ETAS, the seasonal space heating energy efficiency, is the generator's efficiency expressed against primary energy, whereas SCOP is expressed against the electricity consumed. That is why a heat pump shows an ETAS of 135 % and a SCOP of 3.45: both describe the same machine on two different scales, linked by the primary energy conversion coefficient of 2.5 used by the ecodesign framework. Comparing an ETAS with a SCOP without converting is comparing euros with dollars.
ETAS: what exactly are we talking about?
A simple definition of ETAS and how it differs from "full load" efficiency
ETAS, or seasonal energy efficiency, measures a generator's average performance over a heating season. It factors in partial-load operation, stops, and certain auxiliaries. By contrast, "full load" efficiency is a measurement under lab conditions, at rated power. With ETAS, you get closer to a home's actual use.
Which generators ETAS applies to (boilers, heat pumps, water heaters, etc.)
ETAS applies to equipment covered by ecodesign rules and the energy label. This mainly covers boilers (gas, oil, biomass), heat pumps, hybrid systems, and certain "packaged" generators with controls. For domestic hot water, the indicators used are often close to efficiency, depending on the draw-off profile.
Why seasonal efficiency matters on a job site (real use, weather variations)
On site, the unit rarely runs flat out. Weather, flow temperature, controls, on/off cycling and, for a heat pump, defrost phases, change everything. ETAS helps compare solutions on a realistic basis and secure consistency between sizing, emitters, and settings.
The ETAS to SCOP conversion table
Regulation (EU) No 813/2013 defines seasonal space heating energy efficiency, and Delegated Regulation (EU) No 811/2013 uses it for the label. Moving from one scale to the other rests on the primary energy conversion coefficient, set at 2.5, plus a flat correction.
| ETAS shown | Corresponding SCOP |
|---|---|
| 125 % | 3.20 |
| 135 % | 3.45 |
| 145 % | 3.70 |
| 155 % | 3.95 |
| 165 % | 4.20 |
| 175 % | 4.45 |
The rule of thumb worth memorising: 10 points of ETAS are worth 0.25 of a SCOP. That is what lets you settle an argument on the spot when one supplier quotes ETAS and another quotes SCOP, without digging out the documentation. On a reversible unit the same conversation plays out in cooling mode around SEER and its classes.
The energy label class grid
An ETAS figure does not read on its own: Annex II of Delegated Regulation (EU) No 811/2013 turns it into a class, and the scale differs depending on whether the generator works at medium or low temperature. The same product can therefore show two classes depending on the declared regime.
| Class | ETAS, medium temperature application | ETAS, low temperature heat pump at 35 °C |
|---|---|---|
| A+++ | ≥ 150 % | ≥ 175 % |
| A++ | 125 to 150 % | 150 to 175 % |
| A+ | 98 to 125 % | 123 to 150 % |
| A | 90 to 98 % | 115 to 123 % |
| B | 82 to 90 % | 107 to 115 % |
| C | 75 to 82 % | 100 to 107 % |
| D | 36 to 75 % | 61 to 100 % |
| E | 34 to 36 % | 59 to 61 % |
| F | 30 to 34 % | 55 to 59 % |
| G | < 30 % | < 55 % |
Two readings follow straight onto the quotation. A condensing gas boiler tops out around 92 to 94 % ETAS, so class A, and no fuel-fired boiler will ever clear the 98 % that opens class A+. And Regulation (EU) No 813/2013 sets the market floor at 86 % ETAS for a fuel-fired boiler rated at 70 kW or less, which in practice rules non-condensing low-temperature boilers out of new installations.
The two traps that distort the comparison
The first is the water temperature. An ETAS only means something alongside the flow temperature it was established at, typically 35 °C or 55 °C, and the gap between the two is considerable on the same machine. An ETAS quoted without its regime is an unusable figure, and yet that is the form in which it most often travels through sales documentation.
The second is the nature of the figure itself. ETAS is a conventional efficiency computed for a reference season, not a measurement of what the machine will do at your client's house. It says nothing about sizing, about the emitters' actual temperature, or about installation quality, which are the three things that will decide the bill. Two machines with the same ETAS installed differently will consume nothing like the same.
What that changes in practice: ETAS is for comparing products and checking eligibility, not for promising performance. On a quotation, state the ETAS with its flow temperature, taking the value from a catalogue whose technical characteristics are verified with the manufacturers, and talk about savings from the sizing, never from the label alone.
