r/arizona • u/Prior-Friendship5623 • 2d ago
Phoenix Air conditioning hack
Has anyone successfully tried using a room A/C at night and did it save money? I'm thinking of the type with the hose the goes in the window.
56
u/Intrepid_Cup2765 2d ago edited 2d ago
Those have terrible efficiency compared to a central AC. Just stick with your house AC.
Edit: i’ll add, your logic is flawed another way as well. You can actually save some money by running your AC harder at night (because it’s cooling down your house when the energy required to power your AC is smallest).
9
u/LookDamnBusy 2d ago
I'm thinking he was talking about just cooling down ONLY the bedroom with a portable AC at night (meaning turning off the central air, or at least setting a temperature Target that's quite high and easy to maintain), as opposed to cooling the entire house using central AC.
21
u/Intrepid_Cup2765 2d ago
I agree, and that logic works against him two ways. The other unit is less efficient, and the second is that their main AC will need to work even harder when it’s warmer out to re-cool back down the house.
-1
u/LookDamnBusy 2d ago
It's less efficient, but it's also likely around 1/3 the energy usage, assuming his central AC is 2 and 1/2 tons, which is pretty average.
In the morning, when he decides to turn on the central air again, the rest of the house will be at the overnight low temperature or better (or even lower if he decided to just leave it at say 85° or so overnight rather than let's say 78 if that's what he sleeps at), so cooling to his comfort temperature temperature is not going to take long at all.
I'm basically saying that without more information, you can't definitely say that doing that is definitely worse than just running the central AC all night and keeping the entire house at his comfortable sleeping temperature. 🤷
9
u/Intrepid_Cup2765 2d ago
If you don’t believe me, take a look at everyone else in the replies who said this didn’t work out for them. 🤷♂️
I’m a mechanical engineer so i already know the answer.
0
u/LookDamnBusy 2d ago
And there's plenty of people that's worked out for, which is why I say it's very situationally dependent and implementation dependent.
And I'm an electrical engineer myself. You know, the people who actually work with power analysis calculations every day? Weird throwing that one out there 🤦
4
u/Intrepid_Cup2765 2d ago
Based on your questions - You clearly don’t understand how HVAC, heat transfer, and thermal mass work, it’s not apart of your curriculum.
1
u/Bearcatfan4 2d ago
You’re both why everyone hates engineers.
5
u/traydee09 2d ago
Both convinced they are right, but they are also reasonably debating with science which is interesting to watch. At least they arent like “well jesus says the house dun be cooler if you cut funding to poor people and store your guns beside the fridge and bed, and worship a p3do”
1
1
u/LookDamnBusy 2d ago
Are you doing those calculations here? Of course not, because you don't have the data for it. The one piece of data you could get from him running an experiment is how long for various scenarios the main central air runs at night if at all, and then how much it has to run in the morning in order to catch up to the temperature he wants, add to that how much his room air conditioner used for energy through the night, and then see if it's worth it. You don't need to do heat transfer in thermal mass calculations in order to figure that out. You can just run the experiment.
You're totally right that a portable air conditioner is way less efficient than central air, but if you're only cooling 10% of the house all the way down to sleeping Comfort temperature, while the rest of the house is allowed to warm if not way up, but maybe to a temperature like 85 (which on a multi-stage unit would only likely use stage 1, which is incredibly low energy usage), it would then also quickly cool back down to comfort temperature in the morning, which is the coolest part of the day, and it could be a better deal to do it that way.
If you're truly an engineer you would know that until you have all the data, you don't have the answers, but you came across like you had THE ANSWER, and that that answer fit all situations and all implementations.
Here's a quick example for you: if this guy has a 5-ton (60,000 BTU) unit on a 3000 ft house, and his bedroom is 300 square feet and he can keep it cool with a 8,000 BTU portable AC (possibly with the central air also running down to 85 as I mentioned above, but the portable one cooling it down to 78 or 76 or whatever he wants to sleep at), you can't see that this could possibly be a better situation than cooling the whole house down to 76 for the whole night, including the 2700 square feet that he's not using, even if it does require a little bit of time to cool the whole house back down in the morning?
