Heat Pump Sizing: What Installers Need to Know

Cai Williams
1 day ago
2 min read

Correct heat pump sizing is critical to system performance, efficiency, and customer satisfaction. Oversize it, and the system cycles too frequently. Undersize it, and it won’t keep the home warm on the coldest days.

This blog offers a simplified guide to help heat pump installers, especially those newer to the trade, understand the fundamentals of sizing, common pitfalls, and how EPS can support you with system design and advice.

Why Sizing Matters For Heat Pump Installations

Unlike gas boilers, heat pumps are designed to run continuously at a lower temperature. If the system is not sized correctly:

Oversized systems cost more upfront, take up more space, and cycle on/off too often, which reduces lifespan and efficiency.
Undersized systems struggle to meet demand in cold weather, leaving customers cold and unhappy.

Correct sizing ensures consistent comfort, lower running costs, and MCS compliance (where applicable).

What Goes Into Sizing a Heat Pump?

To size a heat pump properly, installers need to calculate the property’s heat loss — essentially, how much heat escapes from a building under peak winter conditions. This helps determine the output (kW) required from the heat pump.

Key factors include:

Room sizes and layout
Property age and construction type
Insulation levels (walls, loft, floors)
Window types and glazing
Air change rate (ventilation/draughts)
Design outside temperature (usually -2°C to -3°C in the UK)

📌 Rule of thumb sizing is not recommended.

Always aim to complete a room-by-room heat loss calculation for accuracy — this is especially important for MCS-accredited installs or BUS-eligible jobs.

Tools Installers Use

Many installers use software tools or spreadsheets that comply with MCS standards to carry out heat loss calculations. Popular options include:

MCS-approved calculators (often built into installer portals)
Manufacturer-provided tools
Specialist design software (e.g. Heat Engineer)

Some experienced installers carry out manual calcs or work with independent designers.

Beyond the Heat Pump: System Design Matters

A correctly sized heat pump is just one part of the equation. Installers should also consider:

Radiator/underfloor sizing (based on flow temp)
Cylinder selection for domestic hot water
Buffer vessels or low-loss headers (where appropriate)
Control strategy and wiring
Flow rates and pipework sizing

System design plays a huge role in overall performance and efficiency — so even if the pump is the right size, the whole setup needs to work in harmony.

How EPS Can Help

EPS works with heat pump installers across the UK to ensure systems are well-matched to each property. While we don’t replace a full design service, we’re always happy to offer advice on: