As the words entail; the heating system is used to provide heat to the building during cold weather. Combination or combi boilers (for short, combis) combine the central heating with domestic hot water (DHW) in one device. In domestic installations, potable water heated for uses other than space heating is also called domestic hot water (DHW). The Vitocell range from Viessmann offers the ideal domestic hot water (DHW) system for every demand & heat source. Domestic Hot Water (DHW) · 1.