Turning the AC on while it's still cool inside vs...

JakeCK

Vs when the gates of hell have already opened up in your living room. Which one is more energy efficient, easier on the AC?

I've heard arguments both ways. That leaving the AC on means it doesn't have to work as hard and the house is already cooled down vs it actually uses less energy to only cool the house down once when needed where as leaving it on all day actually uses more energy. I'm curious if there is actually any data out there to support either strategy?

Today I employed the strategy of closing up the house a little before 7am and setting all the window shakers to 74 and then went to bed. Woke up and couldn't even tell it was 92 outside, and the ac's were actually cycling. Last time I turned them on when I woke up drenched and they ran non stop for 12hours.

STEVEusaPA

It's more about humidity control and comfort.
I have 2 AC/ air handlers, and a boiler with 6 zones of heat-8 thermostats total-haven't touched them in years and they're not programmed. Only time I touch them is to change the batteries.
I'm sure there's data to support your thoughts. After all it's using no energy when it's not running.
I'd rather be comfortable all the time.

ratio

If the sizing is proper, it won't be able to catch up.

I personally use AC for comfort, not efficiency, so it's on whenever it's hot out. I will open the doors & ventilate if it's nice outside.

JakeCK

Actually my thoughts are that I'm not sure which strategy is better in terms of energy usage and efficiency. Turning it on early certainly is more comfortable.

pecmsg

Turning the A/C on with a high heat load inside and high ambients outside puts a tremendous strain on the equipment. If it manages to reduce temperature at all then it is oversized for the load. 

JakeCK

ratio said:

If the sizing is proper, it won't be able to catch up.

I personally use AC for comfort, not efficiency, so it's on whenever it's hot out. I will open the doors & ventilate if it's nice outside.

That is what I did this morning. The past 3 days were very nice. Mid 60s to 70s, very low humidity. Had windows open and enjoyed the fresh air. This morning when I got home from work it was 61 but I knew it was going to rocket up to the low to mid 90s. I closed everything up, closed the curtains, set the ac's to 74 and passed out. This was a little before 7am. When I woke up I had forgot it was even hot outside it was so comfortable. That is until I opened the door to get the mail. I got slapped in the face with a flame thrower.

mattmia2

I'm not sure how much humidity comes in through infiltration with the house closed up, if it makes a difference to run the AC vs opening up windows and letting in humid air then trying to condense the latent heat out again when you close everything up and turn on the AC or if that much humidity comes in through infiltration anyhow.

GroundUp

It could certainly be calculated out by someone willing to do the math, but that person is not me. I've always just left the stat at 70 year-round but last summer I tried the whole turning it off in the morning and back on when I got home thing. In addition to it being miserably hot in the house well past bedtime, the elctric bill almost doubled compared to leaving it on 24/7. I also never open windows for any reason so humidity stays low regardless. That is a big killer from what I've seen- while it may seem nice out and low humidity, it's not. At least in my neck of the woods. I typically only run the AC in my garage when I'm out there and it takes half a day to pull enough humidity out before it actually starts cooling.

ethicalpaul

Honestly, I don't understand this idea of "puts a huge strain on the equipment" or "makes it work hard"

It's a compressor pump and a fan. Is the compressor really "working harder" if its warm inside? Can it tell the difference?

ChrisJ

ethicalpaul said:

Honestly, I don't understand this idea of "puts a huge strain on the equipment" or "makes it work hard"

It's a compressor pump and a fan. Is the compressor really "working harder" if its warm inside? Can it tell the difference?

Take your gauges hook them up and watch them. The higher the load on the system the higher the pressure differential across the expansion valve. The higher the pressure differential the higher the load on the compressor.


To an extent.

But if I had to guess I'd say that's effected more by outdoor temperature than indoor conditions with central air.

But I'm sure evaporator load has an effect. I'm not positive and maybe someone can confirm but I believe with a TXV it should try to maximize evaporator capacity across most conditions.

ChrisJ

On addition that's also why I highly recommend using recirculate in a car if it's hot out after you've cooled the inside off some.

I generally insist on using it above 80.
Pulling 90-100F humid air thru the evaporator with 100+ air cooling the condenser causes very high head pressure and it's both hard on the system and uses more fuel.

ethicalpaul

"hard on the system" not to get a reputation as a contrarian (haha!) but surely the pump is engineered to be able to run for many years at any load that the system might see, right?

