EngineerHub.orgDrawing No. EH–TH–002 // Thermal Engineering & HVAC
Boiler Size Calculator for Homes & Buildings
Reviewed August 2026
Use a quick homeowner estimate or switch to an envelope-based design heat load with walls, windows, roof, floor and air change.
01 // Building heat load
02 // Recommended boiler capacity
Background & method
Space-heating equipment should fundamentally be sized to design heat loss. The advanced mode calculates ΣUAΔT plus an air-change heat-loss term; combi mode separately checks instantaneous hot-water power.
FAQ
Only for a rough first estimate. Final sizing should be based on design heat loss from walls, roof, windows, floor and ventilation/infiltration.
Instantaneous domestic hot-water production can require much more power than the building heat loss, so the DHW flow rate often governs combi-boiler input.
No. Excessive boiler capacity can cause short cycling, poor seasonal efficiency and control problems. Check minimum modulation as well as maximum capacity.