1. Machine
   1. Ineffective use
   2. Inappropriate unit power
2. Material
   1. Failure of fan
      1. Defective fun
      2. Inappropriate installation
   2. Failure of thermostat
      1. Defective material
      2. Inappropriate installation
   3. Failure of electric compression
      1. Defective compression
      2. Inappropriate installation
3. Measurement
   1. Energy consumption
   2. Time required to reach the set temperature
4. Method
   1. natural gas-fired absorption heat pump
   2. air-to-air heat pumps
5. Man
   1. Failure of technician to install it properly
   2. False maintenance
   3. False use
6. Mother nature
   1. Weather conditions
      1. Extreme low temperatures