The marketing materials for air source heat pumps tend to feature impressive efficiency numbers — SPF 3.5, SPF 4.0, sometimes higher. What those numbers mean is that for every unit of electricity the heat pump uses, it delivers 3.5 or 4 units of heat. Genuinely impressive compared to a gas boiler, which at best gives you one unit of heat per unit of energy.
Here’s the thing, though. Those figures come from laboratory conditions. Real-world UK installations frequently tell a different story.
What SPF Actually Is
Seasonal Performance Factor measures efficiency across a whole heating season rather than at one operating point. It’s the number you actually care about — not the coefficient of performance at a specific outdoor temperature on a sunny autumn day, but the average across cold January nights and milder March mornings.
The Energy Systems Catapult’s Electrification of Heat project, which monitored heat pumps in UK homes over multiple years, found median SPFs clustering around 2.5 to 2.8 for many installations. That’s functional — still better than oil heating on a carbon basis, and increasingly better than gas as the grid decarbonises — but well below the 3.5 to 4.0 the industry often quotes.
The difference between a well-installed heat pump running at SPF 3.5 and a poorly commissioned one running at SPF 2.2 is significant. Over a year in a typical three-bedroom home, that gap represents hundreds of pounds in extra electricity bills.
What Causes the Gap
Flow temperature is the biggest lever. Heat pumps are most efficient when they produce water at lower temperatures — 40 to 45°C works much better than 55 to 60°C. But many older properties run radiator systems designed for gas boilers at 70°C+. Installers sometimes compensate by running the heat pump at higher flow temperatures, which works but kills efficiency.
The right solution is to assess whether the radiators are large enough for lower flow temperatures. In a well-insulated home, they often are. In poorly insulated properties, you might need to upsize some radiators or accept lower room temperatures. But running the heat pump hot because no one did the radiator assessment is a commissioning failure, not an inherent limitation of the technology.
Cycling behaviour. A heat pump that’s oversized for a property’s heat loss will reach its target temperature quickly and switch off, only to restart a few minutes later when the temperature drops. Each start cycle is inefficient. An oversized heat pump cycling 20 times a day performs worse than a correctly sized one running continuously. Proper heat loss calculations before installation matter enormously.
No buffer tank. Some systems, particularly those with underfloor heating or very low system volume, benefit from a buffer tank that smooths out cycling. Without it, the heat pump hunts for a stable operating point and never quite finds one.
Defrost cycles in winter. In cold, damp UK winters, the outdoor unit accumulates ice and needs to run a defrost cycle. During defrost, the system temporarily reverses and takes heat back from the hot water, reducing output. Better heat pump designs minimise defrost frequency, but all air-source units do it. Proper positioning of the outdoor unit — avoiding frost pockets, ensuring good airflow — helps.
How to Check Your Own Installation
If you have a heat pump already, most modern units report their operating data via an app or web portal. You’re looking for:
- Average flow temperature: should be below 50°C for most systems
- Number of start/stop cycles per day: more than 6 to 8 cycles suggests potential oversizing or system design issues
- COP readings over time: if your heat pump reports live COP and it’s consistently below 2.5, investigate
The Homely Energy platform (used with several UK heat pump brands) and the Tado integration for Mitsubishi and Daikin units can surface this data. Some installers will provide this on request — good ones monitor performance proactively.
What Good Commissioning Looks Like
An MCS-certified installer doing proper commissioning will:
- Calculate your home’s heat loss first. Room by room, accounting for insulation levels, window sizes, and thermal bridging. This determines what size heat pump you need.
- Set the weather compensation curve correctly. Weather compensation adjusts the flow temperature based on outdoor temperature — lower on mild days, higher on cold ones. Getting this curve right for your specific home and system is essential.
- Balance the radiator circuit. Each radiator should receive the right proportion of flow. An unbalanced circuit means some rooms are too hot, others too cold, and the heat pump works harder than it needs to.
- Set minimum run times to reduce cycling.
- Return after the first winter to review. Honest installers will check real-world performance against the predicted SPF from their heat loss calculations.
If your installer didn’t do a heat loss calculation before sizing your heat pump, that’s a red flag. The MCS Compliance Scheme requires one, but enforcement has been inconsistent.
Getting a Rebalance
If you suspect your existing installation is underperforming, you have a few options. An independent heat pump assessor — separate from your original installer — can review your settings and system design. The Heat Pump Association maintains a list of certified engineers. Some heat pump manufacturers also offer commissioning support services, particularly if the unit is under warranty.
Homely Energy’s optimisation service, and similar offerings from Tepeo and other platforms, can tune the heat pump’s operating schedule without hardware changes and often produce noticeable efficiency improvements on existing systems.
The technology genuinely works. The gap between rated performance and real-world performance is mostly a commissioning and system design problem, not an engineering limitation. With the right setup, SPF 3.5 in a UK home isn’t unrealistic — it’s just not automatic.