|HEATING AND COOLING WITH A HEAT PUMP|
|HEATING AND COOLING WITH A HEAT PUMP
If you are exploring the heating and cooling options for a new house or looking for ways to reduce your energy bills, you may be considering a heat pump. A heat pump can provide year-round climate control for your home by supplying heat to it in the winter and cooling it in the summer. Some types can also provide supplementary hot water heating.
In general, using a heat pump alone to meet all your heating needs will not be economical. However, used in conjunction with a supplementary form of heating, such as an oil, gas, or electric furnace, a heat pump can provide reliable and eco- nomic heating in winter and cooling in summer. If you already have an oil or electric heating system, installing a heat pump may be an effective way to reduce your energy costs.
Nevertheless, it is important to consider all the benefits and costs before purchasing a heat pump. While heat pumps may have lower fuel costs in comparison with conventional heat- ing systems, they are more expensive to buy. It is important to carefully weigh your anticipated fuel savings against the initial cost. It is also important to realize that heat pumps will be most economical when used all year round. Investing in a heat pump will make more sense if you are interested in both summer cooling and winter heating.
In addition to looking at cost, you should consider conve- nience. How much space will the equipment require? Will your supply of energy be interrupted on occasion? If so, how often? Will you need a new ducting system? How much servicing will the system need, and what will it cost?
Becoming fully informed about all aspects of home heating and cooling before making your final decision is the key to making the right choice. This booklet describes the most common types of heat pumps, and discusses the factors If you are exploring the heating and cooling options for a new house or looking for ways to reduce your energy bills, you may be considering a heat pump. A brief section on the cost of operating different types of heat pumps and a conventional electric heating systems is also included.
Energy Management in the Home
Heat pumps are very efficient heating and cooling systems and can significantly reduce your energy costs. However, there is little point in investing in an efficient heating system if your home is losing heat through poorly insulated walls, ceilings, windows, and doors, and by air leakage through cracks and holes.
In many cases, it makes good sense to reduce air leakage and upgrade thermal insulation levels before buying or upgrading your heating system. A number of publications explaining how to do this are available from Natural Resources Canada.
Summer Cooling May Add to Energy Bills
Heat pumps supply heat to the house in winter and cool the house in the summer. However, they require electricity to operate. If you add a heat pump to your heating system or convert from another fuel to a heat pump, and your old system was not equipped with central air conditioning, you will find that your electricity bills will be higher than before.
What is a Heat Pump and How Does it Work?
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 in Canada and around the world for decades. Refrigerators and air conditioners are both common examples of heat pumps.
Figure 1: Basic Heat Pump Cycle
Heat pumps transfer heat by circulating a substance called a refrigerant through a cycle of alternating evaporation and condensation (see Figure 1). 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.
Nevertheless, 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 -18oC contains about 85 percent of the heat it contained at 21oC.
The air-source heat pump, which absorbs heat from the outdoor air in winter and "dumps house" heat into outdoor air in summer, is the most common type found in Canadian homes at this time. However, ground-source heat pumps (also called earth-energy systems), which draw heat from the ground or ground water, are becoming more widely used, particularly in Ontario and Atlantic Canada.
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 substance which 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.
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 releases 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 which forms part of the system for distributing heated or cooled air through the house. It is generally a large compartment immediately above the heat exchanger.
A Btu/h, or British thermal unit per hour, is a measure of 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 18oC. For example, if the average temperature on a particular day was 12oC, 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 1 000 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.
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) is a measure-ment of 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
The 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. This is the point at which the heat pump meets the full heating needs of the house. Below this point, supplementary heat is required. 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.
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 standard for air source heat pumps is CSA C273.3-M1991. CSA has also published an installation standard for add-on air-source heat pumps (CSA C273.5-1980).
The Canadian Earth Energy Association (CEEA) has worked with CSA to publish standards to test the efficiency of ground-source heat pumps, and to ensure that they are installed properly. These standards are CSA C446-1990 and C445-1992, respectively. Minimum efficiency standards are stipulated in both the air-source and ground-source performance standards, and in some jurisdictions in Canada these levels are currently regulated.
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 comparable to 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 imply performance in an application. In the section of this booklet titled "Major Benefits of Earth-Energy Systems" (see page 34), the COP ratings were used in a calculational procedure to estimate HSPF in different regions across Canada. HSPFs are not typically used to express the efficiency of earth-energy systems, but are only used here to enable a comparison with air-source heat pumps.