Coming to Terms With Heat Pumps
Here are some common terms you'll come across while investigating
heat pumps.
HEAT PUMP COMPONENTS
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.
OTHER TERMS
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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