Which statement best describes the effect of outdoor temperature on heating capacity: as outdoor temperature decreases, heating capacity tends to:

Which statement best describes the effect of outdoor temperature on heating capacity: as outdoor temperature decreases, heating capacity tends to:

• A. Increases
• B. Remains the same
• C. First increases then decreases
• D. Decreases

Answer: (D)

The key idea is that outdoor temperature directly limits how much heat a heating system can deliver, especially for air-source heat pumps. When outdoor air is warmer, there’s more thermal energy to extract, and the system can move more heat into the building. As the outdoor air cools, there’s less heat to recover and the refrigeration cycle operates less efficiently (the COP drops), so the system cannot deliver as many BTUs per hour. In practice, this means heating capacity falls with decreasing outdoor temperature. The other options don’t match how heat pumps behave: they don’t gain capacity in colder air, they don’t stay constant, and the common pattern is a monotonic decrease as it gets colder (aside from brief defrost or supplemental-heat effects).

The key idea is that outdoor temperature directly limits how much heat a heating system can deliver, especially for air-source heat pumps. When outdoor air is warmer, there’s more thermal energy to extract, and the system can move more heat into the building. As the outdoor air cools, there’s less heat to recover and the refrigeration cycle operates less efficiently (the COP drops), so the system cannot deliver as many BTUs per hour. In practice, this means heating capacity falls with decreasing outdoor temperature. The other options don’t match how heat pumps behave: they don’t gain capacity in colder air, they don’t stay constant, and the common pattern is a monotonic decrease as it gets colder (aside from brief defrost or supplemental-heat effects).