Comparing two suppliers' quotes, you'll often hear: "ours is 92 %, theirs is only 86 %". Without knowing which 92 %, the comparison is meaningless. The two numbers below, and how to verify the claim.
Instantaneous efficiency vs seasonal efficiency
Instantaneous efficiency (η) is the ratio between heat delivered to the space and gas-input energy at the current operating point. Without context, this only tells you how good the unit is at full load in a lab.
Seasonal efficiency (ηₛ) is the weighted average of efficiencies across a full season: 100 %, 50 %, 30 % load, plus standby. Defined by EU regulation 813/2013 (ErP Lot 1). This number predicts annual gas consumption.
The difference: a condensing unit heater has instantaneous η up to 105 % (condensation extracts latent heat from water vapour), but ηₛ 89 to 96 %, because efficiency drops at 30 % load. A gas radiant heater has instantaneous η 96 %, but ηₛ 84–87 %, because radiant equipment has no continuous modulation cycle.
Always require ηₛ (labelled "seasonal" or "ErP") in a quote, never instantaneous η.
ErP classes for space heaters
EU regulation 2018/1146 (linked to 813/2013) classifies gas industrial heaters from A++ down to G by seasonal efficiency:
| Class | ηₛ (radiant) | ηₛ (unit heaters) |
|---|---|---|
| A++ | ≥ 90 % | ≥ 100 % (condensing only) |
| A+ | 87–89 % | 95–99 % |
| A | 83–86 % | 87–94 % |
| B | 79–82 % | 82–86 % |
| C and below | < 79 % | < 82 % |
From 1 January 2026 all new industrial equipment sold in the EU must be class A or higher. Class B units are being phased out of manufacturer catalogues. Buying B or C means you risk having no spare parts in 5–7 years and being unable to install an identical replacement.
How to verify the claim
Every Solaronics data sheet states:
- Nominal heat output (Qn) in kW at 100 %
- Minimum heat output (Qmin) in kW with modulation
- Instantaneous efficiency at Qn (η at 100 %)
- Seasonal efficiency ηₛ in %
- ErP class (A++ / A+ / A / B)
- NOx class (1–6; regulation caps at class 5 until 2025, class 4 from 2026)
If a supplier omits ηₛ or ErP class in the unit document, demand the technical data sheet or Declaration of Conformity. Without that documentation the unit cannot be installed in projects funded by the Slovenian Eco Fund or EU energy-efficiency subsidies.
Practical impact on the gas bill
A 5 000 m³ warehouse heated by an ηₛ 96 % unit (Solaronics MV condensing) vs ηₛ 84 % (basic warm-air): difference is ~14 % of annual gas consumption. At 4 500 m³ of gas per year that is ~630 m³ × €0.40/m³ = €252 per year saved. Over a 12-year service life: €3 024, before any gas-price increase.
Related: LCC math for condensing units shows the break-even point by annual operating hours.