..."Or how to cool your house for less money and be more comfortable while limiting carbon emissions".
     More attention is paid to CO2 created by the driving habits of Americans than to the carbon impact of heating and cooling our houses. Many are surprised to learn that the latter total is considered to be bigger than the former. But it seems so "do-able" to get a more efficient car and/or drive less, compared to modifying our houses for greater efficiency.
     But there are a good many steps one can take to reduce household emissions; here we discuss how to air condition to save energy and reduce carbon dioxide. First, we discuss the effect of dew point on comfort, and then suggest some smart moves to make.
Dew Point?      Many readers already know that dew point is the temperature at which the water vapor in a given volume of air produces relative humidity of 100%. Two air masses at the same temperature can have very different dew points, and will produce surprising differences in human comfort. For example, if the temperature is 78 degrees and the dew point is 58 degrees, almost everyone feels quite comfortable because there is adequate "evaporative cooling" (meaning that your perspiration dries and cools you). But, a temperature of 72 degrees with a dew point of 70 degrees will make most people feel uncomfortably warm, so they will often set the air conditioning down to 68 degrees and even then sometimes put on a sweater because it feels chilly and damp.
     So what? The problem is that thermostats measure temperature, not comfort, and they cause air conditioners to keep cooling your house only until the set temperature is reached. That sounds OK, except that HVAC companies almost invariably recommend and install roughly twice the necessary "tonnage" of air conditioning equipment, both for new construction and for re-fitting. They do this partly to be sure the house can be kept cool on the worst day of the summer, and (of course) to sell more equipment. The net effect is that there is almost always too much AC "muscle", so the house gets cool long before the air has a chance to dry out, which would have made the residents feel comfortable at higher temperatures.What's To Be Done About It?      When you are selecting new air conditioning gear, always insist on having about half the tonnage that the industry recommends. Or have two small units, with the ability to run one alone. Next, buy one or two dew point meters. You can find them on the web and will offset their cost with the money you are about to save.
     As soon as the temperature gets above 75, check the dew point outside, and if it is above 64, turn on the air conditioner so that it will start to remove moisture from the air. Let it run on a low setting for quite some time as it takes a while to extract the water vapor. A second dew point meter inside, which is less necessary, lets you know how you are doing.
     Ultimately, you can stabilize the air in the house at a low dew point (below 60) and then the temperature can be several degrees warmer without human discomfort. And you save money while reducing GHG's. Our house, in the summer, is often the same temperature as the outside, somewhere in the mid to high 70's, but the relative humidity is 30 or 40 percentage points lower, and it is really comfortable.
     And our air conditioner runs a lot less than it would if we tried to keep it cooler.
     Think of it this way. A too-big AC running 50% of the time and drawing 3000 watts consumes 36 kilowatt hours per day, keeps the house cold and damp and costs over $5 per day. On the other hand a "too-small" AC running even 80% of the time draws 1000 watts, consumes 19 kwh per day, keeps the house warm, but comfortably dry, and costs more like $2.65 per day while saving nearly half the GHG emissions.The net effect is that you can keep your house more comfortable than before by running less cooling for longer periods. And, if you put the capital you saved by not overpowering the AC into more efficient equipment (which does cost more), you can save more than a third of the power for the same tonnage.
     It is a triple win situation. You are more comfortable, you spend less and the environment is improved.
     The United States suffers from several examples of this kind of mindless energy waste; it stems from our energy being so cheap that it almost seemed free. And we failed to recognize early that the carbon dioxide being dumped into the atmosphere was damaging the climate. If we had paid "dumping fees" for that a long time ago, we would be better off today.
   - Douglas Ayer