There are two main types of heat pump: compression heat pumps and absorption heat pumps. Compression heat pumps rely on mechanical (electrical energy); whereas absorption heat pumps also run on heat energy (from electricity or burnable fuels). Thus compression heat pumps run on electricity whereas absorption heat pumps can run on either electricity or natural gas. Deciding which type of heat pump is best for your home depends on the cost of electricity vs. the cost of natural gas; and, also the climate – absorption heat pumps work better in cold climates due to a lower minimum operating temperature.
In effect a heat pump is like a 2 way air-conditioner. It can take heat from outside and heat up a house or it can take heat from inside a house and disperse it outside. Heat is moved from a ‘source’ at a lower temperature to a ‘sink’ at a higher temperature. On a heat pump there are 2 coils depending on whether heat is being pumped into or out of a house one coil is an evaporator and one coil is a compressor. The direction of heat is changed using a reversing valve. This is how a heat pump is different to an A/C unit – air-conditioning doesn’t have a reversing valve.
Typically, heat pumps draw heat from the air or the ground. If a heat pump is used to cool a room it is basically the same as an A/C unit. A fluid or refrigerant is used to absorb the heat in a room as it vaporizes and release it outside when it condenses. An evaporator is inside to absorb the heat and a condenser is outside. It is the reversing valve that allows for the refrigerant to change direction and for the evaporator and condensers to change roles.
The performance of heat pumps is measured in terms of the coefficient of performance or COP. This is the amount of heat moved per unit of input work required; or, in other words the amount of electricity or gas needed to heat or cool. Due to the laws of thermodynamics the coefficient of performance decreases with increasing temperature difference between inside and outside. Thus, one way to improve the coefficient of performance is to improve insulation in a home that way the energy used to maintain a cooled space is decreased. It is normal for heat pumps to have a back-up system when the difference in temperature between inside and outside is so great that the coefficient of performance drops low. This back-up is normally a standard oil, gas or electric heating/cooling system.
The most eco-friendly version of the heat pump is the geothermal heat pump. This type of heat pump uses water as a medium rather than a refrigerant. The ground is used as both a heat source and a sink. Geothermal heat pumps are much cheaper to operate than standard heat pumps. They can be made more efficient if summer heat is stored in the ground for use in the winter and if winter coldness is stored in the ground for cooling in the summer.
Efficiency compared to other types of heating and cooling
As mentioned the COP is dependent on the temperature difference between inside and outside a house. The greater the difference the lower the COP. On a mild day at say 10 degrees Celsius a typical air source heat pump has a COP of 3 or 4. A typical space heater has a COP of just 1. Thus during a mild winter a heat pump is 3 or 4 times more economical than conventional forms of heating. At an outside temperature of about -18 degrees Celsius the COP of a heat pump reaches 1.
The advantage of a geothermal heat pump is that it takes its heat from the ground. The ground temperature 1.5 meters below the ground remains warmer than on the surface in the winter. The average COP of a geothermal heat pump is between 4 and 5.
Another important variable in determining the COP of a heat pump is the size of the heat exchanger: the larger the heat exchanger the more efficient the heat pump is and the higher the COP. This is worth bearing in mind when buying a heat pump because heat exchangers are expensive and the cheaper heat pumps generally have small heat exchangers.
Units of measurement
To describe the performance of a heat pump in cooling mode a measurement of energy efficient ratio (EER) or seasonal energy efficient ratio (SEER) is given in BTU/(h•W). The higher the EER the better the heat pump performs as a cooler.
If you live in a mild climate where the temperature differential between inside and outside does not reach extremes, the best option is an air source heat pump. They maintain a COP that makes them more economical than electric resistance heaters. Air source heat pumps are easy to install and will save you money.
More expensive to install are ground source or geothermal heat pump because installation involves digging trenches. However, for homes located in more extreme climatic conditions the expense is worth it. Ground temperature is more constant and geothermal heat pumps thus maintain a better COP all year round.
The key point about digital thermostats that are used to regulate heat pumps is that they only switch to auxiliary or back up heat when it is absolutely necessary. That is when the temperature difference is so great that the COP drops to 1. The only way to effectively know when this point has been reached is to have a thermostat that can read the temperature both inside and outside a home. The Honeywell Prestige HD has an Outdoor Sensor that allows the thermostat to determine accurately when auxiliary heat is necessary. The Honeywell Prestige HD is a state of the art device that can manage 2 stages of cooling and 3 stages of heating.
The Lux Products HP2110 Heat Pump Thermostat doesn’t have an outdoor sensor but reviews of the device are generally indicative that the thermostat does a good job with heat pumps. The Lux Products HP2110 can manage heat pumps with 2 stages of heating and 1 of cooling.