For the sake of argument and discussion, let’s use a perfectly matched, balanced, and calibrated R-410a home a/c system as an example.
Why do folks say it’s more efficient for said home a/c system to run continuously for 2 hours and then off for 5 minutes instead of cycling on/off much more often to achieve the same interior cooling setting on the thermostat? (I understand how home a/c system work, but I’d like to know why some say longer run times = greater efficiency.)
I’ve heard this claim time and again, but I’d like to know the science behind it. I live in Texas where a/c units run for hours during the day from June through September.
In: Engineering
You want it constantly dehumidifying, which means leaving it running so it can cycle all the house air through it. When it’s really humid in the house you can lose roughly half of your cooling capacity to condensing moisture out of the air, not cooling the air. So when you run constantly and keep the humidity down, you’re using your AC to cool your house air, not just dehumidify it. So you don’t have to use as much energy to cool it down and dehumidify at the same time.
Imagine your AC is that one friend who hates getting out of bed. Once they’re up and moving, they’re fine. But if you keep waking them up every 5 minutes, they’ll be grumpy and inefficient all day!
Also a thing to consider is how much heat the things in your house absorb.
Cooling an environment that’s already heat soaked will require the more energy than keeping the same environment cool and keeping it from becoming heat soaked in the first place.
Combining multiple answers… when you turn on a motor or pump, it takes usually 3 times the energy. Once running, it’s more efficient. Next, 90% of air conditioning is removing the moisture in the air. It takes a bit of time for the consenldenser to remove the water from the air. Afterwards, it’s very easy to cool air after the temperature has reduced, as well as the moisture.
Keep it running for long enough. This is why there are calculations of how large of an AC you need.
No disrespect, but there’s a lot of bad answers here. The energy consumed at startup that lasts for mere seconds is negligible in this discussion unless the system is so poorly sized, it’s cycling on and off constantly. I assume the comparison OP is asking about is between a properly sized unit that runs constantly vs. a properly sized unit that has variable speed and can turn down which runs longer before cycling off. In that case, it has to do with power consumption of the fans and compressor at lower speeds. Power varies with the cube of the speed. If you half the speed, you reduce power by a factor of 8 in an ideal scenario. You can look up the fan affinity laws to learn more. It’s not a given that turning down the unit is more efficient. That will depend on the part load efficiency of the HVAC system and the run time and comparing that to the full load efficiency and run time of the constant volume unit. That covers the efficiency part of your question. In addition to that, a unit that runs longer will typically dehumidify the space better which is a good thing.
You can have a large powerful cooling unit that pushes the temp down then shuts off. Or you can have a more efficient system that runs steady to maintain your temperature all day. It is cheaper to install and run a system that works to keep a temp than it is to install and run a system that cools fast then stops.
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