invention uses water as the working fluid of the heat pump the volume of one kilogram vapor under the pressure of 0 9 bar is 1 869 m 3 and the volume of one kilogram vapor under the pressure of 1 2 bar is 1 428 m 3 therefore in the above example the work done by the compressor to recycle latent heat of vaporation 2250 kj kg of one kilogram vapor during operation of the present invention is that is the energy consumption by the compressor is less than 1 of the recycled heat 2250 kj which is extremely high efficient compared with the prior art of a multi source heat pump using a high temperature high pressure refrigerant the present invention has the following advantages 1 the compressor of the present invention operates under a low pressure and compared with the prior art using a high temperature high pressure refrigerant the present invention has obviously low requirements on the compressor such as power fluid tightness and maintenance and is relatively low cost energy saving and durable 2 the present invention uses water as the working fluid which is much cheaper than the high temperature high pressure refrigerant and is more environmental friendly and 3 the heat pump of the present invention uses a single heat source with highly centralized heat energy so that the volume of the heat pump is smaller than the multi source heat pump in the prior art fig3 to fig5 show a structure of a third embodiment of a liquid heating apparatus according to the present invention a steaming apparatus in a chinese restaurant to steam food such as dim sum is illustrated as an example for explanation the steaming apparatus may also be used to steam other objects as shown in.