Air source heat pumps are efficient space heating systems. However, many people doubt whether air source heat pumps work in winter when the ambient temperature is extremely low. So, I did some research.
Air source heat pumps do work in cold weather. Most air source heat pumps are able to maintain operation at an ambient temperature as low as -15°C (5°F). However, their heating capacity and efficiency drop alongside the ambient temperature.
Although air source heat pumps can work in very cold environments, the decrease in their heating capacity must be taken into account to avoid insufficient heating.
What Temperature Do Air Source Heat Pumps Stop Working?
Just like any other air conditioner and heat pump, air source heat pumps have a certain range of temperature at which they can operate. If the ambient temperature rises or drops beyond the operating temperature limit, the air source heat pump will stop running entirely.
For space heating, a typical air source heat pump has an operating temperature between -15°C (5°F) and 24°C (75.2°F). Hence, if the ambient temperature drops beyond -15°C (5°F), the air source heat pump may stop working.
In other words, an ambient temperature below -15°C (5°F) is considered too cold for a typical air source heat pump.
So, if you expect the outside temperature to drop or even approach -15°C (5°F) in winter, it may not be a good idea to use an air source heat pump as the primary source of heating.
But, there is a category of air source heat pumps that can work beyond -15°C (5°F). Many of them are able to maintain operation down to an ambient temperature of -25°C (-13°F).
Nonetheless, all air source heat pumps suffer from heating capacity and efficiency drop when operating in low ambient temperature environment.
Below is an example of the heating capacity and efficient drop in an air source heat pump based on different ambient temperatures:
|Ambient Temperature||Heating Capacity||Heating Efficiency|
|7°C (44.6°F)||3.6 kW||5.8|
|0°C (32°F)||3.1 kW||5.3|
|-5°C (23°F)||2.3 kW||5.1|
|-10°C (15°F)||2.0 kW||4.5|
|-15°C (5°F)||1.6 kW||3.9|
In short, we need to be aware of the fact that when the ambient temperature drops, the heating capacity and efficiency of air source heat pumps drop as well.
How Efficient is an Air Source Heat Pump in Winter?
When deciding whether to use an air source heat pump for space heating in winter, energy efficiency as well as the heating effectiveness of the air source heat pump play an important role.
Although an air source heat pump can still work when the ambient temperature drops to -15°C (5°F), its heating capacity typically drops about 50%.
So, a 3.6 kW air source heat pump may only be left with about 1.8 kW of heating capacity when the ambient temperature is at -15°C (5°F).
In other words, if you live in a very cold place where the ambient temperature may drop to -15°C (5°F), you need to size the air source heat pump based on 50% of its rated heating capacity.
For example, supposedly a 3.6 kW heating capacity model air source heat pump is sufficient for a 15 m2 (161 sqft) bedroom. But, if the outside air temperature can drop to -15°C (5°F) in winter, a 6.3 kW heating capacity model air source heat pump instead is required in order to ensure adequate heating can be provided in winter. That’s two steps up from the 3.6 kW model!
Meanwhile, the heating efficiency of an air source heat pump can drop about 30% when operating at -15°C (5°F). So, an air source heat pump with a COP of 5 may end up running at a COP of about 3.5 when the ambient temperature is around -15°C (5°F).
Still, an air source heat pump with a COP of 3.5 is more efficient than an electric space heater. However, it may or may not be more cost-effective than a gas furnace depending on the gas price and other factors.
See my post Is It Expensive to Heat with a Heat Pump? (Cost Analysis) for the energy cost comparison of different home space heating systems.
Nonetheless, not everyone needs their air source heat pump to stretch until -15°C (5°F) ambient temperature. For example, if you only need the air source heat pump to run at around 0°C (32°F), you may only see about a 15% drop in heating capacity and a 10% drop in heating efficiency which are still considered acceptable.
Take note that the rated heating capacity and efficiency stated on the specification sheet of air source heat pumps are based on an ambient temperature of 7°C (44.6°F) while assuming the indoor temperature is kept at 20°C (68°F). So, if you want the temperature in your house to be warmer than 20°C (68°F), you may consider using a bigger capacity air source heat pump as well.
Can Air Source Heat Pumps Freeze?
When operating below freezing point (<0°C), the air source heat pump outdoor unit will freeze eventually. However, all air source heat pumps have ways to defrost.
Some air source heat pumps are equipped with an auxiliary heater which is basically an electric resistive heater that melts the ice. Other air source heat pumps may reverse their refrigeration process to transfer heat to the outdoor unit in order to melt the ice.
Air source heat pumps that use an auxiliary heater to defrost are less efficient but offer uninterrupted space heating. Whereas air source heat pumps that reverse the process to defrost are more efficient but they’re unable to provide heating while defrosting.
How Air Source Heat Pumps Work in Winter?
Many people find it hard to believe that air source heat pumps are able to extract heat from the ambient air when it is incredibly cold outside.
Air source heat pumps nonetheless are able to work in winter. To understand the working principle of air source heat pumps works in winter, see my post How Does a Heat Pump Work in Winter? (Simple Explanation).
Air source heat pumps are efficient heating systems. They use less electricity and cause less harm to the environment. However, we must be extra careful when choosing air source heat pumps as they perform differently in cold weather.
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