At what temperature does an air to heat pump become ineffective use considering cost of defrosting the outdoor coils?

Air-to-heat pumps become progressively less efficient as the outside air temperature drops. The exact temperature at which it becomes more cost-effective to use an alternative heating source, such as an electric resistance heater or a fossil fuel-fired furnace, depends on several factors, including:

1. Climate: In colder climates, the temperature at which a heat pump becomes ineffective is typically lower than in warmer climates. For example, in regions where the average winter temperature is below 25°F (-4°C), heat pumps may become ineffective at temperatures below 20°F (-7°C).

2. Heat pump efficiency: The efficiency of a heat pump is measured by its coefficient of performance (COP), which is the ratio of the heat output to the electrical energy input. The higher the COP, the more efficient the heat pump. Heat pumps with higher COPs are more likely to be cost-effective at lower temperatures.

3. Cost of electricity and fossil fuels: The relative cost of electricity and fossil fuels also affects the cost-effectiveness of using a heat pump. When the cost of electricity is low and the cost of fossil fuels is high, it is more likely to be cost-effective to use a heat pump, even at lower temperatures.

4. Amount of heating required: The amount of heating required also affects the cost-effectiveness of using a heat pump. In homes that require a lot of heating, it may be more cost-effective to use an alternative heating source, even if the heat pump is still technically efficient.

As a general rule, it is typically considered to be more cost-effective to use an alternative heating source when the outside air temperature drops below 25°F (-4°C). However, it is important to consider all of the factors discussed above to make a definitive determination.