In the heating cycle, the ground water, the antifreeze mixture or the refrigerant (which has circulated through the underground piping system and picked up heat from the soil) is brought back to the heat pump unit inside the house. In ground water or antifreeze mixture systems, it then passes through the refrigerant-filled primary heat exchanger. In DX systems, the refrigerant enters the compressor directly, with no intermediate heat exchanger.
The heat is transferred to the refrigerant, which boils to become a low-temperature vapour. In an open system, the ground water is then pumped back out and discharged into a pond or down a well. In a closed-loop system, the antifreeze mixture or refrigerant is pumped back out to the underground piping system to be heated again.
The reversing valve directs the refrigerant vapour to the compressor. The vapour is then compressed, which reduces its volume and causes it to heat up.
Finally, the reversing valve directs the now-hot gas to the condenser coil, where it gives up its heat to the air or hydronic system to heat the home. Having given up its heat, the refrigerant passes through the expansion device, where its temperature and pressure are dropped further before it returns to the first heat exchanger, or to the ground in a DX system, to begin the cycle again.
The Cooling Cycle
The “active cooling” cycle is basically the reverse of the heating cycle. The direction of the refrigerant flow is changed by the reversing valve. The refrigerant picks up heat from the house air and transfers it directly, in DX systems, or to the ground water or antifreeze mixture. The heat is then pumped outside, into a water body or return well (in an open system) or into the underground piping (in a closed-loop system). Some of this excess heat can be used to preheat domestic hot water.
Unlike air-source heat pumps, ground-source systems do not require a defrost cycle. Temperatures underground are much more stable than air temperatures, and the heat pump unit itself is located inside; therefore, the problems with frost do not arise.
Parts of the System
Ground-source heat pump systems have three main components: the heat pump unit itself, the liquid heat exchange medium (open system or closed loop), and a distribution system (either air-based or hydronic) that distributes the thermal energy from the heat pump to the building.
Ground-source heat pumps are designed in different ways. For air-based systems, self-contained units combine the blower, compressor, heat exchanger, and condenser coil in a single cabinet. Split systems allow the coil to be added to a forced-air furnace, and use the existing blower and furnace. For hydronic systems, both the source and sink heat exchangers and compressor are in a single cabinet.
Energy Efficiency Considerations
As with air-source heat pumps, ground-source heat pump systems are available in a range of different efficiencies. See the earlier section called An introduction to Heat Pump Efficiency for an explanation of what COPs and EERs represent. Ranges of COPs and EERs for market available units are provided below.
Ground water or Open-Loop Applications
Heating
- Minimum Heating COP: 3.6
- Range, Heating COP in Market Available Products: 3.8 to 5.0
Cooling
- Minimum EER: 16.2
- Range, EER in Market Available Products: 19.1 to 27.5
Closed Loop Applications
Heating
- Minimum Heating COP: 3.1
- Range, Heating COP in Market Available Products: 3.2 to 4.2
Cooling
- Minimum EER: 13.4
- Range, EER in Market Available Products: 14.6 to 20.4
The minimum efficiency for each type is regulated at the federal level as well as in some provincial jurisdictions. There has been a dramatic improvement in the efficiency of ground-source systems. The same developments in compressors, motors and controls that are available to air-source heat pump manufacturers are resulting in higher levels of efficiency for ground-source systems.
Lower-end systems typically employ two stage compressors, relatively standard size refrigerant-to-air heat exchangers, and oversized enhanced-surface refrigerant-to-water heat exchangers. Units in the high efficiency range tend to use multi-or variable speed compressors, variable speed indoor fans, or both. Find an explanation of single speed and variable speed heat pumps in the Air-Source Heat Pump section.
Certification, Standards, and Rating Scales
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 ground-source systems are CSA C13256 (for secondary loop systems) and CSA C748 (for DX systems).
Sizing Considerations
It is important that the ground heat exchanger be well matched to the heat pump capacity. Systems that are not balanced and unable to replenish the energy drawn from the borefield will continuously perform worse over time until the heat pump can no longer extract heat.
As with air-source heat pump systems, it is generally not a good idea to size a ground-source system to provide all of the heat required by a house. For cost-effectiveness, the system should generally be sized to cover the majority of the household’s annual heating energy requirement. The occasional peak heating load during severe weather conditions can be met by a supplementary heating system.
Systems are now available with variable speed fans and compressors. This type of system can meet all cooling loads and most heating loads on low speed, with high speed required only for high heating loads. Find an explanation of single speed and variable speed heat pumps in the Air-Source Heat Pump section.
