Economy 7 and Night Storage Heater Wiring — Off-Peak Heating Circuits, Replacement Controls, and Tariff Metering for UK Electricians

Night storage heaters and Economy 7 tariffs remain a significant part of the UK's heating landscape — particularly in flats, older housing stock, and properties without a gas connection. Millions of storage heaters are still in active use, and as older units fail, electricians are regularly called to replace them, upgrade controls, or wire new systems. Understanding the dual-rate metering system, the specific circuit requirements for storage heaters, and the control upgrades available will help you carry out these jobs safely and efficiently.

This guide covers Economy 7 tariff metering, storage heater circuit design, wiring requirements under BS 7671, replacement procedure, modern control options, and common faults.

What Is Economy 7?

Economy 7 (E7) is a dual-rate electricity tariff that charges a lower unit rate for electricity consumed during a seven-hour off-peak period, typically between midnight and 7 am (exact hours vary by supplier and region — always check with the supplier). During these off-peak hours, storage heaters charge their thermal cores with cheap electricity. During the day, they release the stored heat passively without drawing additional electricity.

Economy 7 is measured by a dual-rate electricity meter, which records daytime consumption and off-peak consumption on separate registers. The meter receives a switching signal (historically via a radio teleswitch or ripple control signal, now increasingly via a smart meter) that activates the off-peak supply.

Economy 10

Economy 10 is a less common variant offering 10 hours of off-peak supply — typically split across the day in three blocks. Some immersion heaters and storage heaters can take advantage of Economy 10, but not all. Confirm the available off-peak schedule with the supplier before specifying the storage heater's charging programme.

Dual-Rate Meters and Switching

Radio Teleswitch (RTS)

Older Economy 7 installations used a Radio Teleswitch — a receiver that picks up a switching signal broadcast by the BBC on Radio 4 Long Wave (198 kHz). The teleswitch activates the off-peak contactor in the consumer unit, connecting the storage heater circuit to the supply during off-peak hours. Many of these systems are still in service.

Important: The BBC Radio 4 Long Wave teleswitch service will be decommissioned on 31 March 2024. Properties still using RTS switching will need to have their switching system updated by the DNO or supplier. If you encounter an RTS unit, advise the customer to contact their supplier immediately.

Smart Meters

Modern smart meters handle Economy 7 switching internally — they automatically record consumption at the appropriate rate and activate off-peak circuits without a separate radio signal. When a customer switches to a smart meter, the old teleswitch and time clock are typically removed and the smart meter performs all the switching functions.

Time Switches

Where a property uses a manual time switch rather than automatic RTS or smart switching, the time switch must be set to the correct off-peak hours for the area and adjusted seasonally (BST/GMT). A mechanical time switch can drift; digital time switches are more accurate and maintain programme through brief power failures via a battery backup.

Storage Heater Circuit Design

Separate Off-Peak Circuit

Night storage heaters must be supplied from a dedicated off-peak circuit — they must not share a circuit with general socket outlets or lighting. In most installations, the off-peak supply is controlled by the dual-rate meter's switching relay, which connects and disconnects the off-peak circuit at the appropriate times.

The off-peak circuit runs from the consumer unit to a distribution board or to individual storage heater outlets. In traditional installations, a separate consumer unit or group of MCBs is dedicated to storage heaters and controlled by the off-peak contactor.

Individual Circuits per Heater

Each storage heater should ideally be on its own radial circuit, protected by its own MCB or RCBO. This simplifies fault diagnosis and allows individual heaters to be isolated without affecting others. Where multiple heaters share a circuit, ensure the combined load is within the circuit's rating and that diversity can be applied.

Load Calculations

Storage heater input ratings typically range from 0.5 kW (small bedroom unit) to 3.4 kW (large living room unit), with most domestic units in the 1.0–2.5 kW range. Unlike conventional heaters, all the charging energy is consumed during the off-peak period — so the simultaneous demand during charging is the full rated input of every heater.

Example: A flat with 3 storage heaters (2.0 kW + 1.5 kW + 1.0 kW) draws 4.5 kW = 19.6 A during charging. This requires a supply capable of supporting the full charging load, not just peak usage.

Apply correction factors for cable grouping, ambient temperature, and installation method per Appendix 4 of BS 7671. For cables installed in thermal insulation, apply the appropriate derating factor — this is particularly important for storage heater circuits in well-insulated modern properties.