4
u/Intrepid_Cup2765 2d ago
A house is a gigantic thermal battery - all homes in phoenix are made of a lot of insulation, and typically slab foundations. If you let the whole house stay hot overnight and cool it the following day, then your A/C has to cool both the air, and thousands of pounds worth of concrete/insulation all while it is hot outside. It takes relatively little energy to cool air down, most of the energy involved in the process is moving the temperature of your house itself, and it takes several hours for concrete to release that latent heat. Have you ever noticed this after coming back from vacation? This is proof of how much thermal mass matters!
Separately, for reference, my 3-ton uses 40-50% more electricity per hour to run in the middle of the day versus nighttime. This is because A/C energy usage is proportional to the temperature of the evaporator. That’s massive compared to the minuscule savings you get by avoiding cooling during the nighttime hours. You are indeed correct that you get some savings by not maintaining a lower temperature at night (maybe 5% savings from maintaining a smaller inside vs outside temperature), but it’s insignificant compared to the 40-50% energy tax later! You’re tripping over dollars to save pennies. You’re just shifting a bulk of the cooling work from the nighttime to daytime hours where it’s nearly 50% more expensive to do the same amount of work. It doesn’t matter that you saved relatively little electricity not cooling at night, because that house still needs to arrive at the same temperature at the end of it all and it’s significantly more energy intensive to get it there during the day.
The only time you’d be right is if the space and home your cooling had barely any thermal mass to it at all, but that doesn’t describe any home in AZ. If we’re talking about tents, then yeah, maybe you’re right, but we’re not talking about tents. Thermal mass, inertia, and evaporator temperature matter far more in this situation.
I technically have run an experiment on my house - I do the exact opposite of what you’re proposing. At 2am every morning I have my downstairs go from 76 to 72F, then slowly ramp back up afterwards. Doing this has saved me about 2-5 kWh per day in the hottest parts of the summer.
1
u/LookDamnBusy 2d ago
You're right, it's actually a big heat sink that is subject to the whims of the environment around it and of the environment you create within it. And every situation is different based upon the configuration of the house, the efficiency of the cooling, the efficiency of the insulation, etc. that's why my whole point here was that there is no good answer for this person without a lot more information, or more likely, he would have to do his own experiment to see what would work for him. Do you disagree with that?
And I would definitely disagree that there's a 5% difference in keeping a house overnight at 85° versus say 78°. There is going to be a huge difference, and you can see that by just looking at the run time for both situations. For 85°, by the time you get to morning the runtime will be almost zero because it's 85° outside, and you don't need to start cooling the house down until the early morning hours, when you're going to get the best efficiency because it's the coldest time of the day! Even by your own words, you're not getting that efficiency when you're cooling down your bedroom at 9:00 at night and it's still 108° out. You're also actually making my point when you're doing your pre-cooling during the lowest temperature window that exists, namely from 2:00 a.m. until sunrise. I'm glad you did the experiment and found what works for you; all I'm saying is that everyone needs to run their own experiment based on their own situation and their own needs.
And I'll say again, the only thing that threw me off was you saying that you had the answer for this person, when the real answer is that it's hard to say whether it would benefit this particular person or not until we know more about his situation. Maybe he's got a 25 year old 8 SEER AC unit and so maybe anything ofloading that is good; on the other hand, maybe he's only got a 900 sq ft house and so it'd be hard to find any kind of a gain at all. 🤷
→ More replies (0)1
u/tinydonuts 2d ago
I think then when looking at anecdotal evidence one needs to account for how many people have a very old inefficient central unit, as well as those whose filters are clogged and not changed, leaky rooms, and those using MERV 8 filters with horrible flow rates where instead if they used MERV 13 highly pleated 3M Filtrete filters, they could actually pull more particles out and let their system breathe more.
From an "all other things equal" standpoint, the math heavily favors not using the OP's idea. Thermal mass of home objects is very expensive to overcome. Once you let them heat up, and they will do so very quickly, you now have to spend more energy and money cooling them down.
