Our boiler has no heat meter. How do we work out the steam heat for UK CBAM?

The technical guide from HM Revenue and Customs (HMRC) gives three methods in falling order of preference: measure heat flows, multiply fuel energy by the boiler's measured efficiency, or use a reference efficiency of 70%. Boiler maker documentation is allowed but less preferred than measured efficiency.

The three methods

1. Method 1, measurement: calculate heat flow from mass flow and the enthalpy difference between the steam sent and the condensate returned. Where condensate is not returned or its flow is unknown, a reference temperature of 90 degrees Celsius can be used.
2. Method 2, proxy from measured efficiency: net heat Q = boiler efficiency x fuel energy input. Energy input is the sum of each fuel's quantity x its net calorific value (NCV), in terajoules (TJ). Measure the efficiency over a reasonably long period so different load states are included.
3. Method 3, proxy from reference efficiency: the same as Method 2 but the efficiency is set at 0.7, which the guide calls a conservative assumption. Method 3 is for when the boiler efficiency is not known. For heat bought from a neighbour the guide uses a different standard value; see the steam answer.

If you only have the boiler maker's number

The guide allows efficiency from the boiler manufacturer's documentation but calls it the less preferred approach. It then asks you to allow for part-load running with an annual load factor: energy input over the period divided by the maximum fuel input if the unit had run at 100% nominal load for the full calendar year.

For a steam boiler, the efficiency should be based on all condensate being returned, with 90 degrees Celsius assumed for the returned condensate if no actual values exist.

Why the choice matters

Fuel emissions are the same whichever method you pick. The method sets how many TJ of heat that fuel is said to produce. Our reading: a lower efficiency means less heat from the same fuel, so each TJ of heat carries more carbon dioxide (CO2), which fits HMRC calling 0.7 conservative.

Example: one boiler, a year, simplified

• Coal burned: 3,000 tonnes x 0.0196 TJ per tonne = 58.8 TJ fuel energy input
• Method 3: 0.7 x 58.8 = 41.16 TJ of heat. Method 2 at a measured 0.78: 0.78 x 58.8 = 45.864 TJ
• Fuel CO2 of 5,562 tCO2 (58.8 TJ x 94.6 tCO2/TJ): Method 3 gives 5,562 / 41.16 = 135.1 tCO2 per TJ of heat; Method 2 gives 5,562 / 45.864 = 121.3
• A process that takes 10 TJ of this heat carries 1,351 tCO2 under Method 3 and 1,213 under Method 2

Coal tonnes, NCV, efficiency and the 94.6 tCO2/TJ factor are example values. This follows the guide's equation 35: fuel emission factor x heat consumed / efficiency.

What Senseible does here

Senseible reads fuel invoices and electricity bills into monthly records. That gives the monthly fuel energy input a heat proxy starts from. The standard product does not measure boiler efficiency or steam (metering is a custom installation on request). A verifier accredited for UK CBAM must still verify the figure.

• Steam bought from a neighbouring plant
• Standard fuel emission factors
• Cogeneration plant split
• UK CBAM for Indian exporters

Sources: HMRC, Carbon Border Adjustment Mechanism (CBAM): technical guide for monitoring, reporting and verification

Last reviewed 2026-11-06. Read from technical guide sections D4.8.2 to D4.8.2.4 on 6 November 2026. HMRC may change the guide before 1 January 2027.