Why do air conditioners and refrigerators have to keep turning on and off to keep the air within a temperature range, instead of adjusting their output to maintain an exact temperature?
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Air conditioners and refrigerators use a cooling process to regulate temperature. Maintaining an exact temperature by adjusting the cooling strength is complex due to changing heat levels indoors. The on-off cycle is simpler and more reliable, as it quickly brings the temperature down when needed. This approach ensures stability and energy efficiency in maintaining a comfortable temperature range.
A lot of air handlers have variable speed options, coupled with a variable speed thermostat.
There ARE variable AC units. They just cost a lot more. More common are two-stage units: ones that can do full blast when needed but do half-blast when only a little cooling is required. Fully variable speed are really nice, because they help you sleep better — no big swings in temperature throughout the night. But, again, they cost more and most people don’t bother spending the $$, since new HVAC systems are already an arm and a leg
Because the cooling process in AC, fridges, etc is vapour compression. There’s a compressor, it’s either on or off.
Variable speed compressors have become available, in recent years, but the simplest, cheapest option is an on/off motor.
The compressor that moves the Freon through the refrigeration system is run at a constant speed to make the system run at best efficiency. You would have to ask the people who design these systems why they don’t make variable speed compressors. I would be delighted to have such a thing on the radio telescopes that I work on, because maintaining a very stable temperature in the equipment rooms is essential to collecting good data.
Refrigerants need to be managed at a specific pressure in order for it to vaporize. Being able to vary the amount of refrigerant kept at pressure takes a bit more machinery so more expensive. The cost of a variable condenser is considerably more so not a lot of call for them in places like houses. However when you move into commercial applications you can use a vrf system that can deliver to multiple indoor units, with less refrigerant sent at times this the need for modulating systems. Or you can use a system that has more than one compressor that can use 2 stages, one for a low amount and second for higher. These are common in commercial.
It operates around a setpoint. It cools a little below the setpoint, then shuts down, then restarts a little above the setpoint. The machine rests a bit in between. A lot of equipment is not meant to operate on a 100% duty cycle, it requires more robust construction, engineering, etc.
I have a window unit in my bedroom. Mine has a low/energy saver setting, works for me.
Continuously variable coolers exist, they’re more complex and thus more expensive so you don’t see them very often.
Have you ridden in a modern car with air conditioning?
Wait wait wait.
I see answers that explain how it’s cheaper to have refrigeration that goes on and off. I don’t know much about refrigerators BUT we just has a new AC system installed that is “variable” speed as in it does the opposite of your question. This AC runs most of the day at varying power/efficiency/stages. It’s a lot more efficient this way and uses less power compared to our old unit.
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