The work took ten minutes to complete professional, offered alternatives if unable to access from my property polite.”. Dhs are also a which? trusted trader so you can rest assured that your boiler installation will be of the highest standard. The first, and most common type, is the gas combination boiler (aka the 'combi') unlike boilers of old, a combi runs both your central heating and your hot water from one unit. Reduce your energy bills by £350*. When you.