Summary

Efficiency labelling for heating products in the UK uses two main systems: SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) for gas and oil boilers, and ErP (Energy-related Products) ratings which provide an A-G scale that now covers both boilers and heat pumps. For heat pumps, the key performance metric is SCOP (Seasonal Coefficient of Performance) rather than rated efficiency.

Understanding these metrics is important for tradespeople because they affect legal compliance (Part L Building Regulations require minimum efficiency thresholds for replacement boilers and new heating systems), sales conversations with homeowners, and the accuracy of running cost comparisons. A homeowner asking whether they should replace their boiler with a heat pump deserves an honest, numerically grounded answer — not a sales pitch.

This article explains SEDBUK and ErP ratings, how to calculate heat pump SCOP, how to perform a genuine running cost comparison, and how to measure flue gas efficiency in the field.

Key Facts

  • SEDBUK 2005 — Seasonal Efficiency of Domestic Boilers in the UK; developed for SAP 2009 energy assessments; expressed as % (e.g. 91.2%)
  • SEDBUK 2009 — updated version used in SAP 2009 onwards; slightly different calculation methodology; same percentage scale
  • ErP Directive — EU Energy-related Products Directive (2009/125/EC); A-G rating scale for boilers and heat pumps; A-rated = ≥90% seasonal efficiency for gas boilers
  • A+ and A++ ratings — heat pumps typically rate A+ or A++ (117–175% nominal efficiency, equivalent to SCOP 1.17–1.75 at nominal conditions, but this is different from real-world SCOP)
  • COP — Coefficient of Performance; instantaneous ratio of heat output to electrical input; measured at a specific test condition (e.g. A7/W35 = 7°C outdoor, 35°C flow)
  • SCOP — Seasonal Coefficient of Performance; average COP over a heating season, accounting for variable outdoor temperatures; the practical efficiency metric
  • SCOP 3.0–4.5 — typical real-world range for domestic ASHP in the UK; well-installed systems with low flow temperature emitters achieve 3.5–4.5
  • Gas tariff — approximately 7.5p/kWh (from April 2025 Ofgem price cap); varies by supplier and tariff
  • Electricity tariff — approximately 25p/kWh (standard rate, April 2025 Ofgem price cap); economy 7 off-peak ~10p/kWh
  • Breakeven SCOP — electricity cost ÷ gas cost = 25p ÷ 7.5p = 3.33; a heat pump needs SCOP > 3.33 to cost less per kWh of heat than gas
  • Wobbe Index — measure of the interchangeability of fuel gases; used to calculate gas rate from meter readings and calorific value
  • Flue gas analyser — combustion analyser measuring CO2%, CO ppm, flue gas temperature, and draft; used to verify boiler combustion efficiency

Quick Reference Table

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Heating System Efficiency Metric Typical Value ErP Rating Running Cost Comparison
Condensing gas boiler (A-rated) SEDBUK/ErP seasonal 89–94% A 100% (baseline)
Non-condensing gas boiler SEDBUK 72–80% E–G 15–25% more than condensing
Oil boiler (condensing) SEDBUK 87–92% A Similar to gas, oil ~9p/kWh
ASHP (good installation) SCOP 3.5–4.0 A+ 75–90% of gas cost
ASHP (average installation) SCOP 2.8–3.3 A 100–120% of gas cost
ASHP with economy 7 tariff SCOP (adjusted) 3.5 at 10p/kWh A+ 40–50% of gas cost
GSHP SCOP 4.0–5.0 A++ 60–75% of gas cost
Electric panel heater — 100% (nominal) — 333% of gas cost

Detailed Guidance

SEDBUK Ratings: Calculation Method

SEDBUK (Seasonal Efficiency of Domestic Boilers in the UK) was developed by the Department of Energy in the late 1990s and adopted for use in the Standard Assessment Procedure (SAP) for building energy assessment.

The SEDBUK calculation accounts for:

  • Full-load efficiency at the rated output (measured to BS EN 13203 or BS EN 15502)
  • Part-load efficiency at 30% of rated output (representing typical steady-state running)
  • Cycling losses (the efficiency penalty for frequent stop/start)
  • Pilot light losses (for boilers with permanent pilot lights)
  • Standing losses (heat loss through the casing and flue when not firing)

The result is expressed as a seasonal percentage: a SEDBUK of 91.2% means that for every 100kWh of gas consumed, 91.2kWh of useful heat is delivered to the heating system.

SEDBUK and Part L compliance: Building Regulations Part L1A and L1B set minimum SEDBUK thresholds for replacement boilers. The current minimum for new natural gas condensing boilers is SEDBUK 86% (or ErP rating A). Boilers below this threshold cannot be installed as a replacement in an existing dwelling without a specific exemption.

