A/C usage Which is best?
 

Forgive me if this has been addressed before but I was curious about a/c usage. I live in south Florida where in the summer my home reaches an interior temp of 85 degrees. I leave the a/c off while I am not home, but it takes hours for the home to cool down to 76 degrees while we sleep.

So which is better, a/c off then r un it solid for 3-4 hours or set it to 79 degrees and have it run throughout the day? To then run for an hour to cool down to 76 in the late evening.

I have an attic fan that keeps the attic at 105 degrees throughout the day. It use to sit at 123 degrees before the fan.

Thanks

I'd set the A/C to a higher temperature while you're gone. Try 6-10°F higher than when you're home for starters, then tweek it until you get the cool down period you want. You'll still save lots of energy, but the temp won't get out of control.
You could also set the A/C to turn on 0.5-1 hour before you plan to return home, that way the temp could be set higher, but you won't feel it.

Turning it off during the day and running it for one four-hour pulse before you get home should be the most efficient approach.

You have to pay for any heat that gets conducted through your walls and into your house. Heat flux approaches zero as the temperature indoors approaches the outdoor temperature. Therefore, you want the solution with the lowest average temperature difference between indoors and outdoors.

Also, I would expect the A/C efficiency to be improved by the larger difference between evaporator temperature and indoor ambient temperature. i.e., just like a furnace, it should be more efficient when the set point is farther from ambient.

For those of us who live in milder climates but still use A/C (e.g. me): I've gotten great mileage from leaving the curtains/blinds closed and the windows open during while I'm away. When I get home, if it's above 80°F in here, I'll turn on the A/C and close the windows. On days following cool nights, I'll leave the windows closed all day.

I'd agree with RobertSmalls. Just as with heat, leave it off as long as possible. Turn it on before you get home to negate the time it takes to cool back down.

It sounds like you are picking up a lot of heat from the attic.
Anything you can do to get it down under 100 up there, should help out.

If the main source of warming inside your rooms is coming from the ceiling,
you might want to think about adding a reflective radiant barrier up there.
AtticFoil® Do-It-Yourself Radiant Barrier Foil Insulation

Assuming your home is pretty air-tight:
If have a dehumidifier that's not a power hog, you could try running it during the day.

If the air indoors is somewhat dry when you turn on the AC, a better comfort level
will be felt much quicker in most cases.

If your home isn't very air tight:
Or if some of your windows are like these..
http://www.2dynamics.com/Windows/Jalousie_Window.jpg

You might want to consider a ceiling mounted heat exhaust fan. (Whole house fan).
Which would suck in outdoor air, but would remove the even hotter air
that accumulates near the ceilings of your living spaces.
Whole House Fans — A Scenario | Brian Irwin || Windermere Real Estate Co. ||

Thank you, my concern was not walking into a hot house, I can handle that.

It was simply the economics between letting the air run when you are not home or letting it run for a 4 hour block before it reaches the preset temp we use.

I am learning quick that attic solutions for keeping the temp down is not easy in South Florida. Our strict hurricane code keeps me from opening holes for fans and such. I will likely just increase the size of the fan I have in the attic now to circulate more air through the exhaust vents along the bottom of the roof line.

Luckily my garage door also has vents along the bottom allowing me to leave the attic door open when the fan is on drawing cooler air from the garage into the attic.

Wow! 76 degrees is COLD! I have mine set at 80 during the day when I am home and 82 when I go to bed.
You do not mention it so I will assume you do not have a programmable thermostat.
Go to Walmart, Lowes or wherever and get one.
If you usually get home at the same time everyday just set it to come on an hour or two before to start cooling the house.
I would set it to about 90 when you are not at home. No need to cool the house when no one is home.
Another thing to focus on is the attic. How much/what kind of insulation do you have?
Might be time to replace or put more insulation.
You need to put a radiant barrier in the attic.
Xringer's suggestion is a good one. You can also buy a radiant barrier paint and paint it on yourself or hire someone to do it.
Paint is your best bet.

Air sealing? Make you house tight then sit back, relax and enjoy comfort town.Run the AC less and stay cooler. This kind of thing is more apparent for us in heating dominated climates. If the house is leaky you're cold in the winter, no matter what you set the thermostat to. A leaky house doesn't feel like a 70 degree day in the summer when the thermostat is set to 70 in the winter. The leaks creates constant convention cycles in any room that abuts an outside wall. The cold air from the walls sinks and the hot air from the room rises and it happens at the wall. There is constant motion of the air in the room due to convection. The same is probably happening to you but in reverse.

What color is your roof? a zinc oxide white roof should be the standard in sunny hot areas, radiant barrer on top of attic insulation should be your next step along with R50 or more of insulation in your attic as that as where a great deal of your heat is coming from.
Awnings, sun blocking shades and shade trees on the south side of your house are also going to do a great deal, when you leave for the day are you leaving your house with every single shade pulled? after all a dark house is a cool house and if no one is home why not pull every shade?

