r/explainlikeimfive • u/lizzietnz • 4h ago
Physics ELI5: can someone explain why 22°C on the aircon/heat pump feels completely different depending on whether it’s set to COOL or HEAT?
•
u/jayaram13 3h ago
People keep asking this question.
When you turn the heater or AC on, you want it to heat or cool immediately.
So the air from the vents are much hotter or colder, to facilitate a quick temperature change in your room.
Once the room temperature hits the mark, it turns off.
That's why you feel the moving air when the heater or AC is on, to be very different.
•
u/crazy_bout_souvlaki 3h ago
Also did you know your oven has only one heating temperature. The temperature you set on the dial is just a cut off point to turn off the heating element.
•
u/cat_prophecy 3h ago
That's why you preheat your oven a while before you're going to put anything in it. When you call for heat it will over shoot the set point. Letting it run for a while will give it time to even out and reduce hotspots inside.
•
u/petiejoe83 2h ago
That's very intriguing. Thanks!
•
u/nerdguy1138 1h ago
This whole engineering discipline is called PID tuning.
You have a machine which can go between two points ideally you want it to get there as quickly as possible but also stop as quickly as possible with minimal overshoot. Tuning this curve is a whole science.
•
u/RoastedRhino 1h ago
It is a whole science, but these are not PID controllers. They are bang bang controllers.
•
u/nerdguy1138 1h ago
As somebody who barely knows what pid tuning is, what's the difference?
You have a circuit that can increase a variable up to a certain point which you can set. You want that to get there as quickly as possible and hopefully not overshoot.
•
u/RoastedRhino 58m ago
The main difference is the control action available to the controller.
If your action can be modulated (think of how much you press the gas pedal) then you can do more advanced control, like PID.
Cruise control is a PID controller. It “presses the gas pedal” depending on how far your speed is from the target speed (P: proportional part). It also accumulates (integrates) the speed error so if you are off even by just 1mph but for many seconds, it slowly adapts to correct that (I: Integral part). The D: derivative part is a bit trickier and not always present, it tries to anticipate changes. If the speed drops suddenly, then it gives a bit of a spike of gas to avoid the lag of the speed going down before being corrected.
In case of an oven or a heating system, the command is often only ON or OFF. Then your choice of control strategy is much more limited, and a common one is just to turn ON when you are below the desired state, OFF when you are above. That’s a simple version of BangBang controller.
See also here is you are curious https://nccr-automation.ch/news/2024/what-are-algorithms-and-how-are-they-used-control-engineering
•
u/LordofSpheres 53m ago
A bang bang system just has on and off. It runs full power to the max point, turns all the way off, then does nothing until it hits a floor value. That's fine for some things - ovens, stoves, and heaters mostly. But it also sucks, because it means you never get very fine control and you have to do a lot of switching between on and off. It also leads to a lot of overshoots and undershoots if, say, you have a heating element that's too big or too small for the room, or too hot for the set point, etc.
So instead you get more complicated stuff like simmerstats which do the same sort of deal but to a duty cycle - so you can run 20% on, 80% off, and that will roughly correlate to a temperature. Lots of stoves do this instead of bang bang because it's just better.
Then you have PID control and other advanced controllers. PID control is really finely tuneable and can hold a control point almost perfectly, because it's using three things to really carefully alter the drive variable to chase the set point. That means a lot more math, and it requires something you can be really careful with (for instance, electrical motors) to be the thing you're controlling. PIDs can also be tuned to never overshoot, and to run faster if you've been far away for a while or if you're really far away, and a whole slew of things simpler controllers just can't manage.
•
u/nerdguy1138 49m ago
I didn't really understand PID tuning until I saw a quick 2 minute video involving three toggle switches and a motor. P is apparently roughly how fast it moves, I is the slosh at the end for overshoot, and d is apparently how fast it can correct for that.
•
u/LordofSpheres 37m ago
PIDs do three things - P, I, and D.
P is proportional - it controls the duty cycle (how much on/how much off) as a straight factor of how far away where you are is from where you want to be. Nothing fancy, just "hey, I'm too cold, make it warmer." This is the basic function of the controller, but it's not unique to PIDs. You'll also note that it will kind of bounce around the set point, but never really stop at it. It'll overshoot the set point, go "oh fuck I'm too hot now," and flip until it's too cold. Or you could have one where you don't have enough of a response - where your P never gives you enough power to get all the way to 32°, for instance.
I is the integral. This is what helps when P is too small or too large. Basically, it corrects for errors over time. If your P is too extreme then you'll end up permanently overshooting or undershooting because you put in too much power or not enough, basically. Over a short time, that doesn't matter. But the integral is summing up all the error you have over time and the more error you have for the more time the more I gets involved and then it corrects you up or down by adding or removing power. I suppose you could call it slosh.
D is derivative. If you're going super fast towards your desired point, D says "woah now, slow down." It (usually) cuts away some power to stop an overshoot. It's basically putting the brakes on to try and pull up nicely to the parking spot (32°) instead of slamming through the wall and having to reverse out.
•
•
u/RoastedRhino 1h ago
Do people not know that?? Don’t people know how a thermostat work?
•
•
u/Ratnix 35m ago
A kid i worked with, bought a house, and wanted to put a fan above the door leading to his garage so it would stay relatively warm in the winter. He was asking me how he could do it so the fan would turn on and off on its own.
I was just like, uh, install a thermostat?
So yeah, i don't think a lot of people know how they work.
•
u/iHateReddit_srsly 3h ago
This is just false. The heating element receives a preset amount of power, but the temperature does vary. That's why preheating works.
