How to Size a Heat Pump for Your Cornwall Home: The Complete Guide
Getting the sizing right is the most important decision in any heat pump installation. An undersized system will struggle on cold days; an oversized one will short-cycle and waste energy. This guide explains how MCS-certified installers size heat pumps for Cornwall homes specifically.
Why Sizing Matters
The most consequential decision in any heat pump installation is getting the size right. An undersized heat pump will run continuously on cold days and fail to maintain comfortable temperatures. An oversized heat pump will short-cycle — turning on and off frequently — which dramatically reduces efficiency and lifespan. A correctly sized heat pump runs for long, steady cycles at low temperatures, achieving a COP of 3.5–4.5.
The Heat Loss Calculation
Proper heat pump sizing requires a BS EN 12831 heat loss calculation — a detailed engineering assessment of how much heat your home loses per hour at the design temperature (in Cornwall, typically −3°C to −5°C). This calculation considers floor area and ceiling heights, wall/floor/roof construction (U-values), window and door sizes and glazing type, air infiltration rate, and orientation and exposure.
MCS certification requires that every heat pump installation is sized using a full heat loss calculation. Any installer who quotes without conducting a proper heat loss assessment is not working to MCS standards — and your BUS grant claim may be invalidated.
EPC Ratings & Insulation First
Improving insulation reduces your heat loss figure, meaning a smaller (cheaper) heat pump is needed. Key upgrades:
- Loft insulation to 270mm: Reduces heat loss by 15–25% in poorly insulated homes. Cost: £300–600.
- Cavity wall insulation: Reduces heat loss by 20–30%. Cost: £500–800. Note: BUS grant requires this done first if recommended on your EPC.
- Floor insulation: Spray foam under suspended timber floors. Cost: £800–1,500.
Typical Heat Pump Sizes for Cornwall Homes
- Victorian terrace, 70m², EPC D: 9–11kW
- Victorian terrace, 70m², EPC C (insulated): 7–9kW
- Semi-detached, 90m², EPC C: 8–10kW
- Detached bungalow, 120m², EPC C: 10–12kW
- Detached house, 150m², EPC C: 12–14kW
- Large detached, 200m²+, EPC D: 14–18kW
Cornwall's mild climate (design temperature −3°C vs −7°C in northern England) means Cornwall homes typically need slightly smaller heat pumps than equivalent properties in colder parts of the UK.
The Oversizing Problem
Some installers routinely oversize heat pumps — a larger unit costs more (good for margin), and efficiency loss may go unnoticed. Signs your heat pump may be oversized: frequent short on/off cycles (under 10 minutes), flow temperatures much higher than 45–50°C, electricity consumption higher than expected. Demand a copy of the BS EN 12831 heat loss calculation before signing any contract.
See our guide: Choosing the Right Heat Pump Brand.
Get a Properly Sized Survey
CCS Heating & Renewables carries out full BS EN 12831 heat loss calculations on every survey as standard. We show you the output and explain exactly why we're recommending the size we are. Book your free heat pump survey today.
Need personalised advice?
Our MCS certified engineers can answer your questions and provide a free, no-obligation assessment for your property.