Reply #2: Well your AC has a much harder job than your heater

[billyskank]

It's all to do with entropy, and the second law of thermodynamics. Entropy is a measure of disorder; the chemical energy in your fuel is relatively ordered (because it's all in one place, ready for use) and so has low entropy, whilst the same energy transformed into heat and diffused through your house is relatively disorded and has higher entropy.

The second law of thermodynamics states that entropy always increases; that is, a system always goes from an ordered state to a disorded state, never backwards. For example, it is impossible to drop a broken plate onto the floor and have it reassemble into a whole plate.

So your furnace works in the direction entropy wants to go: the ordered chemical energy in the fuel becomes disordered heat energy in your house. The AC on the other hand has to work in the opposite direction: its job is to take a disordered state (temperature inside the house and outside is equal) and make it more ordered (temperature lower inside the house than outside). In order to achieve this, it has to do a much greater amount of work to drop the temperature by one degree than your furnace would to raise the temperature one degree.