A really big one, or a really badly insulated one. They're being bought by installers who do finger-in-the-wind heat loss guesstimates.Thank you - can I ask what type of property would require a 30KW open vent boiler in that case? Manufacturesr clearly make them so who is buying them
Heat Geek have produced this handy guide to give a rough idea if what output boiler might be required. It's not a substitute for a full heat loss calculation but at least gets you in the right ballpark