How to save energy running your central A/C

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

My 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. 

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 I 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.  

My house is already pretty well insulated.  I 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.

Central Air Conditioner Coil Shade: 

Using my TED house power monitoring system, I noticed that between the hours of 5-8PM, the duty cycle of the air conditioner goes way up. I also noticed that it was during those same hours that the setting sun is beating down on the air conditioner unit outside.

I decided to build a shade to see if that would improve anything.  

To my surprise, with the tarp shade, the power consumption of the air conditioner while it was running did not change much but the duty cycle of the air conditioner's "on" cycle, shortened.

That worked so well that I spent $100 on a couple sheets of grey PVC lattis and channel pieces.  I built a more permanent trellis shade that looks nice and still shortens the A/C duty cycle during the evening sun hours.

This shade results in a 1.9KWH/day air conditioner energy savings with the added benefit of looking less like a crime scene than the tarp. 

Central Air Conditioner Misting System:  
To cool the outside air temperature, I originally thought of building a swamp cooler around the central A/C unit outside. That cost estimate quickly rocketed up into the $400 range. I'm all for advancing science and mankind's well-being but I don't want to blow that much money this early in the game.

I ended up going with a simple misting design (with 15-20 misting heads) that would allow for more airflow to the A/C coils. There are products on the web that do just that for about $100, but they are for smaller A/C units and adding enough misters for my size A/C unit add up to over $200. Plus I didn't like the toilet flapper float design they used for turning on and off the water supply line. 

For about $45 I built my own misting system that uses an electronic sprinkler valve controlled by the existing HVAC thermostat.

 

The water feeds through from an unused hose bib near by into the sprinkler valve. 

 

How cool and and refreshing is that?  

The misting system reduces the outside air temperature, in turn reducing the pressure of the whole system, in turn reducing the air conditioner's power consumption 850 watts (or 21%) while it is running its cooling cycle. This results in a 4.8KWH/day air conditioner energy savings.

I still need to run a soft water line out to the misting system, before the A/C coil is sentenced to a corroded, rusty fate.

Attic Fans:

A couple of years ago, I installed a thermostat controlled attic fan.  I had read that venting out the heat in the attic will reduce cooling costs, (makes sense). But it also takes energy to spin a large fan for 10 hours every day.

If the house isn't properly air sealed, cool conditioned air from the house will be sucked into the attic space and vented outside. In this case, the central air conditioner will use more energy with an attic fan running.

If the attic is far under ventilated, the attic fan will not be able to draw out enough hot air to make a difference but still draw a lot of power. 

If the attic has superior ventilation (most homes, including mine don't), an attic fan will not do any good either because the attic vents are already doing the an attic fan's job only passively. 

Ever since I air-sealed my home, I have been wanting to quantify the attic fan's performance and usefulness.  I got the chance this weekend when my entire family was away and left me home alone. No stoves cooking, no kids leaving outside doors open, no TV's being left on. Just a relatively constant, measurable whole house power consumption. To help in comparing apples to apples, the weather forecast all weekend was sunny and in the low 90's .

I ran one day with the attic fan on and one day with it off.

Notice how the air conditioner has shorter duty cycles on the day the attic fan is running. 

All things being equal and even with R-38 insulation in the attic, the attic fan lowered the central air conditioner's power consumption by 20%.
Even though my attic fan draws 190 watts when it is running (190x10hrs/day = 1.9KWH), it actually reduced the central A/C usage by 6.6KWH for a net gain of 4.7KWH.
After operating for 2 summer seasons and at an electrical savings of $70/ season, my attic fan just paid for itself last year.

Between the A/C trellis shade (1.9KWH savings), the A/C misting system (4.8KWH savings) and the attic fan (4.7KWH savings), my whole house net energy savings is 11.4KWH/day or a 22% reduction. COOL! 

Shoot, with the energy saved by not using an extra 11.4KWH/day all summer long, I could drive an electric vehicle 3420 miles, or commute in it to work for 5 months, FOR FREE!

Update 7/27/2011:
I have since plumbed my hose bib to the soft water line.  Now the misting system will not corrode the coil nearly as fast; maybe not at all.