A/C usage strategy

Re: A/C usage strategy

There is a problem with the equation to solve this. The variable "human" is not a single variable but variable variable and of course there may be more than one which can result in several setpoint changes within short periods of time.

If we remove this variable for now we can see that there are still other factors to take into account.

The first thing is to not run the A/C at all. Maximum efficiency, lowest power draw.

To keep in the no A/C mode it is best to cool during the night, then seal it off when the temps outside being to increase above those inside. The better insulated the area is the better this will work. So add insulation.

So lets say all the insulation that can be done has been done.

And the area is being cooled at night with lots of fans to cool off everything so the thermal masses inside get to the lowest temperature seen outside.

Now you are starting the day off at the best possible position.

If the objective is to keep the lowest temp possible, then start the A/C right away.

If some temp increase is acceptable then delay turning the A/C. If there is a maximum acceptable temp then the A/C will likely have to be turned on before that point, particularly if the A/C is too small to handle the max heating load.

The greater acceptable temp increase the less power being used.

As for the idea that the A/C is less efficient when moving more heat (drawing more power) that has not been my experience with large units, some very large.

Most A/C units I know of are much more efficient when fully loaded. Part of the energy of starting the unit goes to cooling down and warming up the various parts. This is why units are often undersized it keeps that to a minimum and makes the most of the heat exchangers. Generally the greater the temp differential on them the more efficient (there is graph that will show how each unit operates).

Undersizing also allows the unit to run at rated speed. This means less loss in the system as the percentage of energy (heat/cool) lost through all the various means is smaller than that being moved. Move less heat and a higher percentage is lost (even though it may be the same amount).

From the A/C point of view it likely prefers to run wide open and not throttled back.


During the heating of the day the area may increase to the point that A/C is needed.

Depending on the size of the A/C unit you may want to start it and let it run at 100% for a while an then shut off.

If you can lower the temps such that the continued heating from outside does not result in temps above the acceptable maximum then that is likely going to be best.

If the unit has to start again, then again you will want it to run at maximum for a set period and shut down.

When the outside temp starts to drop heat will still be coming into the area but at some point you want that heat to be removed by fans and ventilation and not by the A/C. The sooner the A/C is shut off and ventilation started the better.

Now we have to put humans back in.

Humans will not like the cold blast of air from the A/C unit only to have it shut off and the room heat up again.

If the cooling is needed for other people, and not equipment, then that, IMO, should decide how the unit is operated.

So I suggest setting it up so it is comfortable, stable temps and then work around that. Start ventilation later in the day so temps are kept comfortable, fans not too loud and so on.

Then work with that. Add insulation (to area and equipment), shade, different paint color and so on.

Bottom line, all the extra heat that gets in has to be moved out, if the A/C is used to do that it likely is going to be most efficient doing that at full load.