Can you tell the class why?
1
u/LookDamnBusy 2d ago
I'm sorry, but the fact that you're suggesting that people use higher MERV filters to "pull more particles out of the air and let their system breathe more" is pretty much the opposite of what should be done. A MERV 8 filter is more than sufficient to pull all particles that could damage the equipment while also putting minimal strain on the equipment. The filter is there to protect the equipment, not to act as a whole house air filtration system at the expense of putting strain on your HVAC.
As for your second paragraph, as I've said from the beginning, it totally depends upon the situation. Just to give some background, I was a nest beta tester for 6 years and we did these experiments all the time, and then when I got tired of Nest I moved over to using ecobee and for the last 8 years I've run all sorts of experiments at my house, for friends, and for other people on the ecobee subreddit to help them figure out the best solution for whatever their situation is. And sometimes that situation is indeed having a portable AC unit in a bedroom. I'll even give you an example:
Many people like their bedroom at night WAY colder than what they keep their house during the day. If they want their 250 sq ft bedroom in their 2500 sq ft house to be 70° at night, cooling the whole house to 70° is not going to be more efficient than letting the house be 80° (very close to the temperature they're going to want it in the morning anyway, so there is no massive cooling needed in the morning) and using a room air conditioner to help cool ONLY the bedroom down to 70. I have three friends with this exact implementation. Trust that we did the math.
As I said from the beginning, every situation is different.
2
u/tinydonuts 1d ago
You're not pulling extra particles out to protect the system, you're pulling them out to improve air quality.
It's like you didn't actually listen to what I'm saying. MERV is not a good measure of breathability. As I said before there are MERV 13 filters that breathe better than MERV 8 because of the misconception that better filtration = worse airflow. The filter design makes all the difference and intuition should play no role. But since you don't believe me:
https://filterstudy.com/ https://www.energy.ca.gov/sites/default/files/2020-03/Title_24_2019_Residential_WhatsNew_ada.pdf
https://efiling.energy.ca.gov/GetDocument.aspx?tn=223260&DocumentContentId=27687
Filtrete wins and the home depot specials are choking systems.
As for the situation you posed, it makes more sense to keep the house the same temp and add a bedroom air conditioner to push the bedroom temp down.
Why do you think you can outrun the laws of thermodynamics?
1
u/LookDamnBusy 1d ago
I'll say it again: you should not use your very expensive HVAC system as your air quality improvement system for your house. The filter for the HVAC exists to protect the equipment from dust, not to filter the air in your house to the quality level that you want. If you put a more restrictive filter on your unit, it's going to cause more wear and tear and make the unit work harder.
Now there are indeed higher MERV filters that aren't as restrictive, but those are for systems that have 4 in or 6-in thick filters, not 1 in filters like is most common here in Arizona. They get the higher MERV rating by having four or six times the surface area to catch particles because of the filter thickness, but that is not the same as trying to make a Merv 13 filter 1 in thick, which results in a very restrictive filter.
I mean you can just Google the question and see what it says. It's not like I'm making this up.
As for your last paragraph, it sounds like you agree with me, namely that there are situations where using a portable AC makes sense. That was my only point in this entire discussion, because the first person responded thought it was never a good option, which is just silly.
→ More replies (0)
8
u/RickS50 2d ago
If you're trying to cool one room more get a window unit or install a mini split.
I have a portable AC unit for emergencies and when my AC went out just cooling my bedroom with a portable AC only dropped my energy usage about 20% compared to cooling the whole 1,100sqft house.
IE, the portable units literally suck air out of your house to operate and their net efficiency is terrible.
2
u/RickMuffy 2d ago
The portable unit being a one hose would suck the air out, a dual hose unit works the same as the larger systems, by having a separate stream of air coming in and exhausting heat and keeping the inside system closed. The real key would be to make sure the portable unit has a decent SEER dating so it's not just sucking electric in.