A variety of sizes of systems are available to suit the Canadian climate. Residential units range in rated size (closed loop cooling) of 1.8 kW to 21.1 kW (6 000 to 72 000 Btu/h), and include domestic hot water (DHW) options.
Design Considerations
Unlike air-source heat pumps, ground-source heat pumps require a ground heat exchanger to collect and dissipate heat underground.
Open Loop Systems
An open system uses ground water from a conventional well as a heat source. The ground water is pumped to a heat exchanger, where thermal energy is extracted and used as a source for the heat pump. The ground water exiting the heat exchanger is then reinjected into the aquifer.
Another way to release the used water is through a rejection well, which is a second well that returns the water to the ground. A rejection well must have enough capacity to dispose of all the water passed through the heat pump, and should be installed by a qualified well driller. If you have an extra existing well, your heat pump contractor should have a well driller ensure that it is suitable for use as a rejection well. Regardless of the approach used, the system should be designed to prevent any environmental damage. The heat pump simply removes or adds heat to the water; no pollutants are added. The only change in the water returned to the environment is a slight increase or decrease in temperature. It is important to check with local authorities to understand any regulations or rules regarding open loop systems in your area.
The size of the heat pump unit and the manufacturer’s specifications will determine the amount of water that is needed for an open system. The water requirement for a specific model of heat pump is usually expressed in litres per second (L/s) and is listed in the specifications for that unit. A heat pump of 10-kW (34 000-Btu/h) capacity will use 0.45 to 0.75 L/s while operating.
Your well and pump combination should be large enough to supply the water needed by the heat pump in addition to your domestic water requirements. You may need to enlarge your pressure tank or modify your plumbing to supply adequate water to the heat pump.
Poor water quality can cause serious problems in open systems. You should not use water from a spring, pond, river or lake as a source for your heat pump system. Particles and other matter can clog a heat pump system and make it inoperable in a short period of time. You should also have your water tested for acidity, hardness and iron content before installing a heat pump. Your contractor or equipment manufacturer can tell you what level of water quality is acceptable and under what circumstances special heat-exchanger materials may be required.
Installation of an open system is often subject to local zoning laws or licensing requirements. Check with local authorities to determine if restrictions apply in your area.
Closed-Loop Systems
A closed-loop system draws heat from the ground itself, using a continuous loop of buried plastic pipe. Copper tubing is used in the case of DX systems. The pipe is connected to the indoor heat pump to form a sealed underground loop through which an antifreeze solution or refrigerant is circulated. While an open system drains water from a well, a closed-loop system recirculates the antifreeze solution in the pressurized pipe.
The pipe is placed in one of three types of arrangements:
- Vertical: A vertical closed-loop arrangement is an appropriate choice for most suburban homes, where lot space is restricted. Piping is inserted into bored holes that are 150 mm (6 in.) in diameter, to a depth of 45 to 150 m (150 to 500 ft.), depending on soil conditions and the size of the system. U-shaped loops of pipe are inserted in the holes. DX systems can have smaller diameter holes, which can lower drilling costs.
- Diagonal (angled): A diagonal (angled) closed-loop arrangement is similar to a vertical closed-loop arrangement; however the boreholes are angled. This type of arrangement is used where space is very limited and access is limited to one point of entry.
- Horizontal: The horizontal arrangement is more common in rural areas, where properties are larger. The pipe is placed in trenches normally 1.0 to 1.8 m (3 to 6 ft.) deep, depending on the number of pipes in a trench. Generally, 120 to 180 m (400 to 600 ft.) of pipe is required per ton of heat pump capacity. For example, a well-insulated, 185 m2 (2000 sq. ft.) home would usually need a three-ton system, requiring 360 to 540 m (1200 to 1800 ft.) of pipe.
The most common horizontal heat exchanger design is two pipes placed side-by-side in the same trench. Other horizontal loop designs use four or six pipes in each trench, if land area is limited. Another design sometimes used where area is limited is a “spiral” – which describes its shape.
Regardless of the arrangement you choose, all piping for antifreeze solution systems must be at least series 100 polyethylene or polybutylene with thermally fused joints (as opposed to barbed fittings, clamps or glued joints), to ensure leak-free connections for the life of the piping. Properly installed, these pipes will last anywhere from 25 to 75 years. They are unaffected by chemicals found in soil and have good heat-conducting properties. The antifreeze solution must be acceptable to local environmental officials. DX systems use refrigeration-grade copper tubing.
Neither vertical nor horizontal loops have an adverse impact on the landscape as long as the vertical boreholes and trenches are properly backfilled and tamped (packed down firmly).
