Sorry my mistake in explanation, they do need to, and thanks for questioning it
There are special clauses in Part G for both vented and unvented thermal stores, the difference is that for various reasons, the installers (though we do anyway ...) don't have to hold a G3 unvented certificate.
It's because the stores don't have a way of
automatically replenishing the water (fluid) in them - they are a sealed system, as such they are considered a part of the central heating system. (They do tend to have large expansion vessels attached
) and also inherently they don't have temperature relief valves for the same reason. (pressure release yes, temperature or combined temperature and pressure - no )
Note: In johnnymc's particular instance because he has an uncontrolled heating source - the wood stove, then some part of his system will need to be vented, the easiest way may be to connect it directly and use an unvented thermal store (he still gets 'mains pressure' hot water of course) and of course (already presumed this was 'understood') - he'd need to fit a tmv on the ho****er outlet - there are other ways though to still be compliant and of course in Ireland the regs may be different anyway.
With biomass, heat pumps, solar thermal, gas and oil boilers (condensing and not in some cases) et al in a multi fuel environment, there are only a very few people out there that fully understand how to design an
effective system with all the correct requirements on flow and return temperatures, then add in the metering requirements for the RHI and you've lost >99% of the boiler fitting companies out there.