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Name: Ronnie
Status: Other
Grade: Other
Location: NC
Country: United States
Date: June 2008


Question:
Does it take more energy to heat a house twenty degrees above outside temperature or to cool a house twenty degrees below outside temperature using a heat pump?



Replies:
Hi Ronnie,

It all depends on the Coefficient of Performance (COP) of the heat pump, and that is dependent largely on the temperature of the heat source or heat sink.

As you probably know, when used to heat a house, a heat pump takes heat from one "source" (such as the colder outside air") that is lower in temperature than the temperature inside the house, and pumps this heat to the inside of the house.

Similarly, when cooling a house, the above is reversed. Heat is pumped from inside the house to the hotter air outside.

Note that heat is always being pumped "uphill" (from a cooler place to a warmer place). The reason for this is clear. First, when you are heating a house, the outside air is by definition colder (otherwise why do you need heat?). Similarly, if you are cooling the house, the outside air will be warmer (otherwise why cool?).

Take heating for example. When the temperature difference between the source (outside air) and the inside temperature, is too great (that is, it is too cold outside) the COP of the heat pump drops of dramatically. Similarly, when cooling, the greater the difference between inside and outside temperature, and the worse the COP is.

The worse the COP, the more energy is needed to pump the same amount of heat. So without knowing the so-called "delta-T" (difference between inside and outside temperatures) for each case of heating and cooling, there is no way to answer your question. For example, it would be unfair to compare energy requirements when heating a house when outside temperature is only 5 degrees less than room temperature, with cooling a house when outside temperature is 10 degrees above room temperature.

However, note that the electrical energy required to drive the heat pump often ends up heating the house, no matter whether you want to heat or cool, because the heat pump is often installed inside the house. In this case, when heating the house, this energy is added to the energy that is pumped from the outside, and as a result, less heat is needed to be pumped, increasing the overall efficiency of the system.

When cooling, the opposite occurs. The electrical energy needed to pump the heat from the house to the outside, remains in the house causing the house to get warmer. The heat pump must work harder because it has to not only pump the normal internal heat to the outside, it also has to pump the heat generated by its own operation out as well.

Hope this was helpful.

Bob Wilson



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