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Which AC consumes more energy: one that runs continuously to keep the house cool at 24C or the one that runs every 20m?

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  • Aug 12th, 2021 9:12 pm
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Jun 27, 2015
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Which AC consumes more energy: one that runs continuously to keep the house cool at 24C or the one that runs every 20m?

So for the same house same time of the day etc, which AC would consume more energy: one that runs continuously to keep the house cool at 24C or the one that runs every 20 minutes?
Of course the above ACs have different cooling capacity (BTU?)-asume the same technology same manufacturer etc, everything else is the same excepting the BTU size
The answer might seem simple but I don't think it is that obvious
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Jun 11, 2010
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CuriousC wrote: So for the same house same time of the day etc, which AC would consume more energy: one that runs continuously to keep the house cool at 24C or the one that runs every 20 minutes?
Of course the above ACs have different cooling capacity (BTU?)-asume the same technology same manufacturer etc, everything else is the same excepting the BTU size
The answer might seem simple but I don't think it is that obvious
Energy wise they're probably similar depending on the efficiency of each unit. However, the one cycling on/off every 20 minutes is probably going to wear out faster and cost more in repair costs or replacement costs over the same timeframe. Ours cycles on probably once an hour most of the Summer, and sometimes that's just to circulate air, there are some days where it runs continuously but it's only on extreme days. If it was consistently running 24/7 it'd be an under powered unit for the home.
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Jun 26, 2019
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CuriousC wrote: One that runs continuously to keep the house cool at 24C or the one that runs every 20 minutes?
Assuming exact same technology and different sizing, you're probably splitting hairs. In theory the one that runs every 20mins would consume more energy, because it is likely reaching its target and then shutting off, where as the one that runs continuously is not powerful enough/sized correctly so if its never turning off, its using less energy by definition most likely because its not taking as much heat out of the house.

The better question for this would be how poorly insulated is your house, or how much air leakage do you have that this happens. Solving your air leakage or insulation issues so you never have the above issue is far better than debating whats marginally more efficient.
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Nov 13, 2019
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barqers wrote: Energy wise they're probably similar depending on the efficiency of each unit. However, the one cycling on/off every 20 minutes is probably going to wear out faster and cost more in repair costs or replacement costs over the same timeframe. Ours cycles on probably once an hour most of the Summer, and sometimes that's just to circulate air, there are some days where it runs continuously but it's only on extreme days. If it was consistently running 24/7 it'd be an under powered unit for the home.
Been trying to gauge my AC activity as well

So on a regular summer day (25-30C), how long does each cycle last? What do you have your temp set to? And how big is the place?

Thanks!
[OP]
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SubjectivelyObjective wrote:
The better question for this would be how poorly insulated is your house, or how much air leakage do you have that this happens. Solving your air leakage or insulation issues so you never have the above issue is far better than debating whats marginally more efficient.
Better for whom?
What is he cost to have the insulation redone vs how much energy I am consuming over the duration of my stay in this house
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CuriousC wrote: Better for whom?
What is he cost to have the insulation redone vs how much energy I am consuming over the duration of my stay in this house
Better for everyone.

Solving air leakage is usually pretty cost effective. Also, properly insulating your attic is also quite cost effective vs the cost savings you will get from heating/cooling. Also having your upstairs being a closer temperature to the rest of the house is an added bonus.

You likely won't be replacing your wall insulation, because its just not worthwhile although it could be improved, but going after your sill plate, air leakage, and attic will get you some good gains for not much cost.

My AC usually doesn't run all that much during the day or really at all. I think our day temp is set to 25.5 and our night temp is set to 22.5 or 23. AC usually clicks on for 2-6 hours (depending on the outside temperature and how much air exchange there has been) in the evening or maybe late afternoon a really warm day, and that's about it.
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Not so easy there Ma…
CuriousC wrote: So for the same house same time of the day etc, which AC would consume more energy: one that runs continuously to keep the house cool at 24C or the one that runs every 20 minutes?
Of course the above ACs have different cooling capacity (BTU?)-asume the same technology same manufacturer etc, everything else is the same excepting the BTU size
The answer might seem simple but I don't think it is that obvious
Running all day would consume more power. That said, the longer a unit runs, the better the dehumidification and the more comfortable it would be. Higher efficiency units would reduce the difference by running a DC Fan Motor in the condenser and a DC motor in the furnace etc.
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If your experience is either one of these OP, then either your AC is not properly sized for your home, or you have a poorly insulated house in the extreme. In either case, it's likely a good idea to have it looked at by a professional for their input.
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fergy wrote: Running all day would consume more power. That said, the longer a unit runs, the better the dehumidification and the more comfortable it would be. Higher efficiency units would reduce the difference by running a DC Fan Motor in the condenser and a DC motor in the furnace etc.
I think start up power in even newer units is quite a bit higher for compressors than running time. I have an old fridge that startup is 17 amps, but running is only around 5 amps.
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It takes 10+ minutes of operation to reach full capacity, so its better to have it sized close and run extended periods of time.
More capacity = greater energy usage instantaneously, and it best it balances out.

