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Energy for Heating Versus Cooling a House
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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Update: June 2012
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