heatonly

Heat-only (sometimes written heat only or heat-only) refers to heating systems and appliances designed solely to produce space heat, and in many contexts to boilers that provide central heating with hot water stored separately rather than on demand. The term is used in building services and HVAC to distinguish these systems from combination (combi) boilers, which supply both space heating and instantaneous domestic hot water, and from heat pumps that may provide both heating and cooling.

A typical heat-only boiler is part of a conventional or regular system that includes an expansion vessel, circulation pump, radiators, and a separate hot water storage cylinder and often a cold water feed tank for open-vented arrangements. These systems are common in older or larger properties where stored hot water and multiple outlets are required. Heat-only appliances may be fueled by natural gas, oil, biomass, or electricity.

Advantages of heat-only systems include straightforward compatibility with hot water cylinders, simpler integration with solar thermal or other heat storage, and predictable performance in multi-bathroom homes. Disadvantages include larger spatial requirements for storage tanks and cylinders, potentially slower hot water response, and more complex pipework compared with combi units.

Installation, maintenance, and safety of heat-only systems are governed by local building codes and industry standards; routine servicing and correct component sizing are important for efficiency and longevity. The term can also be applied generically to devices that produce heat without cooling or electrical generation functions.