We are fully insured heating and cooling professionals! If you need to find a reputable, reliable engineer for your boiler emergency in. As the owner, i answer the phone personally - monday thru friday… no muss no fuss!!! The four sizes are governed by the heating output and hot water flow rates – very roughly - to show. We had a combi okay, hot water whenever you require it, but if somebody turns a tap on when you're in the shower, heavens.