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ASHPs are the most common type of heat pump system because the atmosphere is readily available as a heat source and they are relatively inexpensive compared with other types of heat pumps. They are used in many applications and are available as central systems or split systems (commonly referred to as mini-splits)5. The main drawback of ASHPs is that they become less efficient with colder outdoor temperatures. Fortunately, the average winter temperature in most regions of British Columbia is high enough to allow this equipment to operate efficiently. In colder climates, supplementary heat is required when the outdoor temperature drops below around -8°C for most conventional ASHPs6. Cold climate ASHPs are a costlier alternate option for colder climates. They include features that allow them to operate in heat pump mode at outdoor temperatures as low as - 25⁰C7, albeit with degraded COP and heating capacity a.
Since frost can form on the outdoor coils of ASHPs when the outdoor temperature falls below 7°C, they employ defrost cycles to melt any ice8. Frost forms on the outdoor coils because they are colder than the ambient air that they are absorbing heat from. This issue is more pronounced in coastal areas since these locations tend to have higher levels of absolute humidity than inland areas b.
Defrost cycles either operate the heat pump in reverse or leverage integrated electric residential coils to produce heat for short periods of time, until any frost on the coils has been melted. This is essential to optimal heat transfer and efficiency of ASHPs in colder temperatures.
ASHPs for residential space heating applications are available in two main configurations; centrally ducted heat pumps and mini-split heat pumps. In addition, two of these system types are sometimes combined to overcome certain design challenges. For example, if there is limited duct capacity, a ductless heat pump may be used to heat and cool one area of a home that is primarily conditioned by a centrally ducted ASHP.
A brief description of the different types of ASHPs is provided below. Each type is explored on subsequent pages. A more detailed description of cold climate ASHPs is included as well.
Centrally Ducted Heat Pumps: Refer to whole-house systems with central air handlers, which are employed in homes with central ducting.
Mini-split Heat Pumps: Generally used in homes without central ducting and can be used to condition either a portion of the home or the entire home.
Mini-ducted heat pumps: A variant of mini-split heat pumps with concealed air handlers and ducting used to distribute the conditioned air to multiple spaces in a home.
Ductless heat pumps: Refers to an ASHP with any non-ducted indoor unit, including wall-mount air handlers, floor-mounted consoles, and in-ceiling cassettes.
a Some manufacturers claim that their cold climate heat pumps can operate in heat pump mode down to -30°C.
b As such, the dew point in the coastal areas is higher than the interior, causing condensation to form on the outdoor coils at warmer temperatures. In other words, suspended moisture in the air starts to condense on a warmer surface in coastal areas.