A Resistive Heating Boiler Calculator is a digital tool used by engineers, installers, and property owners to accurately size electric boilers and assess their operational efficiency. Unlike gas or oil boilers that rely on fuel combustion, a resistive electric boiler utilizes electrical heating elements (resistors) to generate heat via Joule heating ( 1. Boiler Sizing Core Formulas
Sizing an electric boiler requires calculating the precise heat energy needed to overcome building heat loss or fulfill domestic hot water (DHW) demands. Building Heat Loss Method To calculate the raw space heating demand ( Qspacecap Q sub s p a c e end-sub ) in kilowatts (kW) based on floor area:
Qspace=Area (m2)×Heat Loss Baseline (W/m2)×Climate Factor×Height Factor1000cap Q sub s p a c e end-sub equals the fraction with numerator Area open paren m squared close paren cross Heat Loss Baseline open paren cap W / m squared close paren cross Climate Factor cross Height Factor and denominator 1000 end-fraction
Heat Loss Baseline: Typically 60 W/m² for well-insulated properties, 70–80 W/m² for average homes, and 100+ W/m² for poorly insulated structures.
Climate Factor: Multipliers ranging from 0.8 (warm zones) to 1.5 (extreme cold regions). Radiator Estimation Method (EDR)
Alternatively, installers use the Equivalent Direct Radiation (EDR) method by evaluating the collective maximum heat output of all installed emitters:
Qboiler≈∑(Output of each Radiator)×1.05 (Pipe Loss Factor)cap Q sub b o i l e r end-sub is approximately equal to sum of open paren Output of each Radiator close paren cross 1.05 (Pipe Loss Factor)
A quick rule of thumb allocates 1.5 to 2.0 kW per standard radiator. Domestic Hot Water (DHW) Demand
If the electric boiler is a combination (“combi”) unit providing instantaneous hot water, the sizing calculation is driven primarily by water flow rates rather than space heating:
QDHW=ṁ×Cp×ΔTcap Q sub cap D cap H cap W end-sub equals m dot cross cap C sub p cross cap delta cap T = Water flow rate (kg/s or L/s) Cpcap C sub p = Specific heat capacity of water ( = Temperature rise (Target hot water temp −negative Incoming cold water temp)
Combi boilers typically scale in tiers based on bathrooms: 24–27 kW for 1 bath, and 28–34 kW for 2 baths. 2. Sizing Electrical Infrastructure Use our boiler size calculator tables – The Heating Hub
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