Horizontal loop installations use trenches anywhere from 150 to 600 mm (6 to 24 in.) wide. This leaves bare areas that can be restored with grass seed or sod. Vertical loops require little space and result in less lawn damage.
It is important that horizontal and vertical loops be installed by a qualified contractor. Plastic piping must be thermally fused, and there must be good earth-to-pipe contact to ensure good heat transfer, such as that achieved by Tremie-grouting of boreholes. The latter is particularly important for vertical heat-exchanger systems. Improper installation may result in poorer heat pump performance.
Installation Considerations
As with air-source heat pump systems, ground-source heat pumps must be designed and installed by qualified contractors. Consult a local heat pump contractor to design, install and service your equipment to ensure efficient and reliable operation. Also, be sure that all manufacturers’ instructions are followed carefully. All installations should meet the requirements of CSA C448 Series 16, an installation standard set by the Canadian Standards Association.
The total installed cost of ground-source systems varies according to site-specific conditions. Installation costs vary depending on the type of ground collector and the equipment specifications. The incremental cost of such a system can be recovered through energy cost savings over a period as low as 5 years. Payback period is dependent on a variety of factors such as soil conditions, heating and cooling loads, the complexity of HVAC retrofits, local utility rates, and the heating fuel source being replaced. Check with your electric utility to assess the benefits of investing in a ground-source system. Sometimes a low-cost financing plan or incentive is offered for approved installations. It is important to work with your contractor or energy advisor to get an estimate of the economics of heat pumps in your area, and the potential savings you can achieve.
Operation Considerations
You should note several important things when operating your heat pump:
- Optimize Heat Pump and Supplemental System Set-points. If you have an electric supplemental system (e.g., baseboards or resistance elements in duct), be sure to use a lower temperature set-point for your supplemental system. This will help to maximize the amount of heating the heat pump provides to your home, lowering your energy use and utility bills. A set-point of 2°C to 3°C below the heat pump heating temperature set-point is recommended. Consult your installation contractor on the optimal set-point for your system.
- Minimize Temperature Setbacks. Heat pumps have a slower response than furnace systems, so they have more difficultly responding to deep temperature setbacks. Moderated setbacks of not more than 2°C should be employed or a “smart” thermostat that switches the system on early, in anticipation of a recovery from setback, should be used. Again, consult your installation contractor on the optimal setback temperature for your system.
Maintenance Considerations
You should have a qualified contractor perform annual maintenance once per year to ensure your system remains efficient and reliable.
If you have an air-based distribution system, you can also support more efficient operations by replacing or cleaning your filter every 3 months. You should also ensure that your air vents and registers are not blocked by any furniture, carpeting or other items that would impede airflow.
Operating Costs
The operating costs of a ground-source system are usually considerably lower than those of other heating systems, because of the savings in fuel. Qualified heat pump installers should be able to give you information on how much electricity a particular ground-source system would use.
Relative savings will depend on whether you are currently using electricity, oil or natural gas, and on the relative costs of different energy sources in your area. By running a heat pump, you will use less gas or oil, but more electricity. If you live in an area where electricity is expensive, your operating costs may be higher.
Life Expectancy and Warranties
Ground-source heat pumps generally have a life expectancy of about 20 to 25 years. This is higher than for air-source heat pumps because the compressor has less thermal and mechanical stress, and is protected from the environment. The lifespan of the ground loop itself approaches 75 years.
Most ground-source heat pump units are covered by a one-year warranty on parts and labour, and some manufacturers offer extended warranty programs. However, warranties vary between manufacturers, so be sure to check the fine print.
Related Equipment
Upgrading the Electrical Service
Generally speaking, it is not necessary to upgrade the electrical service when installing an air-source add-on heat pump. However, the age of the service and the total electrical load of the house may make it necessary to upgrade.
A 200 ampere electrical service is normally required for the installation of either an all-electric air-source heat pump or a ground-source heat pump. If transitioning from a natural gas or fuel oil based heating system, it may be necessary to upgrade your electrical panel.
Supplementary Heating Systems
Air-Source Heat Pump Systems
Air-source heat pumps have a minimum outdoor operating temperature, and may lose some of their ability to heat at very cold temperatures. Because of this, most air-source installations require a supplementary heating source to maintain indoor temperatures during the coldest days. Supplementary heating may also be required when the heat pump is defrosting.
Most air-source systems shut off at one of three temperatures, which can be set by your installation contractor:
- Thermal Balance Point: The temperature below which the heat pump does not have enough capacity to meet the heating needs of the building on its own.
