Understanding RHI Heat Metering; HLA, HLC, and IRMA Explained
If you’ve ever had to replace a heat meter under the Renewable Heat Incentive (RHI), you’ll likely have come across a few acronyms along the way. Terms like HLA, HLC, and IRMA can appear in Ofgem guidance and RHI documentation, but what do they actually mean and when do they apply?
Below, we explain each one, helping you understand how they relate to your biomass installation and ongoing RHI compliance.
Heat Loss Assessment (HLA)
A Heat Loss Assessment (HLA) is used for systems accredited under the RHI after 29th September 2013.
It’s designed to calculate how much heat is lost between the biomass boiler and the eligible end use, such as buildings or drying systems etc. The lower the losses, the higher the proportion of heat that counts towards RHI payments.
With an HLA, losses under 3% are normally ignored, provided all external pipework is properly insulated. The underground heat main is automatically considered well insulated, but above-ground external pipework must meet specific insulation.
To be considered properly insulated under RHI guidance, insulation should generally:
- Have a thermal conductivity (λ-value) of 0.025 W/m·K or better at 40°C
- Be weather-resistant and moisture-proof for outdoor use
- Be installed continuously, with all joints and fittings sealed
- Comply with BS 5422 (the British Standard for thermal insulation of pipework and equipment in the temperature range −40°C to +700°C).
Ofgem typically references BS 5422 in its technical guidance as the measure for “adequate insulation.”
If the heat losses exceed 3%, or there are areas of pipework that don’t meet insulation requirements, those losses are factored in and reduce the eligible heat total.
Heat Loss Calculation (HLC)
A Heat Loss Calculation (HLC) applies to systems accredited before 29th September 2013. It performs a similar function to an HLA but follows the earlier version of Ofgem’s methodology.
The key difference is that with an HLC, the calculated losses are always applied, even if they are under 3%. That means older systems tend to have a slightly lower eligible heat figure compared to those accredited later under HLA rules.
As with HLAs, the purpose is to ensure that only genuine renewable heat use is rewarded under the RHI scheme.
Independent Report on Metering Arrangements (IRMA)
An Independent Report on Metering Arrangements (IRMA) is a formal technical report prepared by a qualified independent engineer. It describes how heat is measured on an RHI-accredited system and confirms that the setup meets Ofgem’s metering requirements.
An IRMA is required:
- When an HLC is carried out (for all pre-2013 installations)
- If calculated losses from an HLA are above 3%
- Where there is any ineligible heat use, for example heating a swimming pool or other non-qualifying use
The IRMA ensures that the system is metered correctly and that any deductions for ineligible use or heat losses are accurately applied.
For most modern systems with well-insulated pipework and no ineligible heat use, an IRMA isn’t normally needed. However, Ofgem can request one at their discretion, so it’s sensible to be aware of the requirement.
Why these reports matter
Understanding whether your system is assessed under an HLA or HLC, and whether an IRMA is required, helps ensure your metering setup stays compliant. When replacing a heat meter, these reports may need updating, especially if any part of the system has changed since the original installation.
Having accurate, up-to-date documentation prevents delays in RHI amendments and avoids the risk of payment suspensions during Ofgem checks.
How Treco can help
Treco’s team has extensive experience supporting customers with RHI heat meter replacements, HLAs, HLCs, and IRMAs. We can help review your setup, identify what type of assessment applies, and manage the submission of any required updates to Ofgem.
Get in touch with the team for advice.