About this calculator This calculator evaluates heat pump heating performance at specific outdoor conditions. It accounts for capacity derating at lower ambient temperatures and determines the resulting supply air temperature rise. Use it to verify heat pump performance at winter design conditions or to determine when auxiliary heat is needed.
Use when You need to evaluate heat pump performance at specific outdoor conditions.
Formula ΔT = Adjusted Capacity ÷ (CFM × 1.08)
Variables
CFM Actual Actual airflow in CFMRated CFM Rated airflow in CFMRated Capacity Rated heating capacity in BTU/HrEntering DB Entering dry bulb temperature °FOutdoor DB Outdoor dry bulb temperature °FAltitude Elevation above sea level in feetUnit Type RTU or split system
CFM Actual
Actual airflow through the unit on this job
Rated CFM
Rated airflow the unit was tested at
Rated Capacity (BTU/hr)
Rated heating capacity in BTU/hr
Entering DB (°F)
Return-air temperature entering the indoor coil
Outdoor DB (°F)
Outside air temperature affecting performance
Altitude (ft)
Elevation above sea level in feet
Unit Type
Leaving Air Temp (°F) —
Temperature Rise (°F) —
Adjusted Capacity (BTU/hr) —
Sensible Capacity (BTU/hr) —

How does altitude and temperature affect heat pump performance?

Heat pump heating capacity decreases as outdoor temperature drops because there is less heat energy available in cold outdoor air for the refrigerant to absorb. Manufacturers publish capacity ratings at the AHRI standard rating point of 47°F outdoor temperature, but actual capacity at winter design conditions (often 0–20°F depending on climate zone) can be 40–60% lower than the rated value. This capacity derating follows a roughly linear relationship with outdoor temperature. Below a certain outdoor temperature, known as the balance point, the heat pump alone cannot meet the building heat loss, and supplemental heating (electric strip heat or gas backup) is required. The balance point depends on the building load, equipment capacity curve, and local design temperature. This calculator applies the derating curve to determine the actual available heating capacity and resulting supply air temperature rise at any outdoor condition.

Related Calculators

Reheat Coil Calculator