Which is the more efficient use of air conditioning a house. Letting the house temp. build up to 86 degrees, and then turning on the AC set to 78 degrees or simply keeping the house at 78 all the time. My gut feeling is that the first is less expensive, but not by much.
It costs less to set the temp up when you are not home, and turn it down when you get home. But remember it will take time to get down to the lower set point, that is unless your system is over sized.
I have seen this question come up several times in reverse, i.e. should you run your heating all the time. Regardless, the answer is the same - don’t run your heating/AC when you don’t need to, it is always a waste and actually quite significantly so, depending on your home’s insulation.
It is more efficient to let your house go to 86 and then cool it to 78. To keep your house at 78 around the clock will mean your AC is running for a lot longer time. I think the difference is significant. In my house in Texas, setting the AC to 78 means it would run almost all day. At 82 it would be off for significant amounts of time. At 86 it might come on only a couple of times.
One reason is that you lose the cold air faster as the temperature difference gets greater. Imagine there’s a hole in the wall that gets bigger as the air gets cooler. When your house is very cool, this hole is very big and lets lot of heat in. When your house is warm the hole is smaller and it takes longer for the heat to come in. Say the outside is 90. Your house will go from 78 to 79 much faster than from 86 to 87.
You can do many things to make your house more efficient at keeping the cold in. Install a radiant barrier in the attic which will mean your ceiling won’t get as hot. Add more insulation. Seal up the ducts in the attic so they don’t leak air. Seal up cracks around windows so that the cool air stays in the house longer.
From a simple heat transfer point of view, you are exactly right. Whether heating or cooling, it takes more energy to maintain the current temperature inside the home than it does to let the home get closer to the outside temperature and then bring it back to the desired setpoint.
From a practical point of view though you don’t always want to do this. Many modern heating systems will turn on a less efficient auxiliary heat if the temperature differential is too great, negating the savings you would otherwise get (on some systems you can turn this behavior off). Also, if your area gets cold enough, you don’t want to let your pipes freeze or they can burst. Sometimes the pipes run in outside walls and depend on heat from inside the house to stop them from freezing, and will freeze and burst before the interior of the house reaches freezing temperatures.
Leaving the AC off in some areas can also lead to trouble, especially since the AC does double duty as a dehumidifier. I’ve heard of people ending up with horrible mold problems from leaving their AC off in areas like Florida.
On a somewhat related issue, I was asking an HVAC guy about A/C maintenance just last week. Advised a very simple test should be performed regularly. Using a simple thermometer you should check the air temp at the A/C air inlet and at an outlet. Ideally, the difference should be 20 degrees with anywhere between 18-22 being acceptable.