Electric Storage Heaters: Night Storage, Lot 20, Economy 7, and Installation Guide

Electric storage heaters remain one of the UK's most common heating systems — accounting for around 8% of homes and a much higher proportion in flats, social housing, and all-electric properties without gas supply. For electricians, they represent a steady workload: new installations in properties coming off night-rate tariffs, replacements driven by the Lot 20 regulation, and fault diagnosis on ageing units.

This guide covers how storage heaters work, the Lot 20 requirements that changed the market from 2018, wiring and installation under BS 7671, Economy 7 tariff considerations, and troubleshooting common faults.

How Electric Storage Heaters Work

Storage heaters charge up overnight using cheaper off-peak electricity — typically Economy 7 or Economy 10 tariffs — and release that stored heat during the day. Internally, they contain high-density ceramic bricks (usually aluminium oxide or magnetite) that can absorb and retain large amounts of heat energy.

The basic operating cycle:

• Charging phase (off-peak, typically 00:30–07:30): The internal heating elements switch on and heat the ceramic core to temperatures of 650–750°C in older units, lower in modern high-retention models.
• Storage phase: Insulation around the core retains heat throughout the morning.
• Output phase (daytime): Heat is released through convection, radiation, or a fan-assisted boost.

Types of Storage Heater

Manual storage heaters (legacy)

The simplest type: an input dial controls how much electricity the heater charges overnight, and an output control (a damper or damper rod) regulates how fast heat is released during the day. No electronic controls, no thermostat. These are the old-style units found in properties built pre-2000. Still functional if the elements and bricks are intact, but no longer compliant for new installations under Lot 20 and often inefficient due to heat loss through basic insulation.

Automatic or electronic storage heaters (transitional)

Introduced from the 1990s onwards. An electronic thermostat or room temperature sensor controls the charging level automatically, adjusting input based on ambient temperature and user set-point. Output is still manual in basic versions. More efficient than purely manual units.

High heat retention storage heaters (HHRS) — Lot 20 compliant

The current standard. Required for all new installations (and replacements of existing heaters) since January 2018 under the Lot 20 Ecodesign regulation. These heaters use superior insulation to reduce standby heat loss to less than 1.5% of stored energy per hour (compared to 5–10% in older units). They incorporate:

• Electronic charge control with room temperature sensing and weather compensation
• Programmable 24-hour time clock or smart controls
• Fan-assisted convection for rapid output boost on demand
• Open window detection (reduces heat loss when a room is ventilated)
• Combination charging — ability to top up using peak-rate electricity if the stored charge is insufficient

Manufacturers offering Lot 20 compliant heaters include Elnur, Dimplex, Creda, and Haverland. Capacity is measured in kWh (storage capacity) rather than kW output rating. Common sizes are 1.0 kWh to 3.0 kWh storage capacity.

The Lot 20 Ecodesign Regulation

Lot 20 refers to Commission Regulation (EU) No 813/2013 (space heaters) and Commission Regulation (EU) No 814/2013 (water heaters), transposed into UK law as retained EU legislation. The space heater provisions relevant to storage heaters came into force in January 2018.

Key Requirements

• Minimum efficiency: All electric storage heaters must achieve a minimum seasonal space heating energy efficiency of 30% (equivalent to a seasonal coefficient of performance of 1.0 when calculated against the European reference climate).
• Heat loss limit: Standby heat loss must be ≤1.5% of the rated storage capacity per hour.
• Electronic temperature control: Must include a minimum Class II thermostat (±1°C accuracy) and 24-hour programming capability.
• Product information: Manufacturers must provide energy efficiency class labels (EU energy label or UK equivalent) and product datasheets.

What This Means in Practice

Old manual storage heaters — even if still working — cannot be replaced on a like-for-like basis with equivalent manual units. Any new or replacement heater must be a Lot 20-compliant HHRS. Practically, this means:

• Recommending a full replacement rather than element repair in older inefficient units
• Advising landlords upgrading properties that Lot 20 compliance is mandatory
• New heaters will require a controller/programmer connection — check wiring requirements before ordering

There is no enforcement mechanism for legacy units already installed and operating, but any notifiable electrical work (which replacement of a storage heater may be, if a new circuit is required — see Part P below) will be inspected under current regulations.

Economy 7 and Economy 10 Tariffs

Storage heaters are designed to exploit off-peak electricity tariffs, where the cost per kWh is significantly lower during overnight hours.

Economy 7

The standard off-peak tariff. Provides 7 hours of cheaper electricity, typically from around 00:30 to 07:30 (exact times vary by DNO region and supplier). The off-peak rate is usually 30–50% lower than the daytime rate, though the daytime rate on Economy 7 is typically higher than a standard single-rate tariff — making Economy 7 only cost-effective if at least 40–45% of electricity consumption is during off-peak hours.

