Most domestic heat pumps sold in the UK are categorised by a kilowatt (kW) rating, often 5kW, 6kW, 8kW or similar. But these figures are widely misunderstood. They are not guaranteed outputs in all conditions. In fact, the stated output is often only achievable under specific test parameters: generally, an outdoor air temperature (OAT) of 7C and a flow temperature of 35C. These conditions are rarely representative of winter heating demand in the UK.
At lower outdoor temperatures, heat pumps lose capacity. A 6kW unit that achieves its full output at 7C may struggle to deliver even 4.5kW at -3C. This drop in output has direct implications for performance. If a heat pump has been sized based on its nominal rating without properly accounting for low ambient temperature performance, the system may be unable to meet the heating demand in colder weather. In such cases, the shortfall is often incorrectly blamed on the technology, rather than the system design or sizing.
Adding to the confusion is the practice among some manufacturers of using the same physical hardware across multiple model ratings. A 5kW and 8kW unit may be mechanically identical, with the only difference being software configuration. This means that a so-called 5kW unit may, under certain conditions, deliver more than 5kW, and an 8kW may struggle to meet that figure consistently in real-world scenarios.
These issues are compounded by changes to how performance estimates are now calculated. Earlier this year, MCS implemented a new standard (MCS 031) that replaces manufacturer-declared SCOP (Seasonal Coefficient of Performance) values with standardised Seasonal Performance Factors (SPFs). These SPFs are fixed for all heat pumps based on flow temperature bands, regardless of the specific unit’s capabilities:
- <35C: SPF 4.0
- 36–40C: SPF 3.8
- 41–45C: SPF 3.4
- 46–50C: SPF 3.1
- 51–55C: SPF 2.8
- 56–65C: SPF 2.5
This approach removes variation between manufacturers and is intended to simplify performance estimates. However, it also masks differences between systems, particularly at higher flow temperatures. For installers and consumers, this creates a massive challenge: the published SPF no longer reflects the real performance potential of a better-designed or higher-specification heat pump.
The core problem remains: the industry lacks clear and consistent communication about how heat pumps perform across the full range of outdoor temperatures. Most consumers (and many installers) still interpret nominal figures as guarantees. In reality, they are conditional benchmarks. The impact of poor sizing is often not seen until outdoor temperatures drop below design conditions, which in the UK typically range from -1C to -3C.
The result is predictable: underperformance, increased use of immersion heaters or backup sources, and customer dissatisfaction. These outcomes are preventable. Correct sizing, based on detailed room-by-room heat loss calculations and accurate climate data, remains essential. Equally, manufacturers and certification bodies must do more to improve transparency around performance data, particularly at low ambient conditions and higher flow temperatures.
Until then, it is likely that homeowners will continue to be misled by kW labels that are, at best, incomplete, and at worst, materially misleading. Why does Brand X’s 8kW unit perform like Brand Y’s 5kW? The lack of standardisation feels intentional, and it’s no wonder so many systems end up undersized, oversized or wasteful. This confusion must end.
My heat pump is a sold as a 6kW. But it does almost the same kW at any flow temp and any outside temp varies between 5.88 (-15) and 7kW (+30).
So strongly suspect it’s really an 8kW with a software gag on output. The 4kW is pretty similar almost flat output of 4kW, but the 8kW output varies by quite a margin, with -15 performance pretty similar to the 6kW unit, at just over 6kW.
In fairness to heat pump manufacturers the output inevitably (unless artificially restricted in software) varies with oat and FT. Furthermore the variation is different between manufacturers and models, depending on compressor characteristics and how they have chosen to tune performance.
There is no way that a single figure can describe this complexity and thus it is unrealistic to expect all models branded as having similar ‘headline’ outputs to have the same output under all relevant conditions. So we either have to dispense altogether with headline figures or accept that they tell only a small part of the story.
To add to the challenge, design outdoor temperatures, even within one country eg UK, vary quite significantly so a 7kW unit in Glasgow may be good for 8kW in Plymouth and probably 5kW in Prague.
For me the current approach of specifying ‘headline’ output at a given set of conditions is a practical one, and in reality probably the only practical one. One might reasonably argue that the specified condition should be at a lower oat than 7C. I don’t know where that figure came from, presumably it’s an EU standard, but if so it’s a bit odd because there can’t be many parts of the EU where this would be a sensible design temperature.
What I do think is missing, however, is the detailed design tables specifying max and min output as a function of ft and oat (including the effect of defrost) Some manufacturers are very good at this (eg Mitsubishi which publishes very comprehensive data), some mediocre (eg Vaillant which does publish detailed tables but only in Czech on its Czech website, and says nothing about defrost) and some dreadful (eg Daikin which hides most of the data unless you are an installer, and Samsung which seems to publish very little also). My view us that the open publication of this detailed data should be mandatory and if it were there would be no excuse for designers to get it wrong.
Think you can find the data, if willing to search, but it should be freely available to anyone, it’s not exactly that much data. 1 table says it all, or 2 tables to include cooling.
A lot of heat pumps are just name badge engineering, the issue is the company willing to slap a badge on someone else product, is just playing, not really that involves or really that bothered.
Our heat pump is a Maxa, they are the actual manufacturer, was and still is badge engineered by many big companies including Viessmann.
That’s my point really, it should be both freely and readily available. As you say it’s just a table (or set of graphs) of cop and output vs ft and oat at max and min modulation frequency, and maybe a couple of intermediate frequencies for good measure. Mitsubishi can manage that, why not everyone?
Sounds like your 6kW unit is really just an 8kW with a software cap. If it holds steady output across temps while the 8kW drops off, that’s a clear sign. Just shows how misleading the kW labels can be.