Finding SEDBUK ratings: The Boiler Efficiency Database (SAP Appendix D) lists all approved boilers with their SEDBUK ratings. Search at https://www.boilers.org.uk.

ErP Energy Labels

The EU Energy-related Products (ErP) Directive introduced standardised energy efficiency labels for heating products, phased in from 2015. The label uses an A-G scale:

ErP Grade Seasonal Efficiency Example Products
A++ ≥150% Ground source heat pumps; some ASHPs
A+ ≥131% Best ASHPs
A ≥90% (boilers) / ≥100% (heat pumps) A-rated condensing boilers; standard ASHPs
B ≥86% Older condensing boilers; poorer heat pumps
C ≥82% Some older condensing boilers
D–G <82% Non-condensing boilers (being phased out)

Note: The >100% efficiency figures for heat pumps appear anomalous but are correct — they reflect the heat pump's ability to deliver more energy as heat than it consumes as electricity, by extracting free energy from the ambient air or ground.

Post-Brexit: The UK retained the ErP scheme after Brexit. Labels remain valid and manufacturers continue to use the same scale in the UK market.

Heat Pump COP and SCOP

COP (Coefficient of Performance) is measured at a single operating point, following EN 14511:

  • Notation: A7/W35 means outdoor air temperature 7°C, flow water temperature 35°C
  • A7/W35 is the standard test point for ASHP; a COP of 3.8 at A7/W35 is typical for a quality modern unit
  • COP drops as outdoor temperature falls; at -7°C, a typical ASHP might achieve COP 2.2
  • COP also drops as flow temperature rises; A7/W55 might yield COP 2.5 on the same unit

SCOP integrates the varying COP across an entire heating season, weighted by the probability distribution of outdoor temperatures in the location:

  • UK climate: EN 14825 defines reference climate conditions for Strasbourg (representing central Europe) and Punta Arenas (cold climate). For UK calculations, the EN 14825 reference climate is used.
  • A UK MCS installation achieving SCOP 3.5 is performing well; a poorly designed system with radiators at 70°C flow temperature might achieve only SCOP 2.0

Calculating SCOP from first principles:

SCOP = Total heat output over heating season (kWh) ÷ Total electrical input (kWh)

This can be measured by the heat pump's built-in heat meter (or an add-on energy monitoring system) over a full heating season.

Factors affecting SCOP:

  1. Flow temperature — the single biggest factor; reducing from 55°C to 40°C can increase SCOP from 2.8 to 3.8 on the same unit
  2. Emitter type — UFH (35–45°C) better than radiators (55–70°C); oversized radiators can run at lower flow temperatures
  3. Outdoor climate — a Scottish highland location will have lower SCOP than a south English location
  4. System design — buffer tanks, correct pipe sizing, and low pressure drop all help

Running Cost Comparison

A fair running cost comparison between gas boiler and heat pump requires consistent assumptions:

Step 1: Calculate annual heat demand

Annual heat demand = Floor area × U-value losses × Degree days

Or use the SAP score method. A typical semi-detached house has an annual heat demand of 12,000–18,000 kWh.

Step 2: Calculate fuel cost

  • Gas boiler: Heat demand ÷ SEDBUK × gas price per kWh
  • Heat pump: Heat demand ÷ SCOP × electricity price per kWh

Example (15,000 kWh heat demand):

System Efficiency kWh fuel needed Cost at tariff
Gas boiler (91% SEDBUK) 91% 16,484 kWh gas 16,484 × 7.5p = £1,236
ASHP (SCOP 3.5, standard tariff) SCOP 3.5 4,286 kWh electricity 4,286 × 25p = £1,071
ASHP (SCOP 3.5, Economy 7 off-peak) SCOP 3.5 4,286 kWh electricity 4,286 × 10p = £429

The time-of-use tariff result illustrates why heat pump owners who shift electricity consumption to off-peak hours can achieve dramatically lower running costs.

Flue Gas Efficiency Measurement

Combustion analysers (flue gas analysers) measure the efficiency of a gas or oil boiler by sampling the flue gases:

Key measurements:

  • CO2% — percentage of carbon dioxide in flue gas; higher CO2% for gas means more complete combustion; ideal range 8–10% for natural gas
  • CO ppm — carbon monoxide concentration; should be below 200ppm CO/CO2 ratio (CO to CO2 ratio below 0.004)
  • Flue gas temperature — higher flue gas temperature = more heat wasted up the flue; modern condensing boilers: 40–70°C; older non-condensing: 150–250°C
  • Stack loss — the efficiency penalty from hot flue gas; calculated as flue temperature minus room temperature divided by a combustion factor

Interpreting results:

  • Combustion efficiency = 100% − stack loss
  • A condensing boiler with 55°C flue temperature and correct CO2 achieves approximately 91–93% combustion efficiency
  • High flue temperature + high CO2 = combustion air too low (rich mixture)
  • Low CO2 + normal flue temperature = too much excess air (lean mixture, inefficient)

Regulatory requirement: Gas Safe engineers must carry out a flue gas analysis on commissioning and during servicing to confirm safe combustion. Results are recorded on the commissioning certificate.

