2. Coming to Terms With Heat Pumps

Here are some common terms you'll come across while investigating heat pumps.

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.

Certification and Standards

The Canadian Standards Association (CSA) currently verifies all heat pumps for electrical safety. A performance standard specifies tests and test conditions at which heat pump heating and cooling capacities and efficiency are determined. The performance testing standards for air-source heat pumps are CSA C273.3 and C656. CSA has also published an installation standard for add-on air-source heat pumps (CSA C273.5-1980).

The industry has worked with CSA to publish standards to test the efficiency of ground-source heat pumps, and to ensure that they are designed and installed properly. These standards are CSA C13256-1-01 and C448 Series-02, respectively. Minimum efficiency standards are in place for air-source and ground-source heat pumps in some provinces and under Canada's Energy Efficiency Regulations.

Efficiency Terminology

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. Both are defined above. However, 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.

Earth-energy systems use only COP and EER ratings. Again, these ratings only hold for one temperature condition and cannot be directly used to predict annual performance in an application. In the section of this booklet titled "Major Benefits of Earth-Energy Systems", the COP ratings were used in a calculation to estimate HSPFs in different regions across Canada. HSPFs are not normally used to express the efficiency of earth-energy systems, but are used here to enable a comparison with air-source heat pumps.

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Source: Natural Resources Canada (NRCan) - Office of Energy Efficiency