One of the newer products to enter the home improvement market is the hot water heat pump (Figure 1), they are used in conjunction with hot water tanks to save energy.
Figure 1 - Hot water heat pump
In most homes the hot water tanks consumes more energy than any other appliance. A hot water heat pump preheats the water before it enters the hot water tank. The intent being that it saves on the energy needed to heat the water.
As with many devices, the hot water heat pump has some advantages and disadvantages. It works best in Southern climates where air conditioning, rather than heating is the primary expense.
The reason for this is that the hot water heat pump uses the ambient air inside a home which is generally hotter than the water entering the home. It takes the ambient air and using a heat exchanger, it takes the heat from the air to warm the water. Once the heat from the air is transferred to the incoming water, it expels the air, which is now cooler than the ambient air in the room. In other words it is an air conditioner, as shown in Figure 2.
Figure 2 - Hot water heat pump diagram
In the summer or in climates that are hot all year around, this is highly advantageous, as not only do you save on the costs of heating water, but you save on the costs of air conditioning your home.
However, in climates where heating the home is the primary energy expense, the hot air which was used to preheat the water tank must be replaced using whatever method is used to heat the home. So while there is savings in energy used to make hot water, there is more expense in heating the home.
Depending on the fuels used for heating the home and for hot water, there still may be a saving in energy cost, however, whether that saving can be rationally justified over the cost of the hot water heat pump is debatable.
Hot water heat pumps are located inside the home, beside the hot water tank. In northern climates, because it is energy advantageous to have the hot water tank in a controlled, heated environment, the hot water tank is usually located in the basement of the home. It is also advantageous to keep all water piping in an environment that does not experience freezing temperatures.
In southern climates the hot water tank is often found in the garage of the home, as it is advantageous for the water tank to be in a, more often than not, hot environment. When a hot water tank is located in a non-air conditioned environment, in a southern climate, the highest possible energy saving will be realized with a hot water heat pump.
Additional information on Heat pumps.