Wiring Requirements Under BS 7671

RCBO or RCD Protection

BS 7671:2018 (18th Edition) requires RCD protection (30 mA) for all final circuits in domestic premises. Storage heater circuits are included. Use a 30 mA RCBO for each individual heater circuit in the consumer unit — this provides both overcurrent and earth fault protection at the device level without nuisance tripping affecting other circuits.

Where the consumer unit uses a split-load arrangement with RCDs protecting groups of MCBs, ensure the storage heater circuits are on an RCD-protected group. RCBOs are preferable for individually derated circuits.

Double-Pole Switching at the Heater

Each storage heater requires local isolation. In practice, this is provided either by:

• A 20 A double-pole switched fused connection unit (FCU) at the heater location — the fuse provides additional protection for the heater's internal wiring; the switch allows local isolation without accessing the consumer unit
• A 20 A double-pole unswitched FCU with isolation at the consumer unit RCBO (acceptable but less convenient)

The FCU fuse rating should match the heater's rated current: a 2.0 kW heater at 8.7 A uses a 13 A fuse; a 3.4 kW heater at 14.8 A uses a 13 A fuse (or specify a 15 A or 20 A fused spur as appropriate). Check manufacturer data.

18th Edition Amendment 2 — AFDD

BS 7671:2018 Amendment 2 introduced requirements for Arc Fault Detection Devices (AFDDs) on final circuits in dwellings. Storage heater circuits are included. Where an AFDD is required, fit an AFDD/RCBO combination unit in the consumer unit. See our guide to Arc Fault Detection Devices (AFDDs) for full Amendment 2 requirements.

Cable Installation

Storage heater cables are typically installed in the wall or under floor — not in cavities with thermal insulation. Ensure the cable is not in contact with thermal insulation unless specifically derating factors have been applied. Use 1.5 mm² or 2.5 mm² twin-and-earth, run in conduit or surface trunking where it is exposed, and clipped directly to a solid wall surface behind the heater.

Earthing

Storage heaters have metal cases that must be earthed. The earth conductor in the supply cable must be continuous from the consumer unit to the heater connection terminals. Check the earth continuity resistance on completion.

Storage Heater Connection Points

Modern storage heaters typically have two connection points at the rear or base:

• Main supply (off-peak): This is the charging circuit — connected to the off-peak supply from the FCU. It draws the full rated input during charging periods.
• Boost (optional daytime supply): Some storage heaters have a secondary connection for a daytime boost element — a small conventional resistive heater to top up output when stored heat is depleted. This is connected to the normal 24-hour supply, not the off-peak circuit.

Where a boost connection is fitted, it requires a separate circuit or connection to a 13 A socket outlet. The boost element is typically 1.0–1.5 kW and can be connected to a spur from the ring main. Never connect the boost to the off-peak circuit — the boost must be available during the day, not only at night.

Replacing an Old Storage Heater — Procedure

Pre-Installation Checks

1. Confirm the off-peak supply is functioning: check dual-rate meter, verify the off-peak contactor/switching is operational, check supply voltage at the FCU during off-peak hours.
2. Identify the circuit serving the old heater at the consumer unit. Check the MCB/fuse rating and verify it is appropriate for the new heater's input.
3. Check cable size — if upgrading from a 1.5 kW to a 3.4 kW heater, the existing 1.5 mm² cable may be inadequate. Either up-rate the cable or specify a heater with a similar input.
4. Confirm the new heater's wall mounting requirements — some modern fan-assisted storage heaters are larger or heavier than older units and may need new wall fixings.

Disconnecting the Old Heater

1. Isolate at the consumer unit and local FCU. Apply lock-out/tag-out where available.
2. Verify dead with a voltage indicator — storage heaters contain capacitors and may not appear dead immediately after isolation.
3. Allow the heater's thermal store to cool if it has recently been charged — some ceramic brick cores retain significant heat.
4. Disconnect supply cable terminals in the old heater. Note L, N, and E connections.
5. Remove the heater — note that older storage heaters with ceramic fire bricks can weigh 50–100 kg. Plan for appropriate lifting equipment or a second person.

Installing the New Heater

1. Fix the new heater bracket or base to the wall.
2. Connect the existing supply cable to the new heater's terminals (L, N, E). Confirm the cable cores are in good condition — replace if the insulation is degraded, brittle, or shows signs of heat damage.
3. Set the heater's internal input selector (if adjustable) to the appropriate setting for the circuit cable size.
4. Commission the controls: set the charge timer (if manual), adjust the input control to the desired setting, and verify boost functionality if fitted.

