Mr. Tight Watt found several excellent references to answer this question about heat pump HVAC systems.
Question: At what temperature is a heat pump not effective and it becomes necessary to run the furnace on electric heat? –James, a Smart Choices reader
Answer: Typically an air source heat pump runs and can heat a home without backup resistance heat down to about the freezing level. A heat pump gets less effective the colder it gets (about 30 degrees F and below) – but it still operates and will continue to do so until it gets close to zero.
Information from other experts
1. Missouri Department of Energy. Electric heat pumps have been available for home heating for more than 30 years. Essentially an air conditioner running in reverse, heat pumps produce two to three units of heat energy for each unit of electrical energy consumed.
A seasonal efficiency rating for heat pumps has been devised by the U.S. Department of Energy (DOE). This rating, known as the Heating Season Performance Factor (HSPF), equals the average heating capacity in Btu-per-hour divided by the power consumption in watts. The efficiency of a heat pump increases with higher outdoor temperatures, therefore, seasonal efficiencies are higher in warmer climates.
Most heat pumps employ the same basic layout and components as the equipment of 30 years ago. With the emphasis in the last decade on energy efficiency, and with the advent of solid state controls, today's heat pump offers marked improvements in efficiency and reliability. New developments in heat pumps, including variable speed compressors and new compressor designs, are improving the HSPFs.
Because heat pumps also provide cooling in summer, consideration should also be given to their cooling-efficiency rating or Seasonal Energy Efficiency Ratio (SEER).
Air-to-air heat pumps are effective in winter at temperatures down to about 30 degrees F. Supplemental heat is necessary at temperatures below that.
Source: Missouri Department of Energy
2. Energy Services Group.The supply air temperature is lower than that of a gas or oil furnace, so the heat pump may run longer to deliver the same amount of heat. When it gets close to 30° outside, the heat pump will run almost continuously. Remember that it is designed to do so.
You actually have two furnaces: the heat pump and the backup coils that look and act just like your toaster or oven coils. The heat pump is very efficient and inexpensive to run, but the backup is not as efficient and is more expensive to run. As the outside temperature drops to about 30°F, the heat pump starts to need help from the backup. Your thermostat will automatically add backup heat as needed, and that proportion increases from no backup at 30° to all backup at close to 0°. Remember that this happens without any help on your part.
Source: Energy Services Group