Thoughts from Phoenix
 

In PHX we have an art of managing our A/C. Our power companies of have time of day plans where power costs more during the hottest time of the day. We invested in a programmable thermostat (about $100 at Home Depot/Lowes). We cool our house down at night (cooling from 90 to 75 is less taxing on the A/C than cooling from 110 to 75). We have the thermostat set for 84 during the day, program the thermostat to start cooling to 78 about a 1/2 before we get home and then down to 75 at bedtime. Fans don't do a lot if you are not in the room, but are a must when you are! Your utility company may also have rebates or discounts on solar screens - windows are a major heat source. Shade trees are also helpful. Make sure to check the weather stripping around doors and windows. Older houses may need to have duct work sealed. Once last thought - make sure to change your air filter at least once a month. Good luck!

Did the OP abandon his thread?

Hes not the most frequent poster. He'll come around I'm sure.

You're in texas. He's in Florida. The humidity in Florida is far, far higher. I remember first stepping foot on The Florida Peninsula at the Miami airport. Felt like stepping into a turkish bath. We keep the thermostat at 73-74.

I would second the radiant barrier suggestion and keeping the thermostat at 84 while out and down to the desired temp by the time You get home.

Don't bother with regular insulation. It will just give off stored heat when You least want it. You want a radiant barrier in the attic. Maybe some regular insulation on top of it to keep dust away so as to limit the reflective barrier efficiency. Possibly coolseal or other white reflecting roof covering for the roof if You have dark shingles/black tar up there.

I'm a bit confused by that statement.
A radiant barrier should be on the underside of the roof decking. Either painted on or a foil that is already glued to the roof decking, I have the latter.
I have seen the rolls of radiant barrier that you staple to the trusses or lay on top of the insulation in the attic.
But either way I have always read that you do not want to have insulation over the radiant barrier.

That's backwards to my understanding as well.

I've always seen
shingles/metal/... - radiant - roof decking - trusses - air gap - insulation - ceiling
or
shingles/metal/... - roof decking - trusses - radiant - air gap - insulation - ceiling

I've never heard it from anyone either. I'm just saying having something on top of the radiant barrier to collect the dust will prolong the effectiveness of the barrier, not sure if it will help insulation much. Otherwise You will be changing the radiant barrier or flipping it on the other side once it collects dust. Maybe cheap air filter material instead?

They do make "radiant barrier chips" which will stay effective even when the top is covered with dust cause You have multiple layers of the stuff, though they are more expensive.

Sorry, but it isn't the easiest thing for me to remember to check in all the time... Anyhow an interesting development. The fan that I was using to circulate the air broke :(. So I figured I would go without for a bit and see what happens.

Now to preface this I still left the opening to my attic through the garage open and my garage door it vented at the bottom. With that said, there has been no change in my attic temperature. Now some of that can be due to the fact that the hottest summer days are behind us now, but I am also questioning the effectiveness of a fan to circulate the air.

Now I am contemplating an all out solar extractor for the roof, I finally saw a company who is rated for the hurricane code in S. Florida. Otherwise I was going to invest in one of those extractors that pulls air from the garage and dumps it into the attic forcing a larger volume of air out through the vents along the lower roof line.

SO TTFN while I continue with my new fish tank in the other thread ..lol

So there is an access panel in the garage to the attic? If so I would get a box fan and build a frame around it so it will force air in. Definitively get a programmable t-stat and test it on the per set points and adjust it from there the Honeywell unit I have even has a setting that will let you select how quickly you want the house to cool when it reaches the new set point.

As long as I had a radiant barrier on the floor of the attic I wouldn't concern myself much with temperature and, as far as ventilation, would just make sure that I have enough bottom vents (soffits/eves) and top vent(s) (Wind turbine Roof Fan) for proper airflow. And I mean TOP vent(s) for proper airflow, not middle.

The attic temp would actually be higher because the radiant barrier would block so much heat back into the attic, but below the barrier and the inside would BE and, more importantly, FEEL much cooler because 5% of HEAT is transferred through conduction, 20% through convection, but 75% through RADIATION

Unlike radiant barrier regular insulation does not block heat. The R value of regular insulation rates resistance to CONDUCTIVE heat transfer and is not applicable to a radiant barrier.

Without a radiant barrier, by the time You are back from work, a lot of is absorbed and STORED in Your insulation and is ready for You to FEEL it's RADIANT glow even though Your thermostat is trying to convince You that the AIR TEMPERATURE is "comfortable". (Unless You have a thermostat that measures "mean radiant temperature" or a "globe thermometer")

BTW, apparently It is OK to put a radiant barrier on top of existing insulation AS LONG AS IT IS NOT A VAPOR BARRIER. (So perforated or chips would do.)