GREEN CLINIC
Sustainability expert David Hilton explains what an air-to-water heat pump is and how you might benefit from installing one in your home
There are a number of variations of the names used for air source heat pumps. Each is specific and delivers a different result. A generic air source heat pump simply extracts heat from the air around the home, enhances it, and moves it to either air or water in the home. An air-to-air heat pump is more commonly referred to as an air conditioning unit that can deliver heat as well as cooling, but an air-to-water heat pump is a device that specifically extracts heat from the air around your home and delivers it to the water in the central heating system or hot water cylinder in the home.
DIFFERENT TYPES
There are also anumber of different versions of airto-water heat pumps. They all move heat from air and deliver it to water, but the most common ones will have afan unit that sits outside the home that extracts the heat from the outside air. The fan unit is usually up to around ametre tall and around one and ahalf metres long and you need free air space around it so that the emitted cold air is not drawn back into the unit. The air can be up to around 12 degrees cooler when it leaves the fan unit. Heat pumps rely on refrigerants but not all heat pumps have the same refrigerant in them. Some refrigerants have the ability to produce higher temperatures of around 65˚C and if two systems are mixed, even higher temperatures are achievable. Now before you rush off and order a ‘high temperature’ heat pump to install in an inefficient old home, be aware that the higher temperatures use more energy to achieve. High temperatures in heat pumps are less efficient than low temperatures.
HOW MUCH TO RUN?
Heat pumps do not actually create heat, they simply move heat from one place to another. The air-to-water heat pump moves heat from air to water. The colder the air is or the hotter the water needs to be, then the less efficient the heat pump is. Air-to-water heat pumps are sized and tested on abenchmark set of conditions —the most common set of conditions is an air temperature of 7˚C and water produced temperature of 35˚C (A7W35). If the air temperature drops below 7˚C or the water needs to be hotter than 35˚C then the efficiency will drop.
The efficiency, known as aCoefficient of Performance (COP), is quoted as aratio of the energy needed to run the heat pump and the heat moved by the heat pump. For example, if you use 1kW of electricity to move 4kW of heat, then the efficiency is 4or 400%. Given that energ
