Heating / Cooling using a Heat Pump
A heat pump is an electrical device that extracts heat from one place and transfers it to another. The heat pump is not a new technology; it has been used around the world for decades. Refrigerators and air conditioners are both common examples of this technology.
Heat pumps transfer heat by circulating a substance called a refrigerant through a cycle of evaporation and condensation. A compressor pumps the refrigerant between two heat exchanger coils. In one coil, the refrigerant is evaporated at low pressure and absorbs heat from its surroundings. The refrigerant is then compressed en route to the other coil, where it condenses at high pressure. At this point, it releases the heat it absorbed earlier in the cycle.
Refrigerators and air conditioners are both examples of heat pumps operating only in the cooling mode. A refrigerator is essentially an insulated box with a heat pump system connected to it. The evaporator coil is located inside the box, usually in the freezer compartment. Heat is absorbed from this location and transferred outside, usually behind or underneath the unit where the condenser coil is located. Similarly, an air conditioner transfers heat from inside a house to the outdoors.
The heat pump cycle is fully reversible, and heat pumps can provide year-round climate control for your home – heating in winter and cooling and dehumidifying in summer. Since the ground and air outside always contain some heat, a heat pump can supply heat to a house even on cold winter days. In fact, air at –18°C contains about 85 percent of the heat it contained at 21°C.
An air-source heat pump absorbs heat from the outdoor air in winter and rejects heat into outdoor air in summer. It is the most common type of heat pump found in Canadian homes at this time. However, ground-source (also called earth-energy, geothermal, geoexchange) heat pumps, which draw heat from the ground or ground water, are becoming more widely used, particularly in British Columbia, the Prairies and Central Canada.
Here are some common terms you’ll come across while investigating heat pumps.
Heat Pump Elements
The refrigerant is the liquid/gaseous substance that circulates through the heat pump, alternately absorbing, transporting and releasing heat.
The reversing valve controls the direction of flow of the refrigerant in the heat pump and changes the heat pump from heating to cooling mode or vice versa.
A coil is a loop, or loops, of tubing where heat transfer takes place. The tubing may have fins to increase the surface area available for heat exchange.
The evaporator is a coil in which the refrigerant absorbs heat from its surroundings and boils to become a low-temperature vapour. As the refrigerant passes from the reversing valve to the compressor, the accumulator collects any excess liquid that didn’t vaporize into a gas. Not all heat pumps, however, have an accumulator.
The compressor squeezes the molecules of the refrigerant gas together, increasing the temperature of the refrigerant.
The condenser is a coil in which the refrigerant gives off heat to its surroundings and becomes a liquid.
The expansion device lowers the pressure created by the compressor. This causes the temperature to drop, and the refrigerant becomes a low-temperature vapour/liquid mixture.
The plenum is an air compartment that forms part of the system for distributing heated or cooled air through the house. It is generally a large compartment immediately above or around the heat exchanger.
A Btu/h, or British thermal unit per hour, is a unit used to measure the heat output of a heating system. One Btu is the amount of heat energy given off by a typical birthday candle. If this heat energy were released over the course of one hour, it would be the equivalent of one Btu/h.
Heating degree-days are a measure of the severity of the weather. One degree-day is counted for every degree that the average daily temperature is below the base temperature of 18°C. For example, if the average temperature on a particular day was 12°C, six degree-days would be credited to that day. The annual total is calculated by simply adding the daily totals.
A kW, or kilowatt, is equal to 1000 watts. This is the amount of power required by ten 100-watt light bulbs.
A ton is a measure of heat pump capacity. It is equivalent to 3.5 kW or 12 000 Btu/h.
The coefficient of performance (COP) is a measure of a heat pump’s efficiency. It is determined by dividing the energy output of the heat pump by the electrical energy needed to run the heat pump, at a specific temperature. The higher the COP, the more efficient the heat pump. This number is comparable to the steady-state efficiency of oil- and gas-fired furnaces.
The heating seasonal performance factor (HSPF) is a measure of the total heat output in Btu of a heat pump over the entire heating season divided by the total energy in watt hours it uses during that time. This number is similar to the seasonal efficiency of a fuel-fired heating system and includes energy for supplementary heating. Weather data characteristic of long-term climatic conditions are used to represent the heating season in calculating the HSPF.
The energy efficiency ratio (EER) measures the steady-state cooling efficiency of a heat pump. It is determined by dividing the cooling capacity of the heat pump in Btu/h by the electrical energy input in watts at a specific temperature. The higher the EER, the more efficient the unit.
The seasonal energy efficiency ratio (SEER) measures the cooling efficiency of the heat pump over the entire cooling season. It is determined by dividing the total cooling provided over the cooling season in Btu by the total energy used by the heat pump during that time in watt hours. The SEER is based on a climate with an average summer temperature of 28°C.
The thermal balance point is the temperature at which the amount of heating provided by the heat pump equals the amount of heat lost from the house. At this point, the heat pump capacity matches the full heating needs of the house. Below this temperature, supplementary heat is required from another source.
The economic balance point is the temperature at which the cost of heat energy supplied by the heat pump equals the cost of heat supplied by a supplementary heating system. Below this point, it is not economical to run the heat pump.
The efficiency ratings for different types of heat pumps use different terminology. For example, air-source heat pumps have seasonal heating and cooling ratings. The heating rating is the HSPF; the cooling rating is the SEER. In the manufacturers’ catalogues you may still see COP or EER ratings. These are steady-state ratings obtained at one set of temperature conditions and are not the same as the HSPF or SEER ratings.