How ETAS is calculated: what makes seasonal efficiency vary
Calculation conditions and operating profiles (shoulder seasons, half-load, stops)
ETAS corresponds to a seasonal efficiency calculated over a typical year. Tests are run at several operating points, including half-load, with shoulder-season phases and stops. Results are weighted by a climate profile (warm, average, cold) and a flow temperature (for example 35°C or 55°C), to get closer to real-world use.
Losses accounted for (standby, modulation, auxiliaries, cycling)
The calculation includes losses that don't show up in an instantaneous COP. It accounts for standby and off periods, auxiliary consumption (circulator, fan), modulation outside the optimal zone, and cycling losses when the unit repeatedly starts and stops. More stable control limits these penalties and improves ETAS.
Data to look for on the energy label and the manufacturer's datasheet
On the energy label, look for the seasonal efficiency (ηs or SCOP), the reference climate, the design load (Pdesignh), and the noise level. On the manufacturer's datasheet, check the COP at several points (A7, A2, A-7), standby consumption, the modulation range, and the control system information.
Choosing and sizing your generators correctly using ETAS
Sizing: avoiding oversizing and short cycling that penalise ETAS
ETAS reflects seasonal efficiency, and therefore real performance over a year. An oversized generator spends its time stopping and restarting. This short cycling drags down efficiency and increases wear. Base your sizing on a rigorous heat-loss estimate and the local design temperature. Target a unit that can modulate down low, close to mid-season needs.
Controls and emitters: the right settings that improve seasonal efficiency
Controls often gain you more than an extra 1 kW would. A well-set heating curve, a modulating thermostat, and emitters suited to low temperature pull ETAS upward. On condensing systems, aim for cold returns. On heat pumps, limit the flow temperature and take care with schedules and setbacks.
Installation and commissioning: points to watch (hydraulics, balancing, flow temperature)
On site, check the flow rates, sludge removal if needed, filtration, bleeding, then balancing. Calibrate the heating curve and the flow temperature ceiling. Fine-tuning at commissioning, with recorded readings, avoids losing ETAS in the field.
ETAS, grants and requirements: what may change in 2026 for your applications
Funding: where seasonal efficiency comes into the supporting documents and datasheets
In funding applications, seasonal efficiency mainly serves to link the installed product to a measurable performance. In 2026, expect more checks on the consistency between the invoice, the product datasheet, the exact reference, and the characteristics (efficiency, output, ratings, etc.). The measure specification remains your compass, whichever scheme is paying. One clear document beats three vague ones.
Certification and inspections: proving the stated performance without piling up documents
Inspections rarely look at paperwork alone. They look for proof on site. Photos, settings, commissioning, and material traceability make the difference. Aim for a file that's light but complete, with a single clear thread. That's the best defence in case of field inspections.
2026 checklist: documents to prepare (label, instructions, certificate, job-site photos)
To avoid back-and-forth, prepare a clean file from the technical visit onward.
- Energy label and manufacturer's instructions.
- Proof of certification, insurance, detailed invoice (brand, model, quantities).
- Completion certificate and, if requested, ETAS document or technical opinion.
- Dated photos before, during, after. Including the nameplate for equipment.
Concrete cases: improving seasonal efficiency on site
Condensing boiler: settings and heating returns to make the most of condensation
Simple goal. Keep the heating return low to condense often. Aim for a return temperature around 50 to 55°C or lower in mid-season. Adjust the heating curve, lower the flow setpoint, open the thermostatic valves, and balance the network to avoid radiators that are "hot at the top, cold at the bottom." Clean maintenance and good settings often gain more than replacing the equipment.
Heat pump: heating curve, buffer tank (if needed) and mid-season optimisation
On a heat pump, the heating curve is your steering wheel. The lower the water temperature, the higher the seasonal COP climbs. In mid-season, avoid on/off cycling by lowering the slope, widening the hysteresis, and favouring stable control. The buffer tank is only useful if you have short cycling or unstable flow rates. Otherwise, it can degrade performance. Keep continuous operation where possible.
Replacement vs whole-house renovation: when ETAS isn't enough to guarantee performance
ETAS gives you a lab snapshot. On the ground, a leaky envelope, oversized emitters, missing MVHR, or poor sizing can crash seasonal efficiency. Before a simple replacement, check losses, airtightness, insulation, and controls. A well-sequenced whole-house renovation, with an energy audit, secures the result and limits callbacks. That's where the real gain is decided.
