Monday, July 25, 2011

Making Your Central A/C Use Less Energy


Isn't central A/C wonderful? Its so cool and comfortable but on the other hand, so darn expensive. 

Our last house had an evaporative cooler on the roof. It's basically a system that draws hot, dry air through water soaked pads. The heat in the air is absorbed when the water evaporates.  Hot, dry air goes in and cool, more humid air comes out. When the humidity is low, it works beautifully. When it's muggy outside, well, that's where the evaporative cooler gets it's “swamp” nickname. In a dry climate like Utah, the swamp cooler is extremely efficient, somewhere on the order of 48 SEER. A typical central A/C is only 10-18 SEER.

Psychometric chart

A properly tuned and maintained swamp cooler will produce air that is a couple degrees above the wet-bulb temperature. So if it's 110 °F outside with only 10% humidity, the wet-bulb temperature will be 68 °F and the air coming out of the swamp cooler will be about 71°F. Not a bad system for a really dry climate.  But in humid climates, Swamp Coolers WON'T WORK!

A few weeks ago, I was pondering the idea of combining a central A/C with a swamp cooler. They do it all the time in the chillers on large commercial air conditioning systems. Why don't A/C manufactures offer it in their products to homeowners?

After a few hours reading on the home HVAC installer forums, I learned that people who ask this question, “Why not wet down a central A/C?” are flogged and burned at the stake so to speak.

Evaporating lots of water on thin metal cooling fins is the quickest way to rust and corrode them. A modest savings of power up front will end up costing the homeowner a new A/C unit in a few years.




Being an exceedingly curious man, I just can't put the issue to bed that easily. Yes, I can see how wetting down cooling fins with hard water is just asking for trouble, not to mention increased maintenance and the potential for flooding problems. But I believe that under the stretched out tent walls of a generalized bad idea lie stringent ruled, brilliant ideas.

To protect the uneducated public from injury and damage, an idea that is dangerous some of the time is stereotyped as a bad idea all the time.

For example, “Never put metal in the microwave”. Now this might seem like good advice to follow because we have all been raised to believe it is true in all cases; that it is dangerous to put metal in the microwave.

But you forget that microwave ovens are made of metal. Some microwave ovens even come with metal shelves and metal temperature probes. “No metal in the microwave oven” is a contradiction. The more correct statement should be, “Never put metal in the microwave oven when the dielectric properties and electrical dimensions of the metal and food pieces will cause excessive heating and current flow across the metal.” To simplify our lives by not having to crunch a bunch of energy density functions and calculations each time you need to re-heat something, it is generally easier just to not put any metal in the microwave oven.

There will always be exceptions to rules and contradictory exceptions to those exceptions.


FYI#1:You can defrost an open can of juice concentrate in the microwave oven, even though the bottom of the can is made of metal.

FYI#2: Re-heating in a microwave oven an Arby's roast beef sandwich shrouded in its metal lined paper wrapping will cause it to catch fire. Right Marv?


Just as with exceptions to the "no metal in the microwave" rule, I believe that under the right circumstances, cooling the A/C coils with water (softened water) is a good idea and will save energy without significantly shortening the life of the A/C system itself. If I am wrong, I get to buy a more energy efficient A/C coil to replace the one we currently have.


It is my goal to confirm the following statements:

  • Removing direct sunlight by shading the outside A/C coil (without limiting airflow) will lower the power consumption of the compressor during those hours of direct sun-light.  
  • Lowering the temperature of the air conditioner coils (outside unit) will lower coolant pressure, lower compressor current draw and make the A/C run more efficiently.
  • Adding powered attic ventilation in a properly air sealed home will make the central air conditioner run less and reduce overall power consumption.

I know that doing the above steps could potentially save energy. How much? Well, that's where experimenting comes in.  

Our house is already pretty well insulated.  We also air sealed major air penetration areas of the home, (recessed lights, plug sockets, windows) and added more passive attic ventilation. These low lying, energy saving fruits are already harvested.