What is HeatpumpMonitor.org?
HeatpumpMonitor.org is an open-source community initiative run under the OpenEnergyMonitor umbrella. Heat pump owners — many of them among the UK's most technically rigorous installers and enthusiasts — connect monitoring hardware to their systems and upload live performance data. The result is a growing, publicly searchable dataset covering everything from seasonal performance factors to flow temperatures on the coldest recorded day.
The dataset is genuinely useful for anyone buying, installing, or simply curious about heat pumps. It answers questions that manufacturer specifications simply can't: how does a particular model behave in mild weather? What does it actually output during a defrost cycle? Is this system's performance typical or outlying?
The central finding: low flow temperature is everything
The single strongest predictor of annual performance (R² = 0.55) across all 600+ systems is the difference between the average flow temperature and the outside temperature. The lower this gap, the higher the SPF. This confirms basic thermodynamic theory — but it's now proven against hundreds of real UK homes.
In practice, the data shows a clear gradient. Systems running with a weighted average flow temperature of around 30°C achieve an SPF of roughly 4.4. At 35°C average flow, that drops to around 3.9. At 40°C, it falls to about 3.4. Every extra degree of flow temperature costs efficiency.
| Weighted avg flow temp | Typical SPF H4 | Assessment |
|---|---|---|
| 30°C | ~4.4 | Excellent — optimally designed system |
| 35°C | ~3.9 | Good — well-tuned radiator system |
| 40°C | ~3.4 | Moderate — room for improvement |
| 45°C+ | <3.0 | Poor — under-sized emitters or wrong settings |
The flow temperature myth: systems run cooler than designed
One of the most striking findings is that most systems actually run at lower flow temperatures than their heat loss calculations predicted. Systems designed for a 45°C flow temperature typically run at around 38°C on their coldest day. Those designed for 50°C typically achieve around 40°C in practice.
The implication is significant: heat loss calculations, particularly for pre-2000 properties, frequently overestimate how much heat a home actually needs — especially when fabric improvements like loft insulation and double glazing are properly accounted for. When weather compensation is tuned to the real demand rather than the calculation, flow temperatures fall and efficiency rises.
Design recommendations from the data
The OpenEnergyMonitor team distils the dataset into clear design guidance. The key thresholds are:
- Target design flow temperatures of no more than 42°C, ideally 37°C, to achieve an SPF of 4.0 or above. This requires properly sized emitters (larger radiators or UFH) and accurate heat loss calculations.
- For moderate-high performance (SPF ~3.5), design flow temperatures should be no more than 45°C, ideally closer to 40°C.
- Use accurate heat loss calculations — methods incorporating air permeability testing and EN 12831-1:2017 consistently show lower actual heat loss than rule-of-thumb estimates. Use this to lower the design flow temperature, not to reduce radiator sizing.
- A modest oversizing margin of 1.2–1.5× heat loss is fine, and may actually be preferable given the reduced max output some heat pumps produce during defrost cycles.
- Commission weather compensation carefully. A significant proportion of monitored systems show sub-optimal weather compensation settings — one of the most common and correctable causes of poor performance.
- Run open loop where possible — no zoning, TRVs at maximum or used only as high-temperature cutoffs, and minimal overnight setback (18–19°C setback vs 20°C daytime).
What this means for your installation
The dataset validates what the best installers already know: brand choice matters far less than system design. A well-designed, properly commissioned heat pump running at 35°C flow in a home with adequately sized radiators will comprehensively outperform a premium unit running at 50°C in a poorly designed system. The data shows SPF H4 values ranging from below 2 to above 5 — almost entirely explained by system design and commissioning, not hardware.
For those buying now: insist on a proper heat loss calculation, ask what design flow temperature your installer is targeting, and make sure weather compensation will be set up and tuned at commissioning. These decisions matter far more than the brand badge on the outdoor unit.