Latent Heat - Secret to Eccotherm Success
There are numerous explanations that attempt to describe heat pump operation, often using crude comparisons with reversible fridges, air conditioners and even bicycle pumps, but all these fall well short of the mark in their understanding of the real process that is involved.
A much better explanation involves a property of matter called latent heat which is the energy that is absorbed by a substance when there is no change in its temperature. This occurs for instance when ice melts or water boils. It takes a lot more energy for such a change to occur than to increase it's temperature.. For example, 539 times more energy is needed to boil water than to raise it's temperature by just one degree and that's why a steam burn is so much more painful than one from boiling water. All that extra energy condenses(unboils) and goes into your skin.
A heat pump uses this powerful property of latent heat where a special substance called a working fluid(refrigerant) changes repeatedly back and forth from liquid to vapour, absorbing vast amounts of energy as it boils and unboils.
Refrigerants are chosen to have boiling points well below the temperature of their surroundings so they can absorb energy when they are vapourised. A typical boiling temperature can be as low as minus forty degrees centigrade, so even on a freezing day the refrigerant "thinks" things are relatively tropical. Some electrical energy is used to move the refrigerant around the heat pump circuit and most of this as well as the energy that the Eccotherm aborbs from the outside air appears as the heat in your water.