“I had a heat pump installed last year, and it’s still costing me more than my old gas boiler!”
It is a common complaint from homeowners who expected lower bills but are paying more. Their frustration is understandable but often misdirected. Getting the right result all round depends on choosing the appropriately sized heat pump output for the home.
Heat pumps can deliver efficiencies of 300–400% (known as the Coefficient of Performance, or COP), compared to a typical gas boiler at around 90%. However, that efficiency is only realised when the system is correctly sized and operated at low flow temperatures: ideally below 45°C for radiators, or even lower for underfloor heating.
Using the wrong size can result in a heat pump with a power output that’s not optimal for the home. Oversizing leads to frequent cycling (short bursts of operation), which reduces efficiency and increases wear over time. Undersizing, on the other hand, may not meet heating demands during peak winter periods and will result in increased energy consumption.
So where are the common areas where heat pump mis-sizing occurs?
Take measure.
Before making a product choice, the critical first stage is to do a proper heat loss assessment of the property. This means assessing the home’s fabric, insulation levels, and measuring all windows, doors, and other areas where heat can escape.
Remember, every detail counts so accurate information is essential. Windows, floor space, insulation – all these details will be needed to get the full picture of a building’s heat requirements.
New builds are simpler; retrofits need a bit more care to account for thermal bridging, solid walls, and inconsistent U-values in older structures.
Homeowners may be unenthusiastic about taking up freshly laid carpet to get access to pipework to measure it, but it is these details which can make a difference.
Occupancy and usage also play an important role. How many people are in the home? How do they interact with hot water and heating? If a home is a shared occupancy or large family with 2-3 children, then hot water demand will be high, especially in the mornings. Therefore a larger hot water cylinder is essential otherwise the final person in line might be left with a cold shower.
Getting the calculator out.
To calculate the size needed, installers can use a variety of online heat loss calculators. There are plenty of great tools available, we commonly use this one however there are plenty available which do the job.
While online calculators can give a good initial estimate, MCS-compliant installations must include a room-by-room heat loss calculation based on BS EN 12831. This ensures each room’s radiator or underfloor circuit is correctly matched to the required output at design temperature.
Our recommendation regardless of the calculator used is to take the brief time to go through the tutorial. It takes only a few hours, but many calculations we review contain basic input errors like mistaking a 1×1 metre window for 10×10 due to a decimal point. It might seem like a silly example, but each calculator works differently and it’s well worth the time understanding how they work before using them.
Understanding peak winter demand.
Heat loss calculators are based on the principal of working out how much energy is needed to heat the home to 21C, with ‘peak demand’ being the coldest it’ll be outside. The lower the temperature, the more energy needed.
Using a UK average winter temperature leads to incorrect sizing. Instead, design temperatures should reflect the local climate, typically based on the 99.6th percentile winter condition. For example, -3°C might be used in southern England, but -5°C or lower is common in Scottish or far northern areas. These values are often sourced from CIBSE weather files.
Details matter.
The factors outlined above are all interdependent. When correctly accounted for, they ensure the system is not just efficient but also comfortable and cost-effective for years to come. For installers, investing the time upfront means fewer call-backs, happier customers, and higher trust in the technology.
Contact us today for more information on heat pumps, or view our products.