Air-Source Heat Pumps

Air-Source Heat Pumps

Understanding Air-Source Heat Pumps

Air-source heat pumps work by drawing heat from the air outside during the heating season and rejecting the heat outside in the summer cooling season.

The air-to-air pump is the most common. It works by extracting heat from the air and then transferring it either outside or inside your home depending on what season it is.

The other type of air-source heat pump is the air-to-water pump. It is commonly used in homes that have hydronic systems for heat distribution and is not common in Ontario.

Ductless mini-split heat pumps have been recently introduced to the Canadian market. The heat pumps are perfect for retrofit homes that have either electric resistance or hydronic baseboard heating. They are free-air delivery, wall-mounted units installed in the individual rooms of a house. A single outdoor section can serve as many as 8 individual indoor wall-mounted units.

Air-source heat pumps are usually bivalent, all-electric, or add-on. The add-on heat pumps have been designed to be used with an additional source of supplementary heat like an electric, oil, natural gas, or propane furnaces. All-electric air-source heat pumps have their own individual supplementary heating system, which is basically an electric-resistance heater. The bivalent air-source heat pumps are a special variety that have been developed in Canada and uses either a propane or gas fired burner to increase the temperature of the air that enters the outdoor coil. This is what makes it possible for the units to function even when the outdoor temperatures are low.


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How Do Air-Source Heat Pumps Work?

Air-source heat pumps have 3 cycles, namely: the heating cycle, cooling cycle, and defrost cycle.

1. Heating Cycle

The heating cycle is where heat is taken from the outdoor air and subsequently “pumped” indoors.

The expansion device is where the liquid refrigerant first passes and changes to a low-pressure liquid/vapor mixture. It then moves to the outdoor coil that serves as the evaporator coil. The heat from the outdoor air is absorbed by the liquid refrigerant, which boils and becomes a low-temperature vapor.

The vapor then moves to the accumulator via the reversing valve, which then collects all the liquid that remains before the entry of the vapor into the compressor. The vapor is subsequently compressed, which reduces its volume and causes it to heat up.

The reversing valve finally sends the now hot gas to the indoor coil, which acts as the condenser during the winter cycle. Heat from the hot gas is then transferred to the indoor air, using the furnace fan, which then causes the refrigerant to condense back into liquid form. The liquid then returns to the expansion device where the cycle is repeated. You will find the indoor coil close to the furnace, in the duct-work.

The outdoor temperature is what determines the ability of the heat pump to transfer heat indoors from the outside air. As the temperature decreases, the heat pump’s ability to absorb the heat also reduces. Water source heat pumps, on the other hand, are not affected by lowering outdoor temperatures, because they utilize water from a well which remains at a constant temprature, regardless of the outdoor temperature fluctuation.

The heating capacity of the heat pump at the outdoor ambient temperature balance point is equal to the house’s heat loss.

The heat pump is only able to supply part of the heat needed to ensure that the living space stays comfortable, and supplementary heat is needed if it drops below the outdoor ambient temperature. Normally this is around -10 degrees Celsius in Southeastern Ontario.

If the heat pump operates in the heating mode in the absence of any supplementary heat, the air that leaves it will be cooler than that heated by a regular furnace. The temperature that furnaces usually deliver air to the living space at is between 55°C and 60°C. Heat pumps sometimes provide air in greater quantities at between 25°C and 45°C and usually operate for extended periods.

2. Cooling Cycle

During the summer, the cycle that has been described above is then reversed to help cool the house. The system now acts the same as central air conditioning. Heat is removed from the indoor air by the unit and it then rejects it outside.

The liquid refrigerant passes through the expansion device and changes to a low-pressure liquid/vapor mixture just as with the heating cycle. It then moves to the indoor coil that serves as the evaporator. The heat from the indoor air is absorbed by the liquid refrigerant and boils thus transforming into a low-temperature vapor.

The vapor moves through to the accumulator via the reversing valve and any liquid that remains is collected and the vapor ends up in the compressor. The vapor is subsequently compressed, which reduces its volume and makes its temperature, again, rise.

Finally, the now hot gas, passes via the reversing valve and into the outdoor coil that acts as the condenser. Heat emanating from the hot gas is then moved to the outdoor air and causes the condensation of the refrigerant back into liquid form. The liquid then returns to the expansion device where the cycle is repeated.

The heat pump also dehumidifies indoor air during the cooling cycle. Moisture contained in the air that passes over the indoor coil then condenses on the surface of the coil and collected in a pan at the coil’s bottom. The pan is connected to the house drain by the condensate drain. This is why annual service is important to ensure the fan is running at the correct CFM. If the CFM is too high the house may cool faster than the humidity can be removed, making it cold and damp. If the CFM its too low, you could have icing or ice spots on the coil. This will lower the efficiency and escalate into complete evaporator freeze ups, which could cause water damage to equipment and personal belongings..

3. Defrost Cycle

If the heat pump is operating in the heating mode and the outdoor temperature drops to either below or near freezing, moisture in the air passing over the outside coil condenses and freezes on it. The amount of frost that builds up will depend on the amount of moisture present in the air as well as the outdoor temperature.

The buildup of frost reduces the coil’s efficiency by reducing its ability to transfer heat to the refrigerant. The frost must be removed at some point. The unit then switches into defrost mode to do this.

The reversing valve first switches the device to cooling mode, which sends hot gas to the outdoor coil to melt the frost. The outdoor fan, which usually blows cold air over the coil will at the same time be switched off to reduce the amount of heat required to melt the frost.

The heat pump will be cooling the air in the duct-work when all this is happening. The heating system would typically warm the air as it is distributed throughout the house.

1 of 2 methods is used for determining when the unit enters into defrost mode. Demand-frost control monitors the airflow, coil or air temperature, refrigerant pressure, and pressure differential that exists across the outdoor coil to detect any accumulation of frost on the outdoor coil.

The time-temperature defrost is triggered and ended by either a temperature sensor found on the outside coil or a preset interval timer. Depending on the climate and system’s design, the cycle is initiated every 30, 60, or 90 minutes.

Unnecessary defrost cycles tend to reduce the heat pump’s seasonal performance. The result is that the demand-frost strategy has greater efficiency because it triggers the defrost cycle if and only when it is actually needed.


Heat pumps are a great way to lower heating costs, saving hundreds of dollars per year over other fuel sources.

If you would like more info or a quote on ultra high efficiency heating and cooling options, contact Furlong HVAC Services today.

We’d be happy to answer any questions and provide you with a system you will enjoy for many years.


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