Air Conditioning

flame-ice-cubes-dreamstime m 7529210-640x425If you didn't know it already, the term Btu stands for "British thermal unit". I don't know what gave the Brit's the right to have their own thermal unit, but let's hope they don't ask for it back any time soon, as it is so much easier to comprehend than the metric equivalents using Joules, Calories, etc.

It's common to see the cooling and heating capacity of air conditioners, refrigerators, furnaces, etc., being shown by their Btu ratings or Tons (1 Ton of refrigeration = 12,000 Btu), but what exactly is this Btu? What does it look like? And does The Queen have one?

Simply put, one Btu is the amount of heat required to be added to, or extracted from, one pound of pure water in order to raise or lower its temperature by one degree Fahrenheit. This is about the same amount of heat as given off by a match that's used to light a fire.

So, if we have one pound of water at 100°F and add 40 Btu to it, its temperature will rise to 140°F. If we removed 40 Btu's from our one pound of water, its temperature will drop to 60°F. If you are all following this so far it's not too surprising, as what I've described above is known as "Sensible Heat", and, as you are already reading this, then obviously you are all very sensible people!

Now, if we keep on adding Btu's to our one pound of water, it will keep rising in temperature at a linear rate of 1°F for each Btu added until it starts to boil, which at sea level will be 212°F (100°C).

At boiling point something very interesting happens....

glare-heat-personIt seems that there's an ongoing quest in some circles for the holy grail of boating comfort: 12 volt air conditioning. Yet few seem to fully appreciate the benefits of the more practical, but far less complex and substantially less expensive alternative: 115v AC air conditioning powered by a DC source through an inverter. Many boats these days already have inverters installed, and if not, there's a wide range of inexpensive models available that are suitable for powering small air conditioners.

This post will focus on small air conditioning systems that are suitable for sleeping cabins or for small boat applications. With a small unit, the DC current draw is low enough that they can usually be powered by an engine alternator whenever the engine is running, as well as from the batteries when required. Of course, any sized air conditioner can be powered by batteries through an inverter, it is just a matter of practicality.

To better understand the differences between the two concepts, some comparisons are shown below between the Climma Compact 4,200 Btu 115v AC unit and the Dometic Cuddy II 3,500 Btu 12v DC model.

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You want to go for an overnight boat adventure, but it's hot out there, even on the water. During the day, no problem, there is wind, a bimini, swimming to keep you cool, but at night, when you want to sleep, the thought of that hot, humid cabin can be a deal killer.

Solution: a small air conditioning system that is perfect for a sleeping cabin, to lower the cabin's humidity and temperature.

With a small 115v unit, the current draw is low enough that it can usually be powered by an engine alternator during the day when the engine is running and then from the batteries, through an inverter, at night. You may not need a generator, if you choose the right air conditioning unit.

There are also 12v air conditioning units available, but only one that is really worth looking at, and even then, the cost may make the venture unattainable.

To better understand the choices, let's compare two units:

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