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
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.
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