Electric Water Heater Design

What Is an Electric Water Heater (EWH)?

Definition of an Electric Storage Water Heater

An Electric Water Heater (EWH) is a domestic hot water appliance designed to heat and store potable water using electrical resistance heating elements.

Unlike gas-fired systems, the electric water heater converts electrical energy directly into thermal energy through an immersion heater installed inside a closed steel tank. The heated water is stored under pressure and supplied on demand to taps, showers, and domestic hot water systems.

Electric storage water heaters must comply with safety and performance standards such as:

EN 60335-1 – General safety requirements for electrical appliances
EN 60335-2-21 – Particular requirements for storage water heaters
EN 12897 – Performance requirements for indirectly heated storage water heaters

Compliance with these standards ensures:

Electrical safety
Thermal protection against overheating
Mechanical and pressure resistance
Material suitability for potable water
Safe operation under normal and fault condition

Difference Between Storage and Instantaneous Water Heaters

Electric water heaters are generally classified into two main types:

1️- Storage Type (Tank Type)

Stores a predefined volume of heated water (10–150+ liters)
Uses thermal insulation to reduce heat loss
Suitable for residential applications
Lower instantaneous power demand

2️- Instantaneous (Tankless) Type

Heats water only when required
No storage tank
Requires high electrical power (6–24 kW typical)
Suitable for point-of-use systems

From an engineering perspective, storage-type heaters require:

- - Tank pressure design
  - Insulation optimization
  - Heat loss calculation
  - Corrosion protection strategy

While instantaneous heaters focus mainly on:

High power density
Flow control
Thermal shock resistance

Applications in Domestic Hot Water Systems

Electric storage water heaters are widely used in:

Apartments and residential buildings
Hotels and small commercial facilities
Off-grid or electric-only installations
Regions without gas infrastructure

Typical installation configurations include:

Closed pressurized systems
Open-vented low-pressure systems
Vertical or horizontal mounting

Their simplicity, reliability, and compliance with EU safety standards make them one of the most common domestic water heating solutions.

Electric Water Heater Working Principle

1 - Electrical Resistance Heating Mechanism

The core operating principle of an electric storage water heater is electrical resistance heating.

When voltage (typically 220–240 V, 50 Hz in EU markets) is applied to the immersion heating element:

Electrical current flows through a high-resistance conductor.
Electrical energy is converted into thermal energy

According to Joule’s Law:

Q = I^2 * R * t

Where:

Q= Heat energy (J)
I= Current (A)
R = Resistance (Ω)
t = Time (s)

The heating element transfers thermal energy directly to the surrounding water by conduction and convection.

2 - Thermal Stratification Inside the Tank

One of the most important physical phenomena in storage water heaters is thermal stratification.

Due to density differences:

Hot water (lower density) rises to the top.
Cold water (higher density) remains at the bottom.

This natural layering allows:

Faster availability of usable hot water
Improved energy efficiency
Reduced mixing losses

Proper tank geometry and inlet diffuser design are critical to maintaining stratification performance.

3 - Temperature Control Using Thermostat

Water temperature is regulated by an adjustable thermostat, typically within a range of:

30°C to 75°C

The thermostat operates by:

Measuring tank temperature via a sensing probe.
Switching the heating element ON when temperature drops below setpoint.
Switching OFF when the setpoint temperature is reached.

This cyclic operation maintains stable water temperature while optimizing energy consumption.

4 - Overheat Protection and Safety Cut-Off

For safety compliance with:

EN 60335-1
EN 60335-2-21

Electric water heaters include an independent thermal safety cut-out.

If abnormal overheating occurs:

The safety device interrupts power supply.
Manual reset may be required.
This prevents tank rupture or fire hazard.

Additionally, pressure relief valves protect against excessive internal pressure caused by thermal expansion.

5 - Energy Flow Summary

The energy transformation process can be summarized as:

Electrical Energy → Resistive Heating → Thermal Energy → Stored Hot Water

System efficiency depends on:

Technical Specifications of a Typical Electric Water Heater (EWH)

The following table summarizes the typical technical characteristics of a domestic electric storage water heater designed for the European market.

General Technical Data (Example: 20 –120L Model)

Rated Voltage :220–240 V

Frequency : 50 / 60 Hz

Rated Power :1.5 – 3.0 kW

Tank Capacity : 10 – 150 Liters

Maximum Working Pressure : 6 – 10 bar

Maximum Operating Temperature :75°C

Heating Element Type : Immersion resistance element

Insulation Type : Polyurethane foam

Protection Class : IPX4

Installation Type : Vertical / Horizontal

Tank Material : Enameled + carbon steel

Corrosion Protection : Magnesium anode

Safety Devices : Thermal cut-out + Pressure relief valve

Electrical Characteristics

Electric storage water heaters are designed in compliance with:

EN 60335-1
EN 60335-2-21

Key electrical characteristics include:

Class I appliance (earth connection required)
Thermostatic temperature control
Independent overheat safety limiter
Terminal block rated for appliance current

Hydraulic Characteristics

Hydraulic design typically includes:

Cold water inlet with diffuser
Hot water outlet from upper tank zone
Non-return safety valve (external group)
Operating pressure up to 10 bar

Performance classification must comply with: EN 12897

Energy Efficiency Classification

Electric water heaters placed in the EU must meet requirements under: ErP Directive 2009/125/EC

Energy labeling depends on:

Standing heat loss
Load profile (S, M, L, XL)
Thermal efficiency

Summary

An electric storage water heater is a pressure vessel with integrated electrical heating, thermal insulation, and safety control systems designed to provide reliable domestic hot water in compliance with European safety and performance standards.