Practical thermodynamics


The following was a response to a friend's email request for help.




I can't get my house below 80 degrees during the day and often only 75 at night while it is set at 72 at night.  Help me figure this one out please.
I'm having similar problems.  The main culprit is your air conditioning.  The reason why it is excessive is because the heat this summer has been both intense and constant (even into the night).  Most people don't realize that electricity costs aren't a linear phenomenon.  There's a tipping point, and this year we moved past it and stayed there.  If it's a little hotter, it doesn't mean that the air conditioner has to work just a little harder.  In fact, at some point, your air conditioner does essentially no good.  Here's how it works:

The heat in your house is collected by freon (or a chemical with similar properties in more recent systems).  The freon is in liquid form because that absorbs heat more efficiently.  The freon (still liquid) is moved outside your house to the compressor.  That's the thing with the fan blowing hot air and running constantly right now.  What happens here is that the freon is allowed to decompress (yes, the name is backwards), and it gives off its heat.  This is exactly what happens when water (e.g., sweat) evaporates off of you to cool you down.  The change from liquid to gas uses whatever available energy there is, and heat is the most available energy, so it is taken out of the environment (i.e., it gets cooler) in order to evaporate the moisture.  (You can feel this even more strongly with rubbing alcohol, perfume, or anything else that evaporates quickly.)  The cool freon is pumped back into your house while the compressor blows away the hot air.

Here's the problem though.  The assumption in the design of this system is that the amount of heat collected inside the house will make the coils hotter than the temperature outside.  Right now, there isn't much of a difference in temperature because the coils are, say, 115 degrees while the air around them is maybe 110 (It wouldn't be nearly this big an issue if it was 85 outside, for example).  The heat doesn't get blown off of the coils because you can't cool anything this side of molten lava by blowing hot summer air on them.  The (still) hot freon goes right back inside and your compressor, blower, and everything else keep right on running because they haven't brought the temperature inside down to what you would like it to be.

Incidentally, in hotter, dryer environments such as in El Paso, TX, the air temperature is similarly sweltering, but they use "swamp coolers" instead of compressor units like ours.  Instead of air, they pour water over the coils, and that (rather than air) carries the heat away.  Unfortunately, these are ineffective in conditions with higher humidity.  They fail even in El Paso when the humidity rises beyond a certain point.


Some tips
Those are all things pertaining directly to the air conditioning itself, but you can do a lot to ensure your house doesn't produce more heat internally than you need to.  For example:
  • Wash your dishes and do your laundry at night.  You're making heat that your air conditioner has to get rid of.  And it has to work harder (i.e., run longer) during the day.
  • Use your clothesline whenever you can.  I put a set of blankets out the other day, and they were dry in a couple of hours. There's too damned much heat available for free (all summer long!) to be buying more from the electric company.
  • If your water heater is indoors (mine is on the back porch since this is a retro-fitted 80 year-old house), get one of those Little Gray Boxes from the hardware store.  This is a timer that will cut back on how often it runs.  Additionally, you can probably cut the thermostat back on it so it isn't getting as hot.  Further, make sure the pipes are insulated.  You will literally burn yourself on them.  That's heat you paid for that's just escaping all the time otherwise.
  • Don't put anything hot into your fridge.  That's like paying twice to cool it down.  Conversely, have some foresight, and let anything frozen thaw on its own before you pay to microwave it.  I used to put tv dinners on the back of my monitor for a couple hours before supper (this was back before I had a flat-screen).  Even with all that heat, they would still be icy most of the time.
  • Keep your fridge filled with stuff.  This makes it run more efficiently.  Also, when you open the fridge, you have cold *stuff* not cold air.  Cold stuff doesn't run out of your fridge.  Cold air does.  (If you have any extra styrofoam -especially half-inch sheets- put those inside the fridge -especially along the walls- to take up space and to insulate it better.)
  • Turn off your lights and other appliances.  In addition to using (read: wasting) electricity, they also produce a lot of heat you are then paying to get rid of.

I changed the filter and I cleaned around the outside unit thing.  Do I need to have someone come look at my central air unit?  Any ideas would be greatly appreciated.
Those are both great things to do.  That increases the efficiency by getting the air flow maximized.  You can also try to keep the compressor cooler by having it shaded (although you don't want to put something above it that impedes the air, obviously).  If it is a dark color, then you probably want to color it lighter (e.g., optical white, ideally).  Some of the lines leading to it ought to have foam insulation on them so the cool isn't lost before it gets into your house.  (Note: One of the lines is leaving your house hot and therefore shouldn't be insulated.  A third line carries electricity; it should be obvious which one that is.)

As far as the rest of the house, one of the most important things is ventilating your attic.  I would have to check yours out to see if it is effective as it is or if there is anything more that can be done.  I would also try to find places where you could shade the house.  If you have shrubs and trees shading the house, then that's that much less heat that makes it into the house that you have to remove.  A light-colored house helps as well, especially a light-colored roof, although no one seems to make anything like that.  Black seems to be accepted, even though it costs Americans literally billions each year.






Copyright 2006 Alexplorer.
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