Boiler Size & Heating Requirement Calculator
Estimate a sensible UK boiler output class by separating space-heating demand from combi hot-water demand.
Typical use: Early-stage boiler sizing, combi vs system-boiler comparison and hot-water demand planning
Using this calculator
Open the interactive calculator to enter your project measurements and adjust the available settings. Review the calculated quantity and buying units before ordering materials. Check product coverage, pack sizes and installation guidance with the manufacturer or supplier.
Planning and practical guidance
What Size Boiler Might Your Home Need?
Learn why bedrooms alone do not determine boiler size and how heat loss, bathrooms, hot-water demand and heating-system type affect the specification.
- Separate the home’s space-heating requirement from its domestic hot-water requirement; they are related but not the same calculation.
- Space-heating demand depends on heat loss through the building fabric, ventilation, room sizes, insulation and design temperatures rather than radiator count alone.
- For a combi boiler, simultaneous hot-water demand and desired flow rate can drive the required output higher than the space-heating load.
- Homes with multiple bathrooms or high simultaneous hot-water demand may be better suited to a system boiler and hot-water cylinder rather than simply a larger combi.
Boiler Types & Efficiency: Combi, System, Regular and Condensing Operation
Understand boiler system types, hot-water arrangements, condensing operation and why lower heating flow temperatures can improve efficiency on suitable systems.
- A combi boiler heats domestic hot water on demand and normally does not use a separate hot-water cylinder; its hot-water flow capability can influence the required boiler output.
- A system boiler normally heats a separate hot-water cylinder while incorporating more system components within the boiler, making it suitable for homes where stored hot water is preferred.
- A regular/heat-only boiler also uses stored hot water and can be found in traditional open-vented systems with additional tanks and components.
- Modern gas boilers are generally condensing appliances, designed to recover extra heat from flue gases when return-water temperatures are sufficiently low.