Testing and Commissioning

1. Carry out insulation resistance test (500 V IR): measure between line and earth, neutral and earth, and line and neutral. Record results.
2. Measure continuity of earth conductor: confirm the protective conductor resistance is low.
3. Test the RCD/RCBO: press the test button; confirm it trips within the required time.
4. Apply supply and confirm the heater charges during the off-peak period (or simulate with the time switch).
5. Issue an Electrical Installation Certificate (EIC) or Minor Works Certificate as appropriate.

Modern Controls and Upgrades

High Heat Retention Storage Heaters

Modern high heat retention (HHR) storage heaters (e.g., Dimplex Quantum, Glen Dimplex Unidare, Elnur Gabarron) are significantly more efficient than older units. They use superior insulation to retain heat for longer, reducing the proportion that leaks overnight when it isn't needed. They also typically feature:

• Electronic charge control: the heater monitors weather forecasts (via built-in sensors or smart controls) and adjusts overnight charge accordingly — preventing the over-charging that wastes energy in older units
• Programmable output: heat is released on a programmed schedule, not continuously throughout the day
• 7-day programmable timers: output matched to occupancy patterns
• Smart home integration (some models): controlled via phone app, compatible with smart tariff signals

When replacing old storage heaters, always recommend HHR models — they significantly reduce energy bills even on Economy 7, and are often included in government energy efficiency schemes (ECO4, Great British Insulation Scheme).

Combination Storage Heaters

Some storage heaters combine a thermal store with an electric panel heater in a single unit — the panel provides instant top-up heat during the day on standard rate when stored heat is depleted. Wiring requires both an off-peak circuit (for the storage element) and a standard 24-hour circuit (for the panel boost) — plan for this when surveying.

Smart Tariffs and Time-of-Use

Smart electricity tariffs (such as Octopus Agile, Go, or Cosy) increasingly offer off-peak periods dynamically priced by the half-hour rather than fixed Economy 7 windows. Some modern storage heaters and smart controllers can interface with these tariffs to optimise charging timing. This is an emerging area — check current controller compatibility with the customer's supplier before specifying.

Common Faults on Storage Heater Circuits

Immersion Heaters on Economy 7

Economy 7 is not just for storage heaters. Many properties without gas also use a dual-element immersion heater in a hot water cylinder, wired so that the main (lower) element charges on the off-peak circuit overnight, and a smaller upper element provides a daytime top-up on standard rate. This is sometimes called an "Economy 7 cylinder" setup.

The immersion heater off-peak circuit wiring is identical in principle to storage heater wiring — a dedicated off-peak circuit, double-pole FCU or switch, 30 mA RCBO, and appropriate cable size (typically 2.5 mm² T&E to a 20 A FCU). See our guide to Immersion Heaters — Wiring, Thermostat Replacement, and Timer Controls for detailed immersion wiring.

Part P Notification

Installing or replacing a storage heater circuit in a dwelling is notifiable work under Part P of the Building Regulations. Self-certify via a Part P registered scheme membership (NICEIC, NAPIT, ELECSA) or notify Building Control before work. Issue an appropriate Electrical Installation Certificate on completion.

Products for Storage Heater Wiring

Key components for storage heater wiring and circuit installation are available from APM Electricals, including switched fused connection units for local heater isolation, RCBOs for dedicated circuit protection, and appropriate cable sizes.

Related Guides

• Immersion Heaters — Wiring, Thermostat Replacement, and Timer Controls for UK Electricians
• Electric Panel Heaters — Circuits, Controls, and Installation for UK Electricians
• RCD, RCCB, and RCBO Protection — Types, Testing, and Selection for 18th Edition Compliance
• Arc Fault Detection Devices (AFDDs) — 18th Edition Amendment 2, Wiring, and Selection
• Consumer Units and Distribution Boards — Split Load, Full RCD, and RCBO Replacement
• Electrical Cable Identification — Colour Codes, Conductor Sizing, and Circuit Protection

Get Your Materials at APM Electricals

Visit us at 24 Western Avenue, Acton, London W3 7TZ or call 020 8702 8080. Same-day collection available. Browse our full range at www.apmi.uk.