A relatively new type of heat pump for residential systems is the absorption heat pump (AHP), also called a gas-fired heat pump. Absorption heat pumps use heat or thermal energy as their energy source, and can be driven with a wide variety of heat sources such as combustion of natural gas, steam solar-heated water, air or geothermal-heated water, and therefore are different from compression heat pumps that are driven by mechanical energy. AHPs are more complex and require larger units compared to compression heat pumps. The lower electricity demand of such heat pumps is related to the liquid pumping only.