Flat / Apartment
Mid-Terrace House
Semi-Detached House
Detached House
Poor (No loft/wall insulation)
Average (Standard insulation)
Excellent (Modern/New Build)
Recommended Boiler Size:
*Note: These are estimates. Always consult a Gas Safe registered engineer before purchasing.
How to Use the Boiler Size Calculator
Choosing the correct boiler size is crucial for both comfort and energy efficiency. A boiler that is too small won't heat your home adequately, while one that is too large will waste energy and increase your utility bills. Our calculator estimates the required kilowatt (kW) output based on your property's specific needs.
Key Factors in Boiler Sizing
Property Type: Larger, detached homes lose heat more rapidly than flats surrounded by other heated units.
Radiator Count: This determines the "space heating" load. Generally, each radiator requires approximately 1.5kW of power.
Hot Water Demand: Combi boilers need higher kW ratings to provide instantaneous hot water to showers and taps. More bathrooms mean a higher required kW for Combi systems.
Insulation: Modern insulation drastically reduces the heat loss, allowing for a smaller, more efficient boiler.
Example Scenarios
1. Small Flat (1 Bed, 1 Bath): Typically requires a 24kW – 27kW Combi boiler. The heating requirement is low, but the boiler needs enough power to heat water for the shower.
2. 3-Bed Semi-Detached (10 Radiators, 1.5 Baths): Usually requires a 28kW – 34kW Combi boiler or an 18kW – 24kW System boiler.
3. Large Detached House (15+ Radiators, 3 Baths): Often requires a 35kW – 42kW Combi or a 30kW+ System boiler with a separate hot water cylinder.
function calculateBoilerSize() {
var radiators = parseFloat(document.getElementById('numRadiators').value);
var bathrooms = parseFloat(document.getElementById('numBathrooms').value);
var propMultiplier = parseFloat(document.getElementById('propertyType').value);
var insulMultiplier = parseFloat(document.getElementById('insulation').value);
if (isNaN(radiators) || isNaN(bathrooms)) {
alert("Please enter valid numbers for radiators and bathrooms.");
return;
}
// Base Calculation for Space Heating (System/Regular Boiler Focus)
// Formula: (Radiators * 1.5kW) * Property Factor * Insulation Factor
var heatingLoad = (radiators * 1.5) * propMultiplier * insulMultiplier;
// Combi Boiler logic focuses heavily on Hot Water flow rates
// 1 Bath: ~24-27kW
// 2 Baths: ~28-34kW
// 3+ Baths: ~35-42kW
var combiSize = 0;
if (bathrooms <= 1) {
combiSize = 24 + (radiators * 0.5);
} else if (bathrooms <= 2) {
combiSize = 30 + (radiators * 0.4);
} else {
combiSize = 35 + (radiators * 0.3);
}
// Cap Combi size to realistic residential ranges
if (combiSize 42) combiSize = 42;
// System boiler sizing is usually lower as it doesn't heat water instantly
var systemSize = heatingLoad + 3; // +3kW for cylinder heating
if (systemSize < 12) systemSize = 12;
// Display Results
document.getElementById('resultContainer').style.display = 'block';
document.getElementById('combiResult').innerHTML = 'Combi Boiler: ' + Math.round(combiSize) + ' kW';
document.getElementById('systemResult').innerHTML = 'System / Regular Boiler: ' + Math.round(systemSize) + ' kW';
// Smooth scroll to result
document.getElementById('resultContainer').scrollIntoView({ behavior: 'smooth', block: 'nearest' });
}