To know more, let’s review the following concepts:
Heating and Cooling with a Heat Pump:
A heat pump is an electrical device that extracts heat from one place and transfers it to another.
Air-source heat pumps perform two roles:
- Draw heat from the outside air during the heating season
- Reject heat outside during the summer cooling season.
How Does an Air-Source Heat Pump Work?
An air-source heat pump has three cycles: the heating cycle, the cooling cycle and the defrost cycle.
The Heating Cycle
During the heating cycle, heat is taken from outdoor air and “pumped” indoors.
- First, the liquid refrigerant passes through the expansion device, changing to a low-pressure liquid/vapor mixture. It then goes to the outdoor coil, which acts as the evaporator coil. The liquid refrigerant absorbs heat from the outdoor air and boils, becoming a low-temperature vapor.
- This vapor passes through the reversing valve to the accumulator, which collects any remaining liquid before the vapor enters the compressor. The vapor is then compressed, reducing its volume and causing it to heat up.
- Finally, the reversing valve sends the gas, which is now hot, to the indoor coil, which is the condenser. The heat from the hot gas is transferred to the indoor air, causing the refrigerant to condense into a liquid. This liquid returns to the expansion device and the cycle is repeated. The indoor coil is located in the ductwork, close to the furnace.
The ability of the heat pump to transfer heat from the outside air to the house depends on the outdoor temperature. As this temperature drops, the ability of the heat pump to absorb heat also drops.
At the outdoor ambient balance point temperature, the heat pump’s heating capacity is equal to the heat loss of the house.
Below this outdoor ambient temperature, the heat pump can supply only part of the heat required to keep the living space comfortable, and supplementary heat is required.
When the heat pump is operating in the heating mode without any supplementary heat, the air leaving it will be cooler than air heated by a normal furnace. Furnaces generally deliver air to the living space at between 55°C and 60°C. Heat pumps provide air in larger quantities at about 25°C to 45°C and tend to operate for longer periods.
The Cooling Cycle
The cycle described above is reversed to cool the house during the summer. The unit takes heat out of the indoor air and rejects it outside. Min Height: auto Min Height: auto Width: auto Width: auto Min Height: auto Min Height: auto Width: 80% Width: 80%
- As in the heating cycle, the liquid refrigerant passes through the expansion device, changing to a low-pressure liquid/vapor mixture. It then goes to the indoor coil, which acts as the evaporator. The liquid refrigerant absorbs heat from the indoor air and boils, becoming a low-temperature vapor.
- This vapor passes through the reversing valve to the accumulator, which collects any remaining liquid, and then to the compressor. The vapor is then compressed, reducing its volume and causing it to heat up.
- Finally, the gas, which is now hot, passes through the reversing valve to the outdoor coil, which acts as the condenser. The heat from the hot gas is transferred to the outdoor air, causing the refrigerant to condense into a liquid. This liquid returns to the expansion device, and the cycle is repeated.
During the cooling cycle, the heat pump also dehumidifies the indoor air. Moisture in the air passing over the indoor coil condenses on the coil’s surface and is collected in a pan at the bottom of the coil. A condensate drain connects this pan to the house drain.
The Defrost Cycle
If the outdoor temperature falls to near or below freezing when the heat pump is operating in the heating mode, moisture in the air passing over the outside coil will condense and freeze on it. The amount of frost buildup depends on the outdoor temperature and the amount of moisture in the air.
This frost buildup decreases the efficiency of the coil by reducing its ability to transfer heat to the refrigerant. At some point, the frost must be removed. To do this, the heat pump will switch into the defrost mode.
- First, the reversing valve switches the device to the cooling mode. This sends hot gas to the outdoor coil to melt the frost. At the same time the outdoor fan, which normally blows cold air over the coil, is shut off in order to reduce the amount of heat needed to melt the frost.
- While this is happening, the heat pump is cooling the air in the ductwork.
The heating system would normally warm this air as it is distributed throughout the house.
One of two methods is used to determine when the unit goes into defrost mode. Demand-frost controls monitor airflow, refrigerant pressure, air or coil temperature and pressure differential across the outdoor coil to detect frost accumulation on the outdoor coil.
Time-temperature defrost is started and ended by a preset interval timer or a temperature sensor located on the outside coil. The cycle can be initiated every 30, 60 or 90 minutes, depending on the climate and the design of the system.
Unnecessary defrost cycles reduce the seasonal performance of the heat pump. As a result, the demand-frost method is generally more efficient since it starts the defrost cycle only when it is required.
Cold-Climate Air Source Pump System
Cold climate air source heat pumps (CC-ASHPs) are high-efficiency heating and cooling systems that have recently become available in North America.
Cold-climate heat pump unit, in today’s heating cooling industry, is the opportunity to replace combustion-based systems. By this system, we can use only electricity even in the regions where older heat pumps couldn’t perform or were not enough.
To clarify, at 47°F, previous generations of heat pumps would start drifting in capacity. The result was the homeowners would feel cool air coming through their vents. So they had to manually switch over to the gas furnace.
TODAY, cold-climate heat pumps are designed to deliver up to 100% rated heating capacity at 5°F and up to 76% capacity at -13°F. As long as a heat pump can make it refrigerant cooler than the outdoor air, it can capture thermal energy and deliver it as indoor heat.
Modern cold-climate heat pumps have variable-speed compressors designed to slow down and speed up to match system capacity with the load detected by indoor unit sensors. This makes heat pumps more energy efficient than fixed-capacity conventional systems but doesn’t mean we can ignore the heat pump’s range. Each variable-capacity system has a minimum and maximum capacity which is published in the manufacturer’s technical literature.
