In recent weeks, I was asked to share some thoughts on defrost cycles in heat pumps, based on the real-world data we collect through Havenwise. What started as a conversation about defrost behaviour quickly turned into a broader reflection on heat pump sizing—and how it affects comfort, cost, and confidence.
Defrosts are a normal part of any heat pump’s operation, yet for many homeowners, they’re unexpected. We see this every winter: when the weather dips into “classic defrost conditions,” we notice a spike in messages from confused or concerned users. It’s a clear reminder that we need to do more as an industry to explain what’s happening and why it’s not a fault.
But there’s a deeper layer here. While defrosts are routine, they do affect performance. Our data shows that when a heat pump enters defrost mode every 30–60 minutes, the system’s effective heat output can drop by around 15% - sometimes more, depending on the brand, flow temperature, and system setup. That drop can have a real-world impact during the coldest periods, when homes are most in need of steady heating.
One challenge for installers is understanding how much of this is already “baked in” to the manufacturer performance data. The reality is: it’s often unclear. Some manufacturers are transparent, others less so. That uncertainty makes sizing decisions harder. Should an installer trust the stated output at face value? Or apply a safety margin—and if so, how much?
For years, the message across the industry has been consistent: avoid oversizing. And rightly so—historically, systems were often vastly oversized, leading to inefficiencies and cycling issues. But that pendulum may have swung too far.
The data we’ve gathered, and what we hear from homeowners, suggests that slight oversizing can actually improve outcomes:
- The most difficult customer scenarios are when systems struggle to meet demand during deep winter.
- Homeowners often expect to increase the temperature on demand, and slightly more capacity enables that.
- Performance (COP) doesn't significantly deteriorate when the system is 10–50% oversized.
- Some of the best-performing systems on [OpenEnergyMonitor](https://lnkd.in/ep3agbQn) are in fact 40–80% oversized—well beyond what many would consider acceptable.
- With the rise of Time-of-Use tariffs, extra capacity becomes an asset for load shifting and reducing running costs.
- There’s no conclusive evidence that modest oversizing shortens system life—especially when it’s done thoughtfully and doesn't lead to rapid cycling.
This doesn’t mean we abandon the principles of good design. Gross oversizing—2 to 3 times the actual heat loss—can still create problems. But aiming for a “just right” margin above design load isn’t just acceptable. In many cases, it’s the best route to comfort, resilience, and long-term efficiency.
And when you factor in the uncertainty around defrost losses and performance data, that margin becomes not just a preference—but a safeguard.
If you’re an installer, engineer, or homeowner navigating the complexities of heat pump sizing, know this: slightly oversizing a system isn’t a failure of design. In fact, it may be the smartest choice you make.
The data tells us that comfort, efficiency, and reliability can all be improved with a little extra capacity—and that’s something we should feel confident talking about.
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