- Economic Balance Point: The temperature below which the ratio of electricity to a supplemental fuel (e.g., natural gas) means that using the supplementary system is more cost effective.
- Cut-Off Temperature: The minimum operating temperature for the heat pump.
Most supplementary systems can be classed into two categories:
- Hybrid Systems: In a hybrid system, the air-source heat pump uses a supplemental system such as a furnace or boiler. This option can be used in new installations, and is also a good option where a heat pump is added to an existing system, for example, when a heat pump is installed as a replacement for a central air-conditioner.
These types of systems support switching between heat pump and supplementary operations according to the thermal or economic balance point.
These systems cannot be run simultaneously with the heat pump – either the heat pump operates or the gas/oil furnace operates. - All Electric Systems: In this configuration, heat pump operations are supplemented with electric resistance elements located in the ductwork or with electric baseboards.
These systems can be run simultaneously with the heat pump, and can therefore be used in balance point or cut-off temperature control strategies.
An outdoor temperature sensor shuts the heat pump off when the temperature falls below the pre-set limit. Below this temperature, only the supplementary heating system operates. The sensor is usually set to shut off at the temperature corresponding to the economic balance point, or at the outdoor temperature below which it is cheaper to heat with the supplementary heating system instead of the heat pump.
Ground-Source Heat Pump Systems
Ground-source systems continue to operate regardless of the outdoor temperature, and as such are not subject to the same sort of operating restrictions. The supplementary heating system only provides heat that is beyond the rated capacity of the ground-source unit.
Thermostats
Conventional Thermostats
Most ducted residential single-speed heat pump systems are installed with a ”two-stage heat/one-stage cool” indoor thermostat. Stage one calls for heat from the heat pump if the temperature falls below the pre-set level. Stage two calls for heat from the supplementary heating system if the indoor temperature continues to fall below the desired temperature. Ductless residential air-source heat pumps are typically installed with a single stage heating/cooling thermostat or in many instances a built in thermostat set by a remote that comes with the unit.
The most common type of thermostat used is the ”set and forget” type. The installer consults with you prior to setting the desired temperature. Once this is done, you can forget about the thermostat; it will automatically switch the system from heating to cooling mode or vice versa.
There are two types of outdoor thermostats used with these systems. The first type controls the operation of the electric resistance supplementary heating system. This is the same type of thermostat that is used with an electric furnace. It turns on various stages of heaters as the outdoor temperature drops progressively lower. This ensures that the correct amount of supplementary heat is provided in response to outdoor conditions, which maximizes efficiency and saves you money. The second type simply shuts off the air-source heat pump when the outdoor temperature falls below a specified level.
Thermostat setbacks may not yield the same kind of benefits with heat pump systems as with more conventional heating systems. Depending upon the amount of the setback and temperature drop, the heat pump may not be able to supply all of the heat required to bring the temperature back up to the desired level on short notice. This may mean that the supplementary heating system operates until the heat pump “catches up.” This will reduce the savings that you might have expected to achieve by installing the heat pump. See discussion in previous sections on minimizing temperature setbacks.
Programmable Thermostats
Programmable heat pump thermostats are available today from most heat pump manufacturers and their representatives. Unlike conventional thermostats, these thermostats achieve savings from temperature setback during unoccupied periods, or overnight. Although this is accomplished in different ways by different manufacturers, the heat pump brings the house back to the desired temperature level with or without minimal supplementary heating. For those accustomed to thermostat setback and programmable thermostats, this may be a worthwhile investment. Other features available with some of these electronic thermostats include the following:
- Programmable control to allow for user selection of automatic heat pump or fan-only operation, by time of day and day of the week.
- Improved temperature control, as compared to conventional thermostats.
- No need for outdoor thermostats, as the electronic thermostat calls for supplementary heat only when needed.
- No need for an outdoor thermostat control on add-on heat pumps.
Savings from programmable thermostats are highly dependant on the type and sizing of your heat pump system. For variable speed systems, setbacks may allow the system to operate at a lower speed, reducing wear on the compressor and helping to increase system efficiency.
Heat Distribution Systems
Heat pump systems generally supply a greater volume of airflow at lower temperature compared to furnace systems. As such, it is very important to examine the supply airflow of your system, and how it may compare to the airflow capacity of your existing ducts. If the heat pump airflow exceeds the capacity of your existing ducting, you may have noise issues or increased fan energy use.
New heat pump systems should be designed according to established practice. If the installation is a retrofit, the existing duct system should be carefully examined to ensure that it is adequate.
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Post time: Nov-01-2022