How a HVAC contractor chooses a portable air conditioner that won"t rip you off
There is a natural tendency for people, especially Americans, to always opt for the biggest and best name brand air conditioner they can possibly afford. The prevalent calculation is, the bigger the window unit, the better it will work. It will cool the area faster, and thus won't have a major impact on your energy bills.
Hold on there.
A HVAC pro knows what the homeowner doesn't. In this case, the correct 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 tied to the size of the room. But this is where things get messy. This is true only in perfect, or ideal, conditions. In other words, this assumes average to excellent insulation in the walls, a good newer window that will be sealed and has a good insulation value. Placing an air conditioner in a room assumes you have to change the original estimation given on the sticker.
For example, a 5,000 btu A/C with a 9.7 EER rating may promise it will efficiently cool about 1500 square feet without a problem. This is roughly equivalent to a 15x10x10 room. That's assuming ten foot ceilings.
So you naturally buy the unit for a 15x10 room in your dwelling, and it fits fine in the window, but it doesn't appear to be working as well as you'd like. Why not?
Simply put, the insulation or window R-value may not be as good as you assumed.
The best way to measure whether or not an A/C unit will work in your room is with the heating estimator calculator in the Home Energy Audit Kit. The kit makes the calculation easy - all you do is give the dimensions of the room and enter your insulation. This is an important step that homeowners always miss - but HVAC contractors never do. Insulation is in your walls to mitigate the heat gain. The higher grade the insulation, the less heat is lost.
So, let's go with the assumption that our room is 15x10x10. Furthermore, let's say that the unit we want is at least 85% efficient,or not high efficiency, and the approximate temperature outside is 85 degrees F. Our target is a cool and comfortable 76 degrees F in the house. And lets assume our insulation is just average.
Opening the calculator, we find that at its most efficient state, the A/C unit would have to output 6728 BTU, or about 34% more cooling power than the unit we have can possibly provide. Therefore, the A/C unit isn't acceptable.
Let's pretend we select a 93% efficient model, or an energy star one. Our btu need goes down by a few hundred to 6260. That's still relatively insufficient.
As you might have guessed, there are 2 culprits here. The first is the insulation. Average insulation is dragging us down by about half in this instance. We know this because if we adjust our insulation grade to "excellent", and keep everything else constant, our BTU need reduces to 3130 - which means under ideal 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 ratings for room size to be under perfect conditions.
So right now you're pondering "Well what can I do to improve the efficiency of the unit?" Well, you can't improve the efficiency of the A/C directly, but you can make the temperature gradient less drastic than the 9 degrees from 85 to 76. In fact, adjusting the temperature requirement to 78 degrees F, and keeping everything else the same gives us a completely different result. 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. Drawing shades will help - and keeping the door to the room closed will prevent cooled air from escaping.
There are a few lessons to take away from this. First, now you can understand why changing the thermostat on your air conditioner is such a bad idea, unless you increase the temperature - the cost for running the unit increases dramatically. You also can see how important good insulation is, and why measuring a room before you go to the store is so necessary. HVAC contractors always make these measurements and calculations prior to going to the store, and then select the best, most efficient model in a price range that will best work in the room.
Hold on there.
A HVAC pro knows what the homeowner doesn't. In this case, the correct 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 tied to the size of the room. But this is where things get messy. This is true only in perfect, or ideal, conditions. In other words, this assumes average to excellent insulation in the walls, a good newer window that will be sealed and has a good insulation value. Placing an air conditioner in a room assumes you have to change the original estimation given on the sticker.
For example, a 5,000 btu A/C with a 9.7 EER rating may promise it will efficiently cool about 1500 square feet without a problem. This is roughly equivalent to a 15x10x10 room. That's assuming ten foot ceilings.
So you naturally buy the unit for a 15x10 room in your dwelling, and it fits fine in the window, but it doesn't appear to be working as well as you'd like. Why not?
Simply put, the insulation or window R-value may not be as good as you assumed.
The best way to measure whether or not an A/C unit will work in your room is with the heating estimator calculator in the Home Energy Audit Kit. The kit makes the calculation easy - all you do is give the dimensions of the room and enter your insulation. This is an important step that homeowners always miss - but HVAC contractors never do. Insulation is in your walls to mitigate the heat gain. The higher grade the insulation, the less heat is lost.
So, let's go with the assumption that our room is 15x10x10. Furthermore, let's say that the unit we want is at least 85% efficient,or not high efficiency, and the approximate temperature outside is 85 degrees F. Our target is a cool and comfortable 76 degrees F in the house. And lets assume our insulation is just average.
Opening the calculator, we find that at its most efficient state, the A/C unit would have to output 6728 BTU, or about 34% more cooling power than the unit we have can possibly provide. Therefore, the A/C unit isn't acceptable.
Let's pretend we select a 93% efficient model, or an energy star one. Our btu need goes down by a few hundred to 6260. That's still relatively insufficient.
As you might have guessed, there are 2 culprits here. The first is the insulation. Average insulation is dragging us down by about half in this instance. We know this because if we adjust our insulation grade to "excellent", and keep everything else constant, our BTU need reduces to 3130 - which means under ideal 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 ratings for room size to be under perfect conditions.
So right now you're pondering "Well what can I do to improve the efficiency of the unit?" Well, you can't improve the efficiency of the A/C directly, but you can make the temperature gradient less drastic than the 9 degrees from 85 to 76. In fact, adjusting the temperature requirement to 78 degrees F, and keeping everything else the same gives us a completely different result. 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. Drawing shades will help - and keeping the door to the room closed will prevent cooled air from escaping.
There are a few lessons to take away from this. First, now you can understand why changing the thermostat on your air conditioner is such a bad idea, unless you increase the temperature - the cost for running the unit increases dramatically. You also can see how important good insulation is, and why measuring a room before you go to the store is so necessary. HVAC contractors always make these measurements and calculations prior to going to the store, and then select the best, most efficient model in a price range that will best work in the room.