Is there any data that shows the pump or system fails faster when it's used to cool a warmer space?

I don't doubt the electrical load is greater...I'm asking about the longevity of the system, thanks!

ChrisJ

ethicalpaul said:

"hard on the system" not to get a reputation as a contrarian (haha!) but surely the pump is engineered to be able to run for many years at any load that the system might see, right? Is there any data that shows the pump or system fails faster when it's used to cool a warmer space? I don't doubt the electrical load is greater...I'm asking about the longevity of the system, thanks!

On that I have no idea.
I view it as the system uses more power.  

 I never hesitate to put a heavy load on a monitor top.  After defrosting I even put a pan of hot water right on the evaporator to drive any oil back up to the compressor and get things circulating good.

JakeCK

GroundUp said:

It could certainly be calculated out by someone willing to do the math, but that person is not me. I've always just left the stat at 70 year-round but last summer I tried the whole turning it off in the morning and back on when I got home thing. In addition to it being miserably hot in the house well past bedtime, the elctric bill almost doubled compared to leaving it on 24/7. I also never open windows for any reason so humidity stays low regardless. That is a big killer from what I've seen- while it may seem nice out and low humidity, it's not. At least in my neck of the woods. I typically only run the AC in my garage when I'm out there and it takes half a day to pull enough humidity out before it actually starts cooling.

I've been tracking my basement dehumidifier run time for a while now. I've found it tracks closely to the outside humidity. Those days I had the windows open it didn't run at all, that is until about 11pm last night as the warm front swept in. I've also found that I can basically shut down the dehumidifier with the large 14k btu window ac as long as it's hot enough not to cause it to short cycle. That window ac effectively cools about 700sq ft. Closing upstairs bedroom doors also affects both the ac and dehumidifier significantly too. I'm guessing it slows down the stack effect.

ratio

ethicalpaul said:

"hard on the system" not to get a reputation as a contrarian (haha!) but surely the pump is engineered to be able to run for many years at any load that the system might see, right?

Nope. This is the biggest difference between my first trade (electrician) & this one. We sized most things to worst case to the point that I'll get in arguments with Sparky about the code exception that allows him to put small wires on large breakers for HVAC equip. This trade is all about balancing one thing against another.

pecmsg

ethicalpaul said:

Honestly, I don't understand this idea of "puts a huge strain on the equipment" or "makes it work hard" It's a compressor pump and a fan. Is the compressor really "working harder" if its warm inside? Can it tell the difference?

Suction pressure goes up
head pressure goes up
Amperage goes up

look at a Compressor performance curve. 

Why do High rise buildings start there chillers in the early morning?  Ambient is low, inside is low, lower utility bills!

EBEBRATT-Ed

It depends where you are located and how often you want to watch the weather. Here in New England most of the time on hot days you keep the house closed up if it's hot out you will be good until noon time then fire the AC up.

BILLHVAC

Humidity is the biggest problem here in New England. The AC takes out humidity first before cooling too much. Last year with the rains I closed up the house and left the AC on for months at a time.
We used to get calls from the 2nd homers who came up on Fridays
, “l can’t get the temp down”, only to find out it had been on for just 2 to
3 hours.
Now with the advent of the mini-splits and the inverter technology it is highly suggested to leave it on. The equipment uses less energy to keep up than to pull the whole place down. Think of your house as a sponge and leave it on.

OttoPilot

I was an industrial utilities engineer for 30 years and part of my job was calculating energy costs.

The biggest determinant of energy use is the difference between inside and outside temperature. If you set your thermostat higher, you use less energy.

So, ANY TIME the house is warmer than the thermostat setting, the house is absorbing less heat than if were at the thermostat setting.

Therefore the answer is... When your house temperature is above the thermostat setting (for any reason) it requires less energy to cool than when it is at the thermostat setting.

But it's also less comfortable during that time.

Most of us use the AC for comfort, not to save money. We could use an outhouse to save utility cost too, but that debate was settled years ago.

ChrisJ

OttoPilot said:

I was an industrial utilities engineer for 30 years and part of my job was calculating energy costs.