Economy 10

Provides 10 hours of cheaper electricity, typically split across two or three overnight/afternoon windows. More flexible for households that use more electricity in the afternoon. Less widely available than Economy 7.

Smart Tariffs

Smart meters have enabled dynamic time-of-use tariffs (such as Octopus Agile or Go) with variable pricing. Lot 20-compliant HHRS with smart controls or OpenTherm/Zigbee connectivity can be programmed to charge during the cheapest overnight windows, potentially reducing running costs further. This is an emerging area relevant to installers working in all-electric new-build properties.

Identifying the Off-Peak Circuit

Economy 7 properties have a two-rate meter (or a smart meter programmed for two rates) and typically a separate consumer unit or timeswitch that controls the off-peak circuit. The off-peak circuit feeds storage heaters, immersion heaters, and sometimes a hot water cylinder. The switching may be done by:

• A radio teleswitching (RTS) signal embedded in Radio 4 LW broadcast (legacy)
• A ripple control signal injected by the DNO
• A time clock on the meter or consumer unit
• A smart meter programmed remotely by the supplier

Always verify the off-peak switching is working correctly before commissioning a new storage heater.

Wiring and Installation: BS 7671 Requirements

Circuit Design

Storage heaters are high-wattage fixed appliances. Each heater must be on a dedicated radial circuit — they must not be connected to ring final circuits. The circuit must be sized to the heater's charge rating, not its output rating.

Typical charge ratings for Lot 20 HHRS:

• 1.0 kWh storage: 0.5–1.0 kW charge element → typically 6A circuit, 1.5mm² cable
• 1.5 kWh storage: 1.25–1.5 kW charge element → typically 10A circuit, 1.5mm² cable
• 2.0 kWh storage: 1.7–2.0 kW charge element → typically 13A circuit, 2.5mm² cable
• 3.0 kWh storage: 2.5–3.0 kW charge element → typically 16A circuit, 2.5mm² cable

Refer to manufacturer data sheets for exact charge element ratings and verify cable sizing against BS 7671 Appendix 4, accounting for installation method, grouping, and ambient temperature correction factors (see Cable Sizing and Current Carrying Capacity).

Consumer Unit Configuration

Storage heaters fed from an off-peak circuit require their own protection in the consumer unit. The off-peak consumer unit (sometimes called the night storage heater board) is typically separate from the main consumer unit and controlled by the meter timeswitch or ripple control. Each heater circuit must have:

• An MCB of the appropriate rating (typically Type B, 6A–20A depending on heater)
• RCD protection — required under 18th Edition BS 7671 Regulation 411.3.3 for circuits supplying socket outlets and recommended for all final circuits in domestic premises. Storage heater circuits in walls or floors will require RCD protection under Amendment 1 (2020) where cables are installed in walls at less than 50mm depth without earthed mechanical protection.
• An RCBO (combining MCB and RCD protection) is the cleanest solution for each circuit in a modern installation.

Wiring Requirements

Standard twin-and-earth cable (flat grey or white, typically 2.5mm²) is the norm for storage heater circuits. The cable run from the consumer unit to each heater position must be:

• Clipped directly or surface-run where accessible
• Protected by conduit or trunking where surface-run in a plastered area
• Run in safe zones (horizontal or vertical from socket/switch positions) where buried in plaster, unless mechanically protected (see Electrical First Fix Guide)

Terminations at the heater end use the heater's terminal block. Most modern HHRS units have a cable entry point at the bottom and require the cable to enter from a patress box or surface conduit box mounted below the heater.

Double Pole Isolation

A local means of isolation is required. For storage heaters, a double pole isolator switch (fused or unfused, typically 20A DP) should be installed adjacent to or at the heater position. Some units have an integral isolating switch. The isolator must be accessible without moving the heater.

Earthing and Bonding

Storage heaters must be earthed. The heater casing connects to the circuit protective conductor (CPC) of the supply cable. Where the heater is in a bathroom or kitchen, check whether supplementary equipotential bonding is required (see BS 7671 Section 701 for bathrooms — storage heaters are unusual in bathrooms but not unheard of in older properties). Main protective bonding requirements apply as normal (see Earth Bonding Guide).

Part P and Notification

Replacement of a storage heater on an existing circuit (same cable, same MCB) is not notifiable under Part P of the Building Regulations — it is classed as like-for-like replacement of a fixed appliance. However, if the work involves:

• Installing new circuits
• Modifying the consumer unit
• Work in a special location (bathroom, kitchen near sink)

...then the work is notifiable and must be carried out by a competent person registered with an approved Part P scheme (NICEIC, NAPIT, ELECSA) or notified to the local authority building control (LABC). Always obtain a Minor Electrical Installation Works Certificate (MEIWC) or an Electrical Installation Certificate (EIC) as appropriate and issue it to the customer.

