Heating Element & Electrical Design (EN 60335-2-21)

1️ - Heating Element Power Selection

The heating power depends mainly on:

• Tank volume (L)

• Heating time , t
  
  

• Temperature rise , ΔT​
  
  

Formula   P = m×Cp×ΔT / t

Where:

• P = Heating power (W)

• m = Water mass (kg) ≈ Tank volume (L)

• Cp = Specific heat of water = 4186 J/kg·°C

• ΔT = Temperature rise (°C)

• t = Heating time (seconds)

Simplified Engineering Formula  P(kW)= V×ΔT /  ( 860×t(h) ) ​

Where:

• V = Tank volume (L)

• ΔT = Temperature rise (°C)

• t(h) = heating time in hours
  
  

Example

50 L heater  ,  15°C → 65°C   then ΔT = 50°C 

Heating time = 1.5 h

P=50×50/ ( 860×1.5 ) ≈ 1.94 ≈ 2 kW = 2000 W

Increasing heating time to 2 hours  then P= 50*50/(860*2) = 1.45 KW ≈ 1.5 KW = 1500 W

2️ - Typical Heating Element Ratings

10 L  : 1.2 kW

30 L  : 1.5 kW

50 L  :  2.0 kW

80 L  :  2.5 kW

100 L:  3.0 kW

The heating power tolorance is : +5%  , - 10%

3️ - Heating Element Surface Load

A very important design parameter.

Formula  : q = P / A​

Where:

• q = Surface load (W/cm²)

• P = Element power (W)

• A = Element surface area (cm²)

Recommended Limits

Standard element  :  8–10 W/cm²

Long life element   :  5–7 W/cm²

Hard water areas   :  3–5 W/cm²

4️ - Electrical Current Calculation

Formula : I= P / V ​

Example : if  2 kW heater  and  230 V supply

I= 2000/230 = 8.7Amper

5️ - Thermostat Requirements (EN 60335-2-21)

Adjustable temperature :  30 – 75°C

Default setpoint             :  60 – 65°C

Accuracy                       :  ±5°C

Reasons:

• Prevent scalding

• Reduce scaling

• Energy efficiency

6️ - Thermal Cut-Out (Safety Thermostat)   EN 60335-2-21

Safety cut-out  : 85 – 95°C

Type                :  Manual reset

Location          :  Near heating element

Purpose:

• Prevent dry heating

• Prevent runaway thermostat failure

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