HVAC2000 Heating / Ventilation / Air Conditioning

THE AIR-TO-AIR HEAT PUMP
(Most residential units are air-to-air heat pumps.)

The air-to-air heat pump resembles the central air conditioning system. There are indoor and outdoor system components. When discussing summer air conditioning systems, these components are often called the evaporator (indoor coil) and the condenser (outdoor coil). This terminology will work for air conditioning (cooling) but not for a heat pump. Like cooling equipment, heat pumps are manufactured in split systems and package systems.

The systems coils have new names when applied to a heat pump. The coil that serves the inside of the house is called the indoor coil. The coil outside the house is called the outdoor coil. The reason is that the indoor coil is a condenser in the heating mode and an evaporator in the cooling mode. The outdoor coil is a condenser in the cooling mode and an evaporator in the heating mode. This is all determined by which way the hot gas is flowing. In winter the hot gas is flowing toward the indoor coil and will give up heat to the conditioned space. The heat must come from the outdoor unit, which is the evaporator.

LAWS OF THERMODYNAMICS

FIRST LAW OF THERMODYNAMICS is a formula for the conversion of heat into work or work into heat. The formula is: 778 foot-pounds of work is equivalent to the heat energy of 1 Btu.

SECOND LAW OF THERMODYNAMICS states that heat will only flow from a body at a certain temperature to another body, which is at a lower temperature. (Heat flows from a warmer to cooler)

So if the outdoor temperature is (example) 40F and the outdoor coil is 10F, heat will be absorbed into the refrigerant in the outdoor coil, then taken inside to reject heat to the conditioned space.