October, 2015

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Heat Pump Insights

Not all air conditioners are heat pumps, but ALL heat pumps are air conditioners. Heat pumps can be air-source or water-source and are generally more efficient than it’s straight-cooled counter part.

So what’s the difference?  In short, a straight-cool AC can only cool a structure while a heat pump can cool and heat a structure in moderate  temperatures. This is why they are popular in southern regions of the country where the winters are mild. Basically, a heat pump “pumps” heat energy from one location to another. It can pull heat energy from the outside and put it inside a structure for heat or pull heat from inside a structure to the outside. A straight-cooled air conditioner requires a furnace or electric heat to heat a structure.

Air to Air (air-source) heat pumps are the most common heat pumps used today. These heat pumps are generally less expensive and are easier to install than other types of heat pumps. They are available in package units or split systems and can use auxiliary heat when the outdoor temperatures fall below the balance point.

Operation

Like air conditioners, heat pumps have two coils (evaporator & condenser), the difference is  the role that each coil plays changes depending on if the unit is calling for heat or cool. For this reason, technicians identify these coils as “indoor coil” or “outdoor coil” instead of evaporator or condenser.

In cooling, a heat pump runs much like a straight-cool ac. The evaporator absorbs heat from a structure and the condenser rejects the heat to the outside. In heat mode, the condenser takes on the role of the evaporator and absorbs heat energy from the outside while the evaporator becomes the condenser.  This is done by reversing the flow of refrigerant by means of a reversing valve.

Balance Point

The lowest temperature at which a heat pump can heat a structure is called the “balance point”. When the balanced point is reached, auxiliary heat (2nd stage heating) is required. Auxiliary heat consists of electric heat strips which are more expensive to operate. In practice, the auxiliary heat is energized 4-5 degrees above the balance point. Some units have an outdoor thermostat that is set to 5 degrees above the balance point. This is to keep the heat strips from energizing until outdoor temperatures reach the set point. It should be noted that if you set your thermostat 4 degrees above room temperature, the heat strips will energize to meet the demand. Also, when purchasing a heat pump, auxiliary heat strips are generally not included and must be purchased separately.