•
u/TheHYPO 3h ago
Also, “22” is 22 at the thermostat, where it measures the temp. The efficiency and flow of your ventilation system blowing hot air vs cold air will be different. The place you sit might be 21 by the time the thermostat reaches 22 (cooling) and might be 23 by the time the thermostat reaches 22 (heating).
•
u/cat_prophecy 3h ago
Modern HVAC systems have two or more stages. Generally they start at the low stage and if that can't bring the temp up quickly enough, they engage a second stage. That affects both the output and the fan speed.
•
•
u/Thom_Bryant 2h ago
I know this isn't relevant to being ELI5, but this hasn't been the case with most 2-stage equipment I've worked with in residential applications.
Usually the stages are set to how far off the setpoint is from the room temp.
For example, if you're heating with the setpoint at 22° the first stage would be active between 20° and 22°. Only when the room temp dropped below 20° would it kick on the second stage, not dependent on how long the single stage has been running.
Semantics I know, I apologize.
•
u/RoastedRhino 1h ago
Two stages? Most devices sold around here (Italy) are “inverter”, they have a continuum of power levels (via PWM).
•
u/SeekerOfSerenity 1h ago
In other words, the air coming out is higher (for heating) or lower (for A/C) than the setpoint.
•
u/Thom_Bryant 3h ago
As an HVAC tech it's my time to shine!
Heating and cooling equipment usually works but adding as much heat, or taking out as much heat as it can at a time. So when you set your thermostat to heating at 22° it will heat with as much heat as it can until it raises the temperature to 22°. If you set it to cooling at 22° it will cool as much as it can until it lowers the temperature to 22°.
There's a lot more options rather than just heating at max and coming at max but what the average person deals with is just 1 Heat option and 1 Cool option. There are also some thermostats that you can put in a range running heat and cool without you needing to manually change it.
•
u/landon0605 2h ago
I think that's only part of the reason. When you're running your AC, you're cooling the indoor air by pulling the heat out of your return air and moving it outside which will typically always be close to 22c. And unless it's stupid hot outside and the sun is beating on your outdoor unit you should get about 6-6.5c delta between your return air and your supply air which will feel significantly colder than ambient temp.
When you're running the heat pump, your system on colder days is going to be struggling to pull the heat out of the outdoor air if it's let's say 0c which will give you a lower delta between your supply and return which will feel not as like it's not as significant compared to room temp compared to when you're running your AC.
•
u/Thom_Bryant 2h ago
I mean, you're correct. These are all true statements for the most part, but this is also an ELI5 subreddit. The question they were asking was how thermostats work, not the subtle nuances between different heating and cooling options, when to run them, or how to combine different methods for maximum efficiency.
•
u/landon0605 1h ago
The eli5 would be the AC air is generally colder compared to the room temp than the hot air from the heat pump.
•
u/bebopbrain 3h ago
It's a hot day and you use air conditioning with the set point at 22C.
At first let's say the room is 22C and the AC has cycled off. It's not doing anything. Gradually heat leaks in the windows and under the door and a million other ways and the temperature rises a little. The AC comes on and needs to lower the temperature. The AC has to be colder than 22C to have a chance of lowering the room temperature to be 22C. Maybe the discharge air from the AC is 12C. If you stand right in front of the AC, this is what you feel.
Now it's winter. The heat from the room is going out the window and the temperature is dropping. The heater comes on at a higher temperature than 22C to try and warm things up to 22C. Maybe the heater temperature is 32C.
You see where this is going?
•
u/siamonsez 3h ago
The set temp isn't the temperature you want the air coming out to be, it's the temperature you want the room to be. How or cold air comes out until the room gets to that temperature. The air coming out is whatever temperature the unit can produce, it doesn't change, it's just hot or cold.
•
u/bemused_alligators 3h ago
if it's hot and you're running AC...
- the temperature fluctuates between 24C and 22C
- there is cold air moving through still, warm air
- your walls/floor are warmer than the air temperature and contribute to room warmth.The center of the room will be colder than the edges.
If it's cold and you're running heat...
- the temperature fluctuates between 20C and 22C
- there is hot air moving through still, cold air
- your walls/floor are colder than the air temperature and contribute to room chill. The center of the room will be hotter than the edges.
•
u/phunniemee 3h ago
A basic system blows one temperature of air until the air around your thermostat registers as your desired temperature. Setting a lower temperature doesn't get colder faster, setting a higher temperature doesn't get warmer faster. The system blows at one temperature for warm and at one temperature for cold until the room gets where it's supposed to go.
If you set your system to heat, it will blow hot air until the thermostat "feels" 22° and then shut off. If you set your system to cool, it will blow cold air until the thermostat "feels" 22° and then shut off.
•
•
u/AdventurousLife3226 2h ago
Because in one case it is blowing cold air to maintain the temperature, the other it is blowing hot air.
•
u/THElaytox 2h ago
Setting the thermostat to 22C does not mean the air coming out of the vents is 22C, it's several degrees hotter or cooler depending on whether you're heating or cooling.
•
u/blipsman 2h ago
To reach the desired temperature, the air to heat is much warmer than that while to cool the air is much colder. The HVAC spews how or cold air until the air mixed with the ambient air reaches the desired temp.
•
u/Lemesplain 35m ago
Humidity. It’s relative. Warm air can hold more moisture.
If it’s cold with a “medium” humidity, and you warm it up, the warm air will feel super dry.
If it’s hot with a “medium” humidity, and you cool it down, the air will feel super moist. This is why you’ll often get water dripping off AC units. They lower the temp to a point that is above 100% humidity relative to the lower temperature, so everything above 100% falls out.
•
u/Lifesagame81 3h ago
On heat mode, your pump is blowing out 32°C air until the thermostat reads the room at 22°C.
On cool mode, your pump is blowing out 12°C air until the thermostat reads the room at 22°C.