnight and 7am but vary between electricity suppliers and regions, contact your electricity supplier for up to date e7 off-peak rate unit costs and on/off times.

Off Peak Immersion and Peak Immersion, What is the Difference?

Peak Immersion Heater

The peak rate immersion heater should be the top of the two immersion heaters on the hot water cylinder connected via a local 20A DP switch marked as “Peak Immersion Heater”, or just “Immersion Heater.”

NOTE: The Peak and Off Peak immersion switches as illustrated above may not be present if an economy 7 immersion heater control unit has been installed, further details of these e7 immersion heater control units are explained further down in this post.

How to use your Economy 7 Immersion heater

1) To efficiently use your economy 7 immersion heater leave the “off-peak” immersion switch turned ON all the time, the power to this will automatically be energised (power tuned on) when the cheaper off peak rate starts and will also automatically de-energise (power turned off) when the cheaper economy 7 rate ends, this is normally during the night hours.

It is worth noting that once the water has reached the set temperature on the immersion heater thermostat, the thermostat will cut the power to the immersion heater element, when this happens it will not be consuming any power (electricity).

2) Leave the “peak” immersion heater switch OFF all the time. Only turn on the peak immersion heater (or press the boost button if you have a controller) if you have run out of hot water during the day to act as a boost, remember to turn it off again.

Economy 7 Day Time Boost

During the peak-rate hours the economy 7 immersion heater will not heat the hot water because your electricity supplier will have turned it off, however there might be a daytime “boost”.

A daytime boost is where your electricity supplier turns on the economy 7 electrical supply for a few hours during the day, check with your electrical supplier for further details.

Economy 7 Immersion Heater Controls

Properties with an Economy 7 electrical supply will have an Economy 7 capable electricity meter which will turn on and off a 2nd (generally smaller) consumer unit (or the e7 side of a combined consumer unit), the economy 7 electricity meter is owned and controlled by your electricity supplier.

The economy 7 on/off times are set by your electricity supplier, the 2nd consumer unit (which is controlled by the economy 7 electrical supply) will in-turn supply the off-peak immersion heater and any economy 7 storage heaters.

Your electricity supplier will turn on the Economy 7 electrical supply to the 2nd consumer unit during the off-peak (cheaper rate) hours, this will in-turn, turn on the off-peak immersion heater to heat the water inside the hot water tank until it reaches its desired temperature, when the desired temperature has been reached the immersion heater thermostat will click-off and the immersion heater element will stop heating the water any further.

To clarify, when the water reaches the temperature as set on the immersion heater thermostat, the thermostat will interrupt (turn off) the electrical supply the immersion heating element, therefore at this point it will not be consuming any electricity.

When the water inside the hot water cylinder has reached its desired temperature and has stopped heating the hot water, while there will still be power to the Immersion heater during this off-peak period, it wont actually be “consuming” any power as the stat has shut-off the electrical supply to the actual heating element.

If the water cools below the immersion heater thermostat setpoint (during the off-peak hours) then the thermostat will activate and start reheating the water, this is why it is important to ensure your hot water tank and associated pipework are well insulated in-order to minimise heat loss.

Fitting a hot water cylinder jacket to your hot water cylinder will help to reduce heat loss from the cylinder, while fitting pipe insulation to the associated pipework will reduce the heat loss from the actual pipework.

Your electricity supplier will turn off the economy 7 electrical supply when the off-peak rate ends thus turning off the power to the economy 7 (off peak) immersion heater and e7 storage heaters.

Economy 7 Immersion Heater Controller

No Hot Water

If you find that you constantly do not have any hot water there are a number of possible causes, assuming you have already checked the off-peak immersion switch has been left on for at least 24hrs,

• Fault on the circuit wiring.
• Faulty/corroded immersion heater element.
• Faulty immersion heater controller/timer (if you have one).
• Faulty electricity meter not turning on the e7 electrical supply.
• Faulty immersion heater thermostat.
• A fault causing the MCB/RCD to trip.
• Loose or burnt out connections at the isolator or immersion.
• Water has overheated and tripped the thermostat safety cut-out.
• *A 13 amp fused spur is fitted and has burnt out or the fuse has blown.
• Something else.

*Most immersion heaters are rated at 3kW which equates to 13.04 amps @ 230 volts, 13 amp switched fused spurs should not be installed or connected to immersion heaters.

Immersion Heater Temperature

As tempting as it may be to adjust the thermostat down to reduce the temperature of the hot water to save money, be sure not to reduce it too low otherwise there could be a risk of legionella growth, for further information visit Legionnaires’ disease: guidance, data and analysis – GOV.UK (www.gov.uk)

Conclusion

Economy 7 water heating can be an effective way to heat the water inside the hot water cylinder in all electric properties when it has been set up properly.

By leaving the off-peak 20A DP Immersion Heater (bottom immersion element) switch switched on all the time will allow the on/off times to be controlled by your electricity supplier at the off-peak cheap rate times.

Once the water reaches its desired temperature the immersion heater will stop heating the water.

An immersion heater will only consume power when the immersion heater is actually heating the water, therefore when it is NOT actually heating the water (thermostat has turned it off) it is not consuming any power during this time, so it will not be costing you anything in money terms, you are only charged for the actual power you consume (excluding the standing charge).

You electricity supplier will turn off the economy 7 electrical supply when the economy 7 off-peak rate ends which is generally around 7am.

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