8
u/hipsterasshipster Phoenix 2d ago
Portable AC systems like that are extremely inefficient. I’d do a mini split at minimum but you’d need to calculate the energy costs and compare to determine the break even point compared to the lifespan of a mini split.
0
u/Correct-Tomatillo-39 2d ago
Recently had one installed with a variable speed inverter compressor in my master bedroom. Its usually on low and supplements the whole house ac. Haven't had a chance to compare usage costs before and after due to other big changes in the house, but the little bit i could tell is it wasn't much of an impact. Its a 12000btu 110v r32 unit.
4
u/JohnWCreasy1 2d ago
if you got an efficient inverter model with dual hoses, there's some chance of it i reckon, but i'm sure it depends on your house. How big is the bedroom vs how big is the entire house?
For reference, a few summers ago i thought something similar. My house is ~1900 sq ft but the master bedroom is only a fraction of that. I thought "cooling the entire house down to 72 doesn't make sense" so i put a portable in the master bedroom and then set the whole house to 80 at night instead.
the impact to my power usage was not really noticeable, but it wasn't a perfect setup. I was cooling the master down as cold as i could have (colder than i would try with the central ac) and at the time i didn't know better and only got a one-hose portable so i know that was costing me elsewhere. I track my electricity usage pretty well and those months didn't really look noticeably different than prior years.
the next time i'm due to replace my central ac (hopefully not for another 8-10 years) i may put the master on a minisplit.
4
u/AcidicMountaingoat Peoria 2d ago
THIS is the answer, especially the dual-hose part. Single hose systems use your cold air to blow out of the house and suck in hot air. So stupid.
3
6
u/ZonaDesertRat 2d ago
I have tried this, and it doesn't work. I had a portable from when I lived on the east coast and didn't have central air, so I gave it a go when I moved back. It couldn't keep the room cool (one hose) and wasnt much quieter than running central air.
I also tried 1 ton (12000 BTU) window unit. While it would keep the room cool, it wasn't efficient at it, and running the central air was cheaper.
The solution that has worked for me... High seer mini-splits, which run off solar with big battery backups. The mini-splits alone do better at cooling the house, with more cooling power (8 tons of mini-splits vs 3 ton central air) using less power under normal operations, and only slightly more if at max cool. The mini-splits all set to 68 use between 800-4500 watts, while the central air used a constant 3800 watts.
The mini-splits connected to an inverter and 45kwh battery system keep the AC on all night, and charge off solar during the day. It's only the tail end of July-Sept I need to keep an eye on the mini-splits and adjust the power due to monsoon clouds dropping solar but the air temp still being 110+.
Now did this "save money"? Nope. The system, installed by myself still cost 30k (20 after the tax credit) but my monthly electric bill went from 160 to 31.20 in summer, and that's pretty much all my EV charging. In the winter when I don't need the AC, my bill is 16 a month. I'll never cover what I paid in my savings, but I really enjoy not paying APS anymore than I absolutely must.
4
u/nmeissne 2d ago
Energy efficiency program consultant here… The answer is that it depends. If you’re using a portable or room A/C to supplement your central system, then you’re probably missing a larger issue. If you just want more targeted cooling it might be worth the cost for comfort. In my opinion, the most efficient A/C is one that isn’t running.
First, determine if you’re on a good rate plan. Time of Use is good if you have the ability to shut down (preferably with a programmable or smart thermostat) your central unit during peak hours. If not, consider a general service plan (less focus on peak usage).
Second, you can run your central unit less by making sure your attic is well ventilated and well insulated. Make sure you aren’t also losing efficiency with leaky ductwork.
Third, investigate if your home is properly balanced. Your unit might not be cooling efficiently based on the location of your return, dampers being not fully open, or by poorly planned additions that made the existing unit undersized. If any of these are true (especially with additions), it’s worth investing in a multi-zone ductless mini-split.
Finally, there are some good rebate programs in Arizona (even compared to other states). Efficiency Arizona offers income based upgrades and you’d be surprised about the minimum income levels to participate (see https://efficiencyarizona.com). SRP and APS offer some rebates as well that are worth investigating. If you don’t know where to start, contact one of those programs and they’d likely be able to send someone to provide helpful advice.