Longer cycles improve comfort, so the t-stat can be set higher, saving more energy.
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[OP]
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Toukolou wrote: If your experience is either one of these OP, then either your AC is not properly sized for your home, or you have a poorly insulated house in the extreme. In either case, it's likely a good idea to have it looked at by a professional for their input.
well do you think that I sized or I put that AC in ??
I am sick and tired of these pros
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CuriousC wrote: well do you think that I sized or I put that AC in ??
I am sick and tired of these pros
Which one do you have?

My inherited AC runs all day on sunny summer days. I increased the attic insulation, which helped, but it still struggles to cool it down. The house is very sunny so next step may be to shade the house.
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CuriousC wrote: well do you think that I sized or I put that AC in ??
I am sick and tired of these pros
You asked what AC function was more energy efficient. Not sure what difference that makes.

There are plenty of resources on this site to source good installers/products. If you want a "good" experience many people here can point you in the right direction (as many have, suggesting that efficiency isn't really what's important).

As it stands, neither choice you offered is particularly desirable in a home.

Bon chance, anyway.
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[OP]
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Toukolou wrote: You asked what AC function was more energy efficient. Not sure what difference that makes.

There are plenty of resources on this site to source good installers/products. If you want a "good" experience many people here can point you in the right direction (as many have, suggesting that efficiency isn't really what's important).

As it stands, neither choice you offered is particularly desirable in a home.

Bon chance, anyway.
I did not ask for an experience but fr an answer so stop trying to sell your buddies
I expect at minimum every person that is an authorized dealer to be able to provide a good sizing of what they are selling
I bought Carrier and the problem is the cooling capacity probably a little bit undersized (it reaches the temperature but it stays there it is not stopping the AC)
I do not mind as it takes out the humidity as someone mentioned here (i would have had to run a humidifier otherwise
Sometimes it gets below that (toward evening)
I did ask the guys to look at it but they sent me an idiot who needed 30min to understand what I was telling him and he needed to see it happening (I had the same 'experience' with two guys) )
Since it does not bother me too much and the bill does not go through the roof (2000sqft bungalow) and since I know I need to install some automated dumpers to balance the flows since the basement is getting too cold, I decided to live with it till I have time to fix it...so no I do not need an "experience" just an opinion on the above
I was just wondering if buying a bigger AC would have resulted in more or less power consumption
I knew that if you need to cool a certain volume of air the capacity will influence the time needed to cool off that volume but there is no way you can change how much power you need to cool off the same volume.
As others pointed out starting the motors very often consumes more than if you just run it at constant speed especially if your cycles are frequent
Probably I am going to use some shades and I am considering solar panels as well to compensate the excess consumption by recycling the very excess of heat that is giving me grief :-))
Last edited by CuriousC on Aug 9th, 2021 11:11 pm, edited 1 time in total.
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I like the question, doubt anyone will have data on what the answer might be other than perhaps it is dependant on many factors. I run my AC at a certain temp 24hours straight, just se the temp and let the thermostat and AC do the rest. I have a 2 speed fan so it runs on low continuously, then when the compressor turns on ramps up the speed to get the house cool.

A fair question as well is it cannot be efficient to run a fan on low speed continuously as that also takes energy, flip side is the entire house has air circulation and perhaps the AC compressor might not turn on as much. I suspect it is much more energy consuming myself, but the entire house seems to be moderated temperature wise and with a split level bungalow that is important to me.
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Ballroomblitz1 wrote: I like the question,
see my signature....some around here called me the mad scientist exactly because of things like this
I do nothing special other than using my brain and refusing to buy all the sales crap that is streamed my way by all sort of sources with more or fewer vested interests
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The indoor temperature is usually proportional to the air pressure,and LEFOO HVAC Pressure transmitters automatically controls the start and stop of the air-conditioning system according to the indoor air pressure, so as to achieve the purpose of reasonable energy saving and prolonging the service life of the equipment
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You mentioned above that your basement is currently getting too cold and that you may be getting automated dampers then why don't you close off or reduce the flow to some of those basement dampers now and see what happens upstairs and the running time of your unit. That's what I would be doing..As it is now, your system is not balanced imo
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I'm not going to answer your electricity usage question - but I'm sure the math could easily be done based on the AC ratings, etc.

What I will comment on is your scenario where one runs constantly, and one runs "every 20 minutes".

A properly sized AC unit should Not run constantly. It also should not run "not long enough" (ok... that was awkwardly worded)...

A properly sized AC unit will run long enough to remove moisture (Humidity) from the air. If it doesn't run long enough while lowering the temp, the air will still be humid and you won't be as comfortable and feel as cool at that temperature. If it needs to run non-stop, that means it can't actually cool the home to the set temperature which also is obviously not good, and would cost more to run non-stop.

For a symptom of uneven temperatures in a home, that is all about balancing the system. You should have manual dampers in all of your vents that you could adjust, but it is hard to "fix" that issue. The idea of some form of smart damper system (reading temps and adjusting them accordingly) is interesting...
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Mar 27, 2021
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I lived through different AC scenarios, lived in a house where the previous owner installed the tiniest AC and house never ever cooled down properly.

A proper size AC can rapidly bring down temperature but what I found most helpful was ecobee's eco+ feature, where it ramps up earlier in the morning if it predicts a hot afternoon, as well as keeps the fan going longer after AC unit is off to continue to circulate cool air from the coils. The downside is while waking up in the morning, the AC comes on and it's freeeeezing cold but during the hot day it doesn't come on at all or less often.
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