We heat and cool the guest house with a heat pump system. So, how does it work? The wise guy answer is “very well,” so far for winter operation at least.

Once you establish a couple of concepts, it’s really pretty straight-forward.

  • First concept: Heat transfers when there is a temperature differential, i.e. from hotter to colder. The key is, it’s all relative, so heat will even move from cool to colder or hot to warm.
  • Next concept: Pressurizing, causes a gas or liquid to release heat, and depressurizing causes heat absorption. When a liquid evaporates, it absorbs a lot of heat. Similarly when a gas condenses into a liquid it gives off substantial heat. You’ve probably noticed that spraying an aerosol causes the can to get icy cold. Maybe you’ve noticed how hot a bike pump can get from compressing air.

A refrigerator is a heat pump. The air inside is cold, but the system cools it further, continuing to pull heat out of already cold air by running it over a coil with an even colder refrigerant. It’s all relative. You may have noticed the heat coming out of the back of a fridge; that’s where the heat goes after being pulled from the air inside the unit.

A refrigerator does heat your house, but only enough to cool the limited air (and therefore available heat) inside. Imagine a refrigerator trying to cool the great outdoors; it would keep pulling heat out of the air all day, but would make only negligible progress against cooling the earth’s atmosphere. That’s a heat pump, an inside-out fridge.


In the diagram, the left hand side represents the outdoor unit and the right the indoor unit (as in the pics above). It’s cold outside in the winter but the refrigerant in the outdoor unit is much colder, so pulls heat from the air and evaporates into a gas. A compressor, pressurizes the gaseous refrigerant back into liquid which therefore becomes hot, hotter than the air inside the house. The indoor unit releases that heat into the house, cooling the liquid. Finally, the liquid goes through a valve that relieves the pressure, so at the lower pressure, the liquid becomes colder than the outdoor temperature. And so on.

In the summer, the system reverses and essentially operates like a super-size refrigerator. It’s a good technology to consider, particularly for more temperate climates. The Cape is relatively temperate because of being surrounded by water, but also, improved heat pump technology in recent years has extended its applicability further north.


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