For those who believe you will have low heater output because the air conditioner is operating, this is not the case. In my experience most vehicles equipped with AC will automatically turn on the AC compressor whenever the "defrost" function is selected, whether you want the AC on or not. One important reason is to bring the incoming outside air through the evaporator core and to cool that air down to near 32*F to squeeze out moisture from the air (cooling the air below 32*F will cause evaporator icing).
When that very dry air leaves the evaporator core, all of the air is directed through the heater core, around the heater core, or in any combination of blending by passing through and around the heater core simultaneously to get the desired air temperature.
The heater core by design easily raises the incoming outside air temperature from well below freezing to well over 120*F on a medium fan setting if all of the air is being directed through the heater core.
Once the outside air temperature falls well below 32*F, the air conditioning compressor will begin to cycle less frequently due to the pressure / temperature relationship of the refrigerant. When the outside air temperature is 20*F below zero, for example, the AC compressor will have stopped cycling altogether because the refrigerant pressure is too low to operate the system and is not needed anyway. Even the 20*F below zero air will easily be heated by a properly operating HVAC system.
- John
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