Installation Step-by-Step

Before You Start

1. Isolate the off-peak consumer unit — verify dead with a two-pole voltage tester and proving unit.
2. Check existing cable sizing against the new heater's charge rating — many older installations used 1.5mm² and a new HHRS may have a larger element.
3. Confirm Economy 7/off-peak switching is operational by checking with the customer that they are on a time-of-use tariff.
4. Measure the heater recess or position — HHRS units have specific clearance requirements above and at the sides.

Installing the Heater

1. Fix the back plate or mounting brackets to the wall at the correct height (most units specify a minimum 150mm above floor level).
2. Feed the supply cable and any control wiring (room temperature sensor, programmer) through the cable entry.
3. Connect L, N, and E to the heater terminal block. On HHRS with a separate fan circuit, there may be a second terminal block for boost fan supply (usually on peak-rate supply).
4. Clip or recess the heater body onto the back plate.
5. Fit the cover panel and controls.
6. Restore power and commission per manufacturer instructions — typically involves setting the time clock, setting minimum and maximum charge levels, and configuring the room temperature set-point.

Multiple Heater Installations

Where multiple heaters are being installed (typically in a flat or house conversion), plan the circuit layout to minimise cable runs. Label each circuit clearly in the consumer unit. A floor plan with heater positions and circuit numbers will be useful when commissioning and for the landlord's records.

Fault Diagnosis and Common Problems

Heater Not Charging (Cold in the Morning)

• Check Economy 7 supply is live during the off-peak period (use a voltmeter or plug-in energy monitor at midnight)
• Check MCB in the off-peak consumer unit — test by switching off and on
• Check the heater's input or charge control setting is not at minimum
• Test heating elements for continuity and insulation resistance (Megger test at 250V, expected >1 MΩ) — elements fail open-circuit with age
• On modern HHRS, check for error codes on the display and consult manufacturer service guide

Heater Losing Heat Too Fast (Empty by Mid-Afternoon)

• Ceramic brick core may be damaged or cracked (reduced thermal mass)
• Output damper or fan may be set too high
• Insulation blankets around the core may have deteriorated — common in heaters over 15 years old
• Check if Economy 7 night window is being used fully (some meter configurations only give 5–6 hours effective charging)

Heater Not Releasing Heat (Hot All Day But No Output)

• On manual heaters: check the output damper is not stuck closed (often a bi-metallic strip or rod mechanism)
• On HHRS: check the boost fan is operational and fan circuit is powered (peak-rate supply)
• Check for obstructions to air flow inlet/outlet — do not install heaters with output louvres against walls or furniture

RCD Tripping When Heater Charges

• Test insulation resistance of element — earth leakage through degraded element insulation will trip an RCD
• If IR test is low (<1 MΩ at 250V, or <0.5 MΩ at 500V), the element should be replaced
• Check RCD sensitivity — storage heaters with fan boost can cause nuisance tripping on 30mA RCDs if there is a small inherent leakage; consider an RCBO per circuit

Controls Not Working (HHRS)

• Many HHRS units have a 24-hour time clock that must be set correctly after a power cut
• Open window detection may be inhibiting charge if there is a rapid temperature drop (e.g., near an external door)
• Check for loose connections at control board terminal blocks — vibration can loosen connections in older units

Sizing Storage Heaters for a Property

A common rule of thumb for storage heater sizing is to use the property's calculated heat loss, then size the heater storage capacity to cover 8–10 hours of off-peak charging. As a guide:

• Small bedroom (under 10m²): 0.5–1.0 kWh storage heater
• Average bedroom (10–15m²): 1.0–1.5 kWh
• Living room/dining room (18–25m²): 2.0–3.0 kWh
• Open-plan living space (25m²+): Two heaters or a 3.0 kWh+ unit with supplementary panel heater

This is approximate — actual sizing should account for insulation quality (cavity wall, loft insulation), glazing, ceiling height, and orientation. For larger projects (HMOs, blocks of flats, housing association properties), carry out a proper heat loss calculation using SAP methodology or BS EN 12831.

For supplementary heating in rooms where storage heater output is insufficient in late afternoon, a panel heater or convector heater on the peak-rate supply can be added — but advise customers that peak-rate electricity is expensive and this should be a last resort.

Storage Heaters vs Other Electric Heating Options

Key Products for Storage Heater Work

APM Electricals — Plumbing, Electrical & Heating Supplies
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