How to calculate the correct heat pump size for your Dorset property: heat loss calculations, capacity requirements, avoiding undersizing/oversizing mistakes.
A heat loss calculation determines how much heat your property loses to the outside environment at your region's coldest design temperature (typically -3°C for Dorset). This calculates the exact heating capacity (kW) your heat pump needs to maintain 20-21°C indoors during the coldest winter weather.
This is done room-by-room considering each room's specific construction, exposure, and usage.
Cavity vs solid walls, insulation type/thickness, U-value (W/m²K)
Single/double/triple glazing, frame material, total glazed area per room
Insulation depth (mm), type, vaulted vs flat ceilings, room in roof
Solid concrete, suspended timber, above unheated space, insulation
Draughts, air changes per hour, building age/construction quality
Length, width, ceiling height, external wall exposure
North/south facing, wind exposure, sheltering from other buildings
Local climate (-3°C for Dorset), target indoor temperature (20-21°C)
Professional installers use approved software to ensure accuracy:
Warning: Avoid installers who size heat pumps using "rules of thumb" like "100W per m²" or property size alone. This leads to incorrect sizing in 60%+ of cases.
8kW heat pump (Daikin 8kW or Mitsubishi 8.5kW). Slight oversizing buffer accounts for real-world variations.
12kW heat pump (LG 12kW or Vaillant 12kW). Higher capacity needed due to poor insulation. Consider rear wall external insulation to reduce heat loss.
10kW heat pump (Panasonic 9kW or Mitsubishi 11kW). Excellent insulation = low heat demand despite large size. Ideal for underfloor heating.
14kW heat pump (LG 14kW or Mitsubishi 14kW). Exposed location requires +15% capacity buffer. Cold-climate model recommended.
Notice: Two properties with identical floor area (e.g., both 110m²) can require vastly different heat pump sizes (7.8kW vs 11.2kW in examples above) depending on construction, insulation, and exposure. This is why professional heat loss calculation is essential.
| Property Type | Floor Area | Typical Heat Loss | Heat Pump Size |
|---|---|---|---|
| 2-bed flat (modern) | 60-80m² | 4-6 kW | 5-7 kW |
| 3-bed semi (1980s-2000s) | 90-120m² | 7-9 kW | 8-10 kW |
| 4-bed detached (modern) | 130-180m² | 8-12 kW | 10-14 kW |
| Victorian/Edwardian (solid wall) | 100-140m² | 10-14 kW | 12-16 kW |
| Large period property | 180-250m² | 14-18 kW | 16-20 kW |
These are rough estimates based on typical Dorset properties. DO NOT use this table to size your heat pump. Insulation levels, window area, ceiling height, exposure, and dozens of other factors dramatically affect actual heat demand. Always require professional room-by-room heat loss calculation before purchase.
Result: Wrong size selection, poor performance, customer dissatisfaction, wasted money.
Result: Perfect size, optimal performance, maximum efficiency, happy customers.
Free comprehensive heat loss calculation with every quote. We'll determine the exact heat pump capacity your property needs for optimal performance.