Homeowners are becoming increasingly aware of how their heat pumps behave during winter, particularly when systems appear to stop working temporarily. The culprit? Defrost cycles… normal, necessary and often misunderstood. But are the energy losses and output drops they cause actually included in manufacturers’ performance figures?
The answer isn’t as straightforward as it should be.
Are Defrost Cycles Included in COP and SCOP Figures?
The short answer: sometimes. But not always, and that’s a potential problem.
Grant says their ErP and MCS testing protocols (EN14511 and EN14825) may include defrosts, but only if one happens to occur during the three-hour transient test window. If it doesn’t, the performance loss isn’t reflected. Grant openly admits that lab-based COP results often don’t reflect real-world operation.
Ideal Heating claims their output data does account for defrost losses.
Viessmann also states that defrost is factored into their testing, and highlights a specific mitigation strategy: their Hydro AutoControl system. It uses stored thermal energy to help maintain comfort during defrost events, reducing performance dips. It’s smart, but are all models and manufacturers doing this? No.
And that’s the broader issue. As Graham Hendra (@grahamh) and Karl Richardson both pointed out to me, there is no requirement for manufacturers to disclose how defrost is treated in their performance data. Some do. Many don’t. The result: installers are left making educated guesses, either taking figures at face value or adding unknown safety margins that could lead to oversizing.
Sune Nightingale (@sune) noted that the EN14511 standard allows for both steady-state and transient testing. Without controlled humidity in the test environment (something rarely required) results often don’t match UK winter conditions. This, he argues, casts doubt on how relevant COP and SCOP figures really are for British households during the coldest parts of the year.
Havenwise agrees: “It’s not always clear what’s accounted for in the manufacturers’ output sheets. This complicates the life of an installer. Do they take the stated output at face value or add a margin – but how much? Too much, and they risk oversizing.”
Real-World Data: Defrost Losses Are Small… Until They’re Not
This is where independent monitoring data becomes critical.
Open Energy Monitor has tracked hundreds of systems across the UK through its Heat Pump Monitor project. Trystan Lea, one of the team behind it, says that across a full year, energy lost to defrosts is usually in the range of 2-3% of total heat output, enough to lower SPF slightly, but not dramatically.
However, the picture changes during cold snaps.
In one example from Adam at Heat Geek, real-time system monitoring showed defrost losses of 9.2% during a specific 36-hour window. That’s nearly one-tenth of the system’s energy vanishing into thin air (literally) just to keep the outdoor unit frost-free.
Havenwise confirms this pattern: “When a heat pump defrosts every 30-60 minutes, it’s no surprise that heat output drops. We see defrosts reducing output around 15% compared to stated figures, sometimes significantly more, depending on manufacturer, flow temperature and other conditions.”
This is not a trivial issue for homeowners, especially those with systems sized tightly against calculated heat losses. A 15% drop in output can mean the difference between staying warm and being cold, particularly in poorly insulated homes during freezing weather.
The Hidden Issue: Inflated Output Claims?
Trystan Lea highlights another layer to the issue: many heat pumps do not achieve their stated maximum outputs when defrost cycles are frequent. This, he argues, is a more serious problem than defrost energy losses themselves.
“Historically, heat loss calculations included overly high air change rates,” he explains. “That created generous margins for error. But those margins are disappearing.”
This is due to changes in UK industry standards. MCS 023 is moving toward mandatory use of the EN12831-1:2017 standard, which encourages tighter, more accurate heat loss calculations, especially when paired with blower door tests. It’s a good step forward for design accuracy, but it leaves little room for real-world performance shortfalls during cold weather.
In short: we’re designing systems with tighter tolerances based on theoretical outputs that may not be achievable when defrosts are frequent.
Is Oversizing Still a Dirty Word? Not Anymore
The industry has spent the last few years preaching the gospel of correct sizing. Oversizing, we were told, reduces efficiency, leads to short cycling and shortens lifespan.
But real-world data (and rising consumer expectations) are changing that narrative.
Havenwise makes the case for modest oversizing: “It’s totally OK (and in my opinion, preferred) to slightly oversize a heat pump. The homeowner will be more comfortable, running costs will be lower, and nobody has proven that system lifetime is negatively impacted.”
Their argument is supported by data and experience:
- Comfort matters: When homes can’t stay warm during defrost-heavy periods, the result is stress, discomfort and health risks – especially for vulnerable residents.
- Flexibility is important: Homeowners want the ability to turn up the thermostat when needed. Slight oversizing allows for that.
- COP remains stable: There’s little evidence to suggest that modest oversizing harms efficiency.
- ToU tariffs change the game: Oversized units can run hard during off-peak hours and coast through high-tariff periods, cutting costs without sacrificing warmth.
And crucially, as Havenwise notes, oversizing doesn’t always mean excessive cycling, especially if the controls and system volumes are managed properly.
So Where Does That Leave Us?
Here’s the reality:
- Defrost losses are small on average, but can spike significantly in cold weather.
- Output drops due to defrost are real, and often unaccounted for in sizing or performance data.
- Manufacturers aren’t always transparent and defrost handling varies by brand.
- Oversizing is back on the table and it may be essential for ensuring winter comfort in UK homes.
The challenge now is communication. Homeowners need to understand that defrosts are normal. Installers need access to clearer data. And the industry needs to drop its one-size-fits-all approach to sizing.
Because in the real world, on a January morning, in a draughty semi, at 2C and falling, comfort isn’t about lab conditions. It’s about whether your heat pump is still delivering heat when you need it most.