Calculating Required BTU (Heat Output)
Choosing the correct radiator size requires
calculating the required British Thermal Unit (BTU) output and factoring in heat loss.
The Simplified Formula: BTU = Room Volume x Insulation Factor x Temperature Factor
1. Room Volume (The Baseline)
Measure the room's Length, Width, and Height.
Multiply these dimensions together to get the cubic volume.
Impact: A larger volume requires a proportionally greater amount of heat (a higher BTU).
2. Insulation Level (The Heat Retention Factor)
Insulation dictates how quickly heat escapes. Poor insulation requires a significantly higher BTU.
Condition: Modern building standards, insulated cavity walls, good roof/floor insulation.
Impact: Requires the lowest base BTU.
Average Insulation (x 1.3):
Condition: Standard double-glazing, typical 1980s–2000s construction.
Impact: Increases BTU requirement by ~30%.
Poor Insulation (x 1.5+):
Condition: Older buildings, solid brick walls, uninsulated cavity walls, or uninsulated roof space.
Impact: Increases BTU requirement by ~50% or more.
3. External Walls (The Exposure Factor)
Heat loss occurs mainly through walls exposed to the outside air.
0 External Walls: Internal room (sheltered); minimal heat loss through walls.
1 External Wall: Standard exposure; calculation applies.
2 or More External Walls (Corner Room): Highly exposed; add 10–20% to the final calculated BTU.
North-Facing Rooms: These are typically colder; consider adding an extra 10% adjustment.
4. Windows (The Glazing Factor)
Windows are a major source of heat loss. The number, size, and type of glazing must be considered.
Triple Glazing: Minimal or no BTU adjustment needed.
Modern Double Glazing: Standard calculation applies.
Old Double Glazing: Add 5–10% to the final calculated BTU.
Single Glazing: Add 15–25% to the final calculated BTU to compensate for major heat loss.
Large Glazed Areas (Bay Windows, French Doors): May require an additional percentage adjustment due to the sheer surface area.
5. Final Radiator Selection
Match or Oversize: Select a radiator (or combination) whose total BTU output matches or slightly exceeds the calculated requirement (aim for 5–10% extra).
Avoid Undersizing: It's always better to have an oversized radiator that you can turn down than an undersized one that can never fully heat the room.
Placement: Install the radiator on the coldest wall, typically under a window or on an exterior wall, to counter cold-air movement.
Style for Space:
Use a Vertical Radiator in small or awkward spaces to meet high BTU needs without using up floor space.
In very large rooms, consider using two smaller radiators to distribute heat evenly and prevent cold spots.