Wobbe Index and Gas Rate Calculation

The Wobbe Index is used to calculate the heat energy rate from a gas meter reading:

Gas meter calculation:

Power (kW) = Gas flow rate (m³/h) × Calorific value (kWh/m³) × Volume correction factor
  • UK natural gas calorific value: approximately 10.35–10.45 kWh/m³ (varies by region and season)
  • Volume correction factor: 1.02264 at standard UK conditions
  • Wobbe Index for UK natural gas: approximately 51.41 MJ/m³

Using the Wobbe Index: When verifying whether a gas appliance is operating at its rated input, divide the gas rate (m³/h) measured at the meter by the appliance rated input (kW) × 3.6 ÷ calorific value. If the ratio is significantly different from 1.0, the gas valve or injector may need adjustment.

Frequently Asked Questions

What SCOP do I need for a heat pump to save money vs gas?

At April 2025 Ofgem price cap rates (gas 7.5p/kWh, electricity 25p/kWh), the breakeven SCOP is 25 ÷ 7.5 = 3.33. Any heat pump achieving SCOP above 3.33 costs less to run than a gas boiler. A well-designed ASHP system with UFH or oversized radiators typically achieves SCOP 3.5–4.0, making it cheaper to run than gas. A poorly designed system with radiators at 65°C flow temperature might achieve only SCOP 2.5–3.0, making it more expensive.

Does the ErP rating tell me how much it costs to run?

The ErP rating provides a comparative efficiency indicator but does not directly tell you running cost. You need to know the annual heat demand of the building, the SCOP of the heat pump (or SEDBUK of the boiler), and the fuel tariff. The ErP label is useful for comparing products of the same type but should not be compared across fuel types without converting to cost.

How do I check my gas boiler is at its correct efficiency?

Use a calibrated flue gas analyser (Kane, Testo, Anton are common brands). Insert the probe into the flue immediately after the heat exchanger (before any silencer or condenser). Take a reading after 5 minutes at steady state. Check CO2% against the manufacturer's specification (typically 8–9.5% for modern condensing boilers on natural gas). Check flue gas temperature and confirm it matches commissioning data. Record the combustion efficiency reading on the service record.

What is the BUS grant and how does it affect payback?

The Boiler Upgrade Scheme (BUS) provides £7,500 towards the installation of an ASHP (and £7,500 for ground source heat pumps). This grant dramatically reduces the payback period. A typical ASHP installation costing £12,000 becomes £4,500 after the grant — and if running costs save £200/year versus gas, the payback period is around 22 years without the grant but just 6 years with it. Running cost savings are more modest at standard electricity tariffs; they are significantly better with time-of-use tariffs.

Why does my ASHP show COP 4.0 on the datasheet but only 2.8 in practice?

The datasheet COP is measured at a single test point (A7/W35) — 7°C outdoor air, 35°C flow temperature. If your system is running at 55°C flow temperature (e.g. to supply radiators sized for a gas boiler), the real-world COP drops to around 2.5–3.0. Additionally, the datasheet COP doesn't include defrost cycles, controls losses, or system heat losses. Real-world SCOPs are consistently below datasheet COP figures, typically by 0.5–1.0.

Regulations & Standards

  • Building Regulations Part L1B — conservation of fuel and power in existing dwellings; minimum efficiency thresholds for replacement boilers

  • EU ErP Directive 2009/125/EC (retained in UK law) — Energy-related Products Directive; ErP label requirements for boilers and heat pumps

  • BS EN 14511:2018 — air conditioners, liquid chilling packages and heat pumps; COP test methodology

  • BS EN 14825:2018 — air conditioners, liquid chilling packages and heat pumps; SCOP calculation methodology

  • SAP 2012 Appendix D — Standard Assessment Procedure boiler database and SEDBUK calculation method

  • Boiler Efficiency Database — UK boiler SEDBUK rating lookup

  • Ofgem: Domestic Gas and Electricity Tariff Checker — Ofgem energy price cap data

  • Heat Pump Association: SCOP and COP Explainer — Heat Pump Association

  • CIBSE AM15: Heat Pump Systems — Chartered Institution of Building Services Engineers

  • MCS: Heat Pump Performance Data — Microgeneration Certification Scheme

  • wet ufh controls — UFH as an efficient emitter for heat pumps

  • thermal stores — thermal store for combined heat storage and DHW

  • heat pump wiring — electrical installation for heat pumps