One of the most important design details for successful application of air-to-water heat pumps is keeping the required load water temperature as low as possible. Low load temperatures allow the heat pump to operate at high COPs in heating mode.
Space heating distribution systems that can provide design heating output using supply water temperatures no higher than 120ºF allow the majority of currently available air-to-water heat pumps to deliver reasonably good performance. Systems that can operate at even lower supply water temperatures will further improve heat pump performance.
Distribution systems that supply each heat emitter using parallel piping branches rather than series configurations are also preferred because they provide the same supply water temperature to each heat emitter.
Examples of heat emitters and other techniques that allow air-to-water heat pumps to provide good performance include:
• Heated floors with low-resistance coverings
• Radiant wall and ceiling panels
• Generously sized panel radiator systems
• Fan-assisted panel radiators
• Fan coils (allowing for heating and cooling)
• High-output fin-tube baseboard
• Existing cast iron radiators
• Reducing building heating loads, or adding a heat emitter to existing systems
This article discusses these heat emitter and design options in more detail.