There is a natural tendency for people, especially Americans, to always opt for the most expensive and most efficient window unit A/C they can possibly afford. The prevalent thought is, the more efficient the air conditioner, the better it will work. It will cool the room faster, and thus save you money.
Not true.
A HVAC pro knows what homeowners forget. In this circumstance, the best air conditioner should be selected for the room it will be placed in.
Under most conditions, the btu, or cooling power of the unit is directly proportional to the dimensions of the room. But where homeowners start to go astray. This is true only in perfect, or ideal, conditions. Basically, this assumes average to excellent insulation in the walls, a good quality window that will be sealed and has a good insulation value. Placing an air conditioner in your house makes you revise the original estimation given on the sticker.
For example, a 5,000 btu room A/C with a 9.7 EER rating may promise it will efficiently cool about 1500 square feet efficiently. This is about a 15x10x10 room. That's assuming 10 foot ceilings.
So you obviously purchase the unit for the appropriate room in your dwelling, and it fits fine in the window, but it doesn't appear to be working as efficiently as you'd thought. Why is that the case? What could be wrong?
In practice, the insulation or window rating may not be as good as you assumed.
The most accurate way to determine whether or not an air conditioner will work in your room is with the heating estimator calculator in the Home Energy Audit Kit. It makes the calculation trivial - all you do is find the dimensions of the room and enter the insulation. This is a critical step that homeowners always miss - but HVAC contractors never do. Insulation is in your walls to mitigate the heat loss. The higher grade the insulation, the less heat is lost.
So, let's assume that our room is 15x10x10. Furthermore, let's say that the A/C unit we want is approximately 85% efficient (so its not a high efficiency energy star model) and the mean temperature outside is 85 degrees F. We want it to be a cool and comfortable 76 degrees F in the house. And lets pretend our insulation is just average.
Opening the calculator, we find that at its most efficient state, the air conditioner would have to output 6728 BTU, or about 34% more cooling power than the unit we bought can ever output. Therefore, the window unit we have isn't sufficient.
Let's say we select a 93% efficient model, or an energy star one. Our btu need goes down a little to 6260. That's still relatively inefficient for our needs.
As you can see, there are 2 problems here. First, the insulation. Average insulation is reducing overall efficiency by about half in this case. We know this because if we modify our insulation grade to "excellent", and keep everything else constant, our BTU need reduces to 3130 - so under our conditions, the 5000 btu A/C unit will work fine.
The second culprit is the misleading advertising of the unit. You should always assume the manufacturer's estimations to be under ideal conditions.
So right now you're wondering "Well what can I do to improve the efficiency of the air conditioner?" Unfortunately, you can't improve the efficiency of the A/C in itself, but you can force the unit to work less by turning up the thermostat from 76. In fact, adjusting the temperature requirement to 78 degrees F, and keeping everything else the same saves a lot of energy. Now, we need 5233 btu for an 85% efficient unit, and 4869 btu for a 93% efficient unit. These are both within range for the 5000 btu unit.
Keep in mind you can also mix the cooled air with a ceiling fan. Closing shades will help - and keeping the door to the room closed will prevent cooled air from escaping.
There are important lessons to learn from this experiment. First, now you can see why changing the thermostat on your air conditioner is not a good idea, unless you adjust up the temperature - the cost for running the unit increases incredibly. You also can see how vital good insulation is, and why finding the volume of a room before you go to the store is so important. HVAC contractors always make these measurements and calculations before going to order, and then select the top, most efficient model in a price range that will best work in the room.
David Andersen has been saving on energy since 2003 and is currently researching energy efficiency techniques. David is a firm believer in monitoring energy costs and energy conservation. He is the owner of a free home energy saving blog, with exclusive valuable information no homeowner can be without ==> http://www.utilitybillbusters.com
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