Chapter 7 of the government's RHI announcement says
As RHI payments will be made on the basis of heat meter readings, the Government will require participants to use meters that have met certain standards to provide assurance that payments are based on accurate measurements and to consumers that their devices are reliable. Tampering with meter components (to influence the meter reading) constitutes fraud and will be dealt with accordingly.
A wide variety of different types of heat meters are available commercially. The Government will allow those who wish to claim the RHI to choose to use any heat meter that meets the Class 2 requirements listed in Annex MI-004 of the EU Measuring Instruments Directive (MID) 2004. Requirements for measuring steam are covered separately.