I learnt that most HVAC systems chill the compressor to 15°C then the thermostat inside the house will regular fan speed to achieve the desired internal temp.
If the AC unit is always chilling to 15°C, why is it said that the AC unit works harder to maintain 22°C than 25°C?
I am also referring to maintaining that temp, rather than the initial stage of overcoming heat soak and getting to target temp!
for reference: https://www.canstarblue.com.au/electricity/air-con-temperatu…
Thanks
Actron service tech.
It was news to me, as i thought it would only work as hard as it needs to
HVAC systems work as hard as they need to, that's why they stop when they reach temperature. Their controllers/thermostat might over or undershoot depending on how well they're designed.
just guessing but i assume if it's 30c outside, your house will quickly/easily lose it's 'coolth' at 22c, compared to 25c.. the greater than span, the quicker the house heats up.. the more energy required to cool it.
(could refer to thermodynamics/entropy etc… carnot cycle.. https://en.wikipedia.org/wiki/Carnot_cycle
engineer/phys will be able to explain)
It doesn’t work harder to get to 22, just longer. Most of them kick in and out depending on the air temp coming into the system. If it gets to 25 and you’ve set it to 25 it’ll stop chilling, if you’ve set it to 22 it’ll keep going until it reaches 22.
I'd you set your room temp to 22 the fan will run faster and transfer more heat into the air handler heat exchanger. This requires a higher flow of refrigerant from the outside unit and thus faster compressor speed resulting in increased energy consumption.
it might be always chilling to 15 degrees, but it will have to work harder to keep it at 15 degrees if you keep adding lots of heat (by asking it to cool to 22 for example). if you only ask it to cool to 25, there will be less heat that needs to be dissipated to keep it at 15 degrees, i.e. less energy used. Fancy systems use inverters that can scale the compressor power up and down, more primitive systems just switch the compressor/fan on and off.
If you wanted to heat a bath you wouldn't add a bucket of water at the desired temperature, you add hotter water which is why the temperature of your indoor coil is lower than your target temperature. If you want to make the bath hotter you add more buckets of hot water.
In terms of maintaining the lower temperature you need to consider the heat load on your system. If you lived in an esky, the AC would get to temperature and stay there without much effort, it just has to overcome the 80 watts of heat that each human occupant is generating. Your house has lots of heat ingress through your windows, walls and ceiling/roof. So the greater the difference between the outside and inside temperature, the harder your system has to work. If your indoor temperature set-point is lower, then the load is higher, even when maintaining.
You may have got some wires crossed talking to your source, the compressor will get hot because it is compressing the fluid. (When you compress something, it heats up aka putting energy into the system. You can hit small piece of metal with a hammer a few times and feel it)
The cold part would be the evaporator (looks like a car radiator and the room air blows over it to cool down). In the evaporator, the liquid in your AC (refrigerant) enters a larger sealed volume/ space and evaporates into a gas to fill that space. Changing from a liquid to a gas takes a lot of energy so it cools down (same principle as you sweating on a hot day and the sweat evaporating off your body and you feel cooler)
So back to your question, the evaporator may always be chilled to 15C but some AC units will chill it all the way down to sub 0C
Say your room is 27C.
Room air is blown over the evaporator which is 15C.
Room air cools down to say 22C and is blown out of the Air conditioner
This chilled air is then blown back into the room and chills other 27C air
But there is much more room air than chilled AC air. So the room air chills down a little to say 26C but the AC air quickly warms up to match the room air
Rinse and repeat this cycle until your room is as cold as you set the AC remote to, say 25C, then the whole system turns off until the room temp rises back to 27C. Then the whole process starts again.
It may take (pulling a number out of thin air), 20 cycles to get the room down to 25C from 27C
So, if you want to cool the air from 27C to 22C, it will take more cycles, so the AC is running longer and using more power
I'm running out of time on my lunch break so I'll update with a part 2 later if anyone wants more info or something I said doesn't make sense. I'll also actually answer your question; "If the AC unit is always chilling to 15°C, why is it said that the AC unit works harder to maintain 22°C than 25°C?"
Thanks for the detailed reply.
Part 2, energy boogaloo
"If the AC unit is always chilling to 15°C, why is it said that the AC unit works harder to maintain 22°C than 25°C?"
I can't talk for everyone but my interpretation is along the same line as many other comments here, it's because there is a bigger difference between the room temp (27C, what the air is) and what you set the AC to (22C, what you want to change it to)
Think of it like going for a run, running 10km is 'harder' than running 5km
You may then ask, " but 22C is closer to the 15C the evaporator is at, shouldn't that be what I compare against?"
Energy (and that's all temperature is, a measurement of energy) moves from a high point to the low point, until everything equals out (like water, it flows downhill, not uphill).
So we shouldn't compare 15C (low point) to 22C (high point) because that's the wrong way
Instead, compare 22C to 15C. Or in your case;
Going from the hot room (say 27C) to 25C, is less than 27C to 22C
Thinking more on your question, are you thinking because the AC is chilling to 15C, whats the difference between 25C or 22C? I've already paid to cool to 15C dammit!
saucesource?