Eli5: What really happens when you change the temperature in an AC (Cooling) ?
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Most thermostats are pretty simple, and when set to cooling, operate under a logic that could be described as “is the current temperature higher than the set point? If so, run AC, otherwise wait.”..
Changing the set point just changes the point where the thermostat will call for the air conditioning system to run.
You change what target temperature the cooling system has. Exactly how the system handles it will depend on the system. Normally a system with a temperature setting has a temperature sensor in some location. It could be somewhere in your house, it could be where the AC sucks in the air that should be cooled.
The control system is design so the sensor temperature is the same as you set temperature and it needs to cool more to if the temperature is too high. If it is too cold you stop cooling or cool at a lower rate.
Some have if I am not mistaken just one power setting and the cooling system is either on off, it that is true for any fan too the result is that system has to be running a larger part of the time or a shorter part of the time depending if the new temperature is lower or higher.
Other systems have a colling compressor that can run at different rates. Then the heat exchanger for the internal air a bit cooler or warmer.
If you have a classic single-stage air conditioner, not much. Your thermostat just controls whether or not the AC is on or off. If it’s too much above the desired temperature, it goes on, and once you get down to the desired temperature, it turns off. It doesn’t actually output differently depending on the desired temperature, it just stops outputting when it reaches the target temperature.
But there’s also something called a two-stage air conditioner. Basically, it can run the actual air conditioner at reduced power in a more efficient mode. It will do this if it’s closer to the target temperature. If it’s further away from the target temperature, it runs at full power to get cooler faster.
So if you make a sudden change to the desired temperature on your thermostat, on a single-stage AC it can trigger it to turn on, but if it’s already on it just changes the threshold for it to turn off. For a dual-stage AC, you could also cross the threshold to change it from high-efficiency to full power mode.
In older thermostats, there was actually a mercury switch couples to a bimetallic strip. You turned the coil of bimetal a certain amount (gauged on the outside). As the temperature rose, the strip expanded until the mercury switch completed the circuit, thus turning on the A/C. As the room cooled, the switch contracted, re-coiling until the switch opened, thus turning off the A/C. In a heating/cooling combo unit, there was a switch on thermostat that controlled whether the mercury switch was connected to your A/C or your heater and reversed the whole on/off part of the circuit pathway (so heat came on when it was supposed to).
Nowadays, it’s all done with sensors, and there’s *usually* a built-in delay, so that the A/C or heat aren’t constantly cycling every few seconds (that’s bad for the systems). Usually about 5 minutes.
ELI5 Answer:
When you change the temperature on the thermostat, the brain of the A/C thinks “you want it colder, I am going to have to work more to cool your room down”. This causes the A/C unit to run longer to meet the temperature you set it at.
Not ELI5 answer:
Without getting into refrigeration thermodynamics, it varies significantly from system to system. In most in-home A/C systems, when the temperature is set lower, the fan will ramp up to get more air to move over the refrigerant coils thus cooling a higher volume of air per unit time. Sometimes the compressor on the refrigerant circuit has the ability to ramp up as well and will send more cold refrigerant to the coils thus allowing you to cool more air, colder.
Most units will stop or slow down when the room temperature is what you set at the thermostat.
More specifically, can I tell my girlfriend that she doesn’t need to turn the AC to 60F when it’s too hot, because it doesn’t cool it down faster just longer??
Pretend there’s a little demon living in your thermostat (let’s call him Maxwell). The thermostat controls when Maxwell feels hot. If he feels hot he turns on the AC. When he’s comfortable again he turns it off.
Another demon controls the heater the same way, but I don’t know his name.
Hot things want to be cold things, so the hot things give their energy to the cold things so they can both be the same. In an air conditioner the hot thing is the air, the cold thing is a refrigerant inside the big metal box outside, and the giving of energy is called the Rankine cycle. In a perfect setting if you were to leave an air consider permanently on, the air you’re cooling would eventually get to the temperature of the refrigerant. That is normally too cold for people so using a thermometer, your thermostat, the unit shuts off at certain measurement. When you change the temperature on the thermostat, you just change where that shut off point is.
There are two main types of thermostats: analog and digital.
***Digital*** is the simplest to explain ELI5-style. It’s basically a small, low-power computer that watches an electronic thermometer. You tell it what temperature you want, and it turns on the AC if the temperature in the room goes above that number. When it drops below that number, it turns the AC off.
***Analog*** thermostats rely on the property of various metals to expand as they arm up and contract as they cool down. Two main designs of analog thermostats are bi-metal and mercury.
Bi-metal thermostats have two types of metal strips sandwiched together. The different metals expand and contract at different rates so as they heat up, the sandwich bends and then straightens again as they cool back down. So, when the metal sandwich heats up, it bends and touches a contact that tells the AC to turn off. When it cools down, the metal straightens and stops making contact which tells the AC to turn off. When you move the temperature knob, you’re changing how far the metal has to bend before it makes contact.
Mercury thermostats use a long metal strip wound in a spiral. On the end of spiral, sits a glass bulb with a little bit of mercury and two electrical contacts. As the metal spiral heats up, it expands and makes the bulb tilt so that the mercury bridges the gap between the two contacts in the bulb. When that happens, it turns the AC on. As the air cools, the wire spiral contracts and tilts the bulb in the other direction and the mercury stops bridging the gap between the two contacts and the AC knows to turn off. When you turn the temperature dial, you’re changing how far the spring has to expand before the bulb tilts enough for the mercury to bridge the contacts.
In a house, it’s pretty simple. Some kind of temperature controlled switch, either electrical or mechanical.
Cars get a little bit more complex. Engine heat is always available and used to warm the air when you turn the dial on the most basic systems. It does this with the AC compressor going in most cases.
Depending on the vehicle and the type of system it has, it could be set to a temperature and the computer opens blend doors as required to redirect warm and cold air to keep the temperature consistent while controlling the compressor.
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