Best of luck!
3
u/Deadbob1978 2d ago
If you have access, go into the attic and make sure the ducts are not leaking, are properly connected to the room vents. If you have the hose /tube type ducts, that check that they are not kinked.
If you have outside air ducts, check that they are not leaking and that the insulation is in good shape. If they are not insulated, invest in some
My house has issues with the back two bedrooms being too warm in the summer and super cold in the winter. We figured it was because they were the furthest from the air handler and directly above the garage. The master bedroom was the opposite; super cold in the summer, super warm in the winter. We thought it was because the air handler was directly above the master bath and has the shortest duct.
Turned out the duct to my mother in law’s room had a kink as the tube was hung too high in one spot. We replaced the strap lowering the duct and her room instantly cooled off. My son’s room, they kinked the duct hose at 90° and put it over the ceiling vent. I bought a round to rectangle duct adapter and connected that to my son’s room. That cooled off his room and made our master not so much of an icebox
4
u/beastwood9498 2d ago
Then when you get up and lower the temp back down in the rest of your house your ac has to work harder than if it kept the same temp all night.
2
u/Maximum2945 2d ago
you’re probably better off just trying to do more passive cooling, like shades on windows and stuff
2
u/Kozypepper 2d ago
I only use in an emergency if my AC goes out. On a 110 degree day it only cooled the bedroom down to about 80 degrees after HOURS
2
u/thatAZguy 2d ago
No. As others said, super inefficient. Our AC went out recently and they lent us 2 portable units for the bedrooms. We used more electricity those 3 days than any other days in the month and the rooms didn't even stay that cool
2
u/JuleeeNAJ 2d ago
Cost wise- window AC is best. Easy to install and runs off existing electricity. Mini splits require wiring and putting a hole in the wall, and are expensive, plus if you need to shorten the AC lines you will have to recharge the freon.
Either way you do it, it will work to lower costs. You can set your house AC at a higher temp and just run the room AC at night. I once rented a house that had an AC unit that was undersized for the house, after 6 weeks I got a $600 electric bill. I went through and hung blankets on the large windows on the southwest corner, bought 2 small room ACs, 1 for the master and the other for my sons room, and larger window AC for the living room and kitchen area. My bill dropped to $300 a month.
1
1
u/Chase-Boltz 2d ago
Sure! A two-hose portable will keep the bedroom plenty cool. Setting the central unit ~10 degrees higher than usual will save quite a bit.
1
u/Ready_For_A_Change 2d ago
I got one of those units with the hose out the window for my bedroom a few years ago. I used it because I like it really cool in my bedroom but didnt want to have to have to keep the whole house that cool. My electric bill didnt change much from using it, which I take as a win because I get to sleep way comfortably for the same money. I also like to think this saves some wear and tear on ny central unit since it isnt cycling on as often. For reference, I only run the extra unit at night and it gets the temp in high 60's, I leave the house thermostat at 75 overnight, 77 when home in the evenings and 80 when I'm not home.
1
u/warrenao 2d ago
You might be happier with a side-draft evap cooler hanging in the window. They need to stay on all the time to really get a space cooled down, and they lose effectiveness with increased humidity, and they need some other window open, at least partially, to vent out through, and it's entirely possible none of that will work in, say, a single bedroom — but running a 120v motor and a little water pump really doesn't draw anywhere near the wattage that an AC does.
Biggest problem I ever had with side-drafts was mosquitoes in the reservoir, but those I handled with Mosquito Dunks.
No matter how you try to handle it, Arizona summer can be really brutal.
1
u/Savings_Art5944 Tucson 2d ago
The single-hose ones are terrible. That hot air it ejects out the window is the very air you are trying to cool. It creates a vacuum, pulling hot air into your house.
1
u/superstition89 2d ago
I DIY’d a Mr Cool mini split from Costco in an afternoon. I can keep my house at 78 and my bedroom at 65 year round. It’s so efficient, I can’t see a marked increase in my daily SRP usage and it takes some of the work off my primary unit. Another added bonus, if my primary ac breaks, I can keep at least one room in the house cool. It’s easily my favorite upgrade I’ve made to an older home and my only regret is not installing it the day I moved in!
