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Heat Pump

When you think about cooling a hot building, you probably don't think of heat pumps. In fact, the words "air conditioner" are likely the first things that come to your head unless you're tight with your pennies. Then you might go with "window fans." As it turns out, a heat pump can both heat and cool, and in some applications, it's preferred to separate heating and cooling systems.

Simply put, a heat pump is a device that uses a small amount of energy to move heat from one location to another. Not too difficult, right? Heat pumps are typically used to pull heat out of the air or ground to heat a home or office building, but they can be reversed to cool a building. In a way, if you know how an air conditioner works, then you already know a lot about how a heat pump works. This is because heat pumps and air conditioners operate in a very similar way.

One of the biggest advantages of a heat pump over a standard heating ventilating and air conditioning (HVAC) unit is that there's no need to install separate systems to heat and cool your home. Heat pumps also work extremely efficiently, because they simply transfer heat, rather than burn fuel to create it. This makes them a little more green than a gas burning furnace. And they don't just heat and cool buildings. If you've ever enjoyed a hot tub or heated swimming pool, then you probably have a heat pump to thank. They work best in moderate climates, so if you don't experience extreme heat and cold in your neck of the woods, then using a heat pump instead of a furnace and air conditioner could help you save a little money each month.

Heat Transfer and Air-Source Heat Pumps

There are many different kinds of heat pumps, but they all operate on the same basic principle -- heat transfer. This means that rather than burning fuel to create heat, the device moves heat from one place to another. There's a key to making this all happen -- heat naturally flows downhill. This means that it tends to move from a location with a high temperature to a location with a lower temperature. Pretty simple. What a heat pump does is use a small amount of energy to switch that process into reverse, pulling heat out of a relatively low-temperature area, and pumping it into a higher temperature area. So heat is transferred from a "heat source," like the ground or air, into a "heat sink," like your home.

One of the most common types of heat pumps is the air-source heat pump. This modern technology takes heat from the air outside your home and pumps it inside through refrigerant-filled coils, not too different from what's on the back of your fridge. The air source variety is pretty basic, and you'll find two fans, the refrigerator coils, a reversing valve and a compressor inside to make it work.

This system is more commonly known as an air-air heat pump, because it takes heat from outdoor air and transfers it to indoor air ducts. With the right kind of modifications, air-source systems can also work with other types of indoor heating systems.

The key to allowing the air-air heat pump to also cool is the reversing valve. This versatile part reverses the flow of the refrigerant, so that the system begins to operate in the opposite direction. So instead of pumping heat inside your home, the heat pump releases it, just like your air conditioner does. When the refrigerant is reversed it absorbs heat on the indoor side of the unit and flows to the outside. It's here that the heat is released, allowing the refrigerant to cool down again and flow back inside to pick up more heat. This process repeats itself until you're nice and cool.

Air-Source, Ground-Source, and Absorption Heat Pumps

By now, you've learned that air-source heat pumps use an outdoor fan to bring air over refrigerant-filled coils. Two sets of these coils transfer this heat indoors, where it's then blown away from the coils by a second fan, and distributed through your home as cool goodness. Some air-source heat pump systems consist of a single packaged unit containing both sets of coils in one box. This box is then installed on the roof of a building with the ductwork extending through the wall. You'll see a lot of larger systems for commercial buildings installed in this way. Home heat pumps are usually split systems with an outdoor and an indoor component installed through the wall. Depending on the type of system, there may be one or more indoor components to distribute heat.

Ground-source heat pumps are a little different. They absorb heat from the ground or an underground body of water and transfer it indoors, or vice versa. The most common type of ground-source heat pump transfers heat directly from the ground by absorbing it through buried pipes filled with water or a refrigerant. These liquid-pumping pipes can be either closed-loop or open-loop systems, and they operate pretty much exactly how they sound. In a closed-loop system, the same refrigerant or water circulates through the pipes repeatedly. In an open-loop system, water is pumped out of the underground water source, like a well or a man-made lake. From there, the heat is extracted from the water, and that water returns to the well or surface lake. More water is then pumped from the well to extract more heat in a continuous open loop.

If that's not enough to blow your mind, consider theabsorptionheat pump -- air-source pumps that are powered by natural gas, solar power, propane or geothermal-heated water, rather than by electricity. Absorption pumps can be used for large-scale applications, but are now available for homes on the larger side. The main difference between a standard air-source heat pump and an absorption pump is that instead of compressing a refrigerant, an absorption pump absorbs ammonia into water, and then a low-power pump pressurizes it. The heat source then boils the ammonia out of the water, and the process starts all over again.

When you go to check out an absorption heat pump, it helps to know how they're rated. Manufacturers rate them using a measurement called a coefficient of performance (COP), which sounds pretty complicated. All you need to know is to look for a COP above 1.2 for heating and above 0.7 for cooling. And don't worry, we'll discuss ratings for standard heat pumps a little later.

Air-source, ground-source and absorption heat pumps are the most common kinds of heat pumps, but they won't work in every situation. Read on to learn about special kinds of heat pumps.

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