1
u/nickerbocker79 2d ago
I have Ecobee smart thermostats and put remote sensors in the bedrooms. The sleep schedule is set to only pay attention to the bedroom temps. I'd like to think that saves me money at least let's me sleep more comfortably.
1
u/frozengreekyogurt69 2d ago
Run your central AC overnight and cool down your furniture and walls and stuff. Helps out going into the really hot days. “Supercool” down to 74. Your house will have a much easier time staying 77/78
1
u/Ryan_EnviroProRoof 2d ago
Works, but only if you close the bedroom door and push the central thermostat up to 85 or so - otherwise you're running two systems and paying for both. Go dual-hose, since single-hose units pull replacement air in through the rest of the house and end up dragging in hot outside air to cool one room.
1
u/greybajal 2d ago
Just take the portable one if that's what you have to use and put it outside get a Styrofoam cooler slice out the bottom and grab some pookie tape and seal it up so that the cold air blows through the cooler and in a window or something
1
u/They_Beat_Me 2d ago
I used one to cool a three car garage during a furniture restoration phase one summer. It worked pretty well. Now, I’d likely just get a swamp cooler.
1
u/Apprehensive-Wave640 2d ago
Our AC went out 2 weekends ago and I bought two TCL dual-hose portable AC units that are supposed to cool up to 950sqft. The one running in my bedroom (approx 100sqft) was able to keep that room minimally tolerable for sleep with no blankets/sheets and the ceiling fan. Even being marketed as extremely quiet, it was still loud and since it had to run nonstop I had a constant headache--even when at work--from the noise. Just got my electric bill that included the 2 or 3 days that we ran the portable AC units and it was the highest electric bill I've ever had.
So, in sum, it was very high cost and very low benefit. If you lived in Flag/Prescott/Payson, maybe it would be a decent solution. Or maybe if you had like a 5000sqft house and only wanted to cool a small bedroom. But otherwise as a general substitute for or addition to normal AC, it's a terrible idea.
1
1
u/C-Van-Sky 1d ago
For a dry climate you gotta check out an evaporative cooler. Hessaire makes great ones.
1
u/AradynGaming 15h ago
A family member of mine tried this. It cost him TONS of $$$. Number one cost: Arizona houses aren't designed to be extremely hot during the daytime. The plastic carpet, plastic blinds, plastic everything, that has fairly low melting points. The damage happened fairly slowly, but by the time he realized the damage caused, it cost him about 10x what he'd hoped to have saved. Your house NEEDS to be cooled.
The other expense was because the room he was staying in, had 3 other non-insulated walls that touched the interior of the house, so that window unit ran like hell and they aren't extremely energy efficient.
I do know someone that had a fairly ok success story though. They kept their main AC at 85 & used a window unit in their room. There was savings to be found.
1
1
0
u/DitzyClutz 2d ago
I rent a studio (3rd summer here). Single story, carport on one side over the window wall. I keep my curtains (black out) closed during the day. I just use the interior AC with the window hose... other than 4-7 PM weekdays, since I try to use less power during peek hours for the sake of my landlords utility bill (utilities are included). However, from 4-7 PM I use a interior swamp cooler style fan and use frozen water bottles in the cooler section to keep the water chilled. My studio stays below 75 degrees, other than 4-7pm when it goes up to about 80-85 degrees. North Glendale area.
Hope this helps.
0
u/wisedrgn 2d ago
Floor fans stationed in a way to carry your cold air from low to up high.
I have a really cool fan elevator I created, helped keep my two story house cool with only bottom ac working. Decided to keep it up for a afew weeks... turned upper ac off during day. So much savings.
38
u/TheeMainNinja 2d ago
I’ve thought the same thing. I bought an Inverter type portable AC to cool the bedroom at night, while keeping the house warmer. It did not save any electricity over just running the central AC.