The biggest determinant of energy use is the difference between inside and outside temperature. If you set your thermostat higher, you use less energy.

So, ANY TIME the house is warmer than the thermostat setting, the house is absorbing less heat than if were at the thermostat setting.

Therefore the answer is... When your house temperature is above the thermostat setting (for any reason) it requires less energy to cool than when it is at the thermostat setting.

But it's also less comfortable during that time.

Most of us use the AC for comfort, not to save money. We could use an outhouse to save utility cost too, but that debate was settled years ago.

Picture it......

It's 2AM, 40 degrees out and heavy rain.
You wake up and you have to go. you have to go, NOW, not in 5 minutes, you have to run out squeezing your cheeks together.........

Yeah, I'm thankful for my bathroom.


I suppose an outhouse was still better than just going outside in the pouring rain but you still had to make it to and from the outhouse that smelled less than fantastic.

bburd

@ChrisJ I have done that 2 AM dash to the outhouse, through deep snow, in 10°F weather. This was in a cabin in the woods in New Hampshire that lacked the invention that saved our ancestors from such hardship: the chamber pot.

Of course, some poor soul had to empty the thing in the morning. 😳

BennyV

Cycling on and off is harder on old single-stage systems than just running to cool down a hot house. The answer really depends on the building and how the system is zoned for sleeping, as that's going to drive the usage in a residential setting, since it needs to be cooler at night to sleep comfortably. It also depends on whether electric rates are different at different times of the day. Lastly, modern variable-speed inverter-driven heat pumps are more efficient just running all the time matching to the load, but they still have to do more work keeping the building cooler, so again, it depends.

fixitguy

I have been in this business for 25 years and have discovered much to my surprise that people can do without heat but they will not live without air conditioning. A AC license is money in the Bank

ChrisJ

fixitguy said:

I have been in this business for 25 years and have discovered much to my surprise that people can do without heat but they will not live without air conditioning. A AC license is money in the Bank

Wouldn't that depend greatly on the location?

For example I highly doubt it applies to Minnesota.  

scott w.

Live in a large brick home built in 1928. The outer walls are plaster over clay block. When it gets hot during the day 85 plus and sun shines on the stone roof and brick exterior the brick and block heat up and release the heat after sunset. If the outside evening temperature is 65 degrees or below I can open up all 32 windows in the house at night and cool it down to the mid sixties, Windows are then closed in early morning and that is good for most of the day until sun sets. This saves energy. If nights and days are humid and really hot, windows are kept closed and the air conditioning is used. If the heat builds in the house because the air is turned off and goes above 80 it takes forever to cool the place down. Even when it reaches thermostat setting at 75 or 76 and shuts off it still is stuffy and have to lower the thermostat to 74 to get the place comfy so I try not to let the interior temp ever to go above 78, degrees.

EdTheHeaterMan

JakeCK said:

Turning the AC on while it's still cool inside vs. when the gates of hell have already opened up in your living room. Which one is more energy efficient, easier on the AC?

To summarize:
@STEVEusaPA said "It's more about humidity control and comfort."
@ratio said "If the sizing is proper, it won't be able to catch up."
@JakeCK said "my thoughts are that I'm not sure which strategy is better"
@pecmsg said "high heat load inside and high ambients outside puts a tremendous strain on the equipment."
@GroundUp said "It could certainly be calculated out by someone willing to do the math"
@ethicalpaul said "I don't understand this idea of 'puts a huge strain on the equipment' or 'makes it work hard' "
To which @ChrisJ replied "Take your gauges hook them up and watch them"
Then @EBEBRATT-Ed chimed in with this "It depends where you are located"
@BILLHVAC added "We used to get calls from the 2nd homers who came up on Fridays
, “l can’t get the temp down”, only to find out it had been on for just 2 to
3 hours."
to which I say , I feel your pain! I also worked in a vacation home community
But the most interesting response was from @OttoPilot who contributed these words of wisdom "We could use an outhouse to save utility cost too,"
Then @bburd thought it necessary to share "I have done that 2 AM dash to the outhouse, through deep snow, in 10°F weather. This was in a cabin in the woods in New Hampshire that lacked the invention that saved our ancestors from such hardship"
Which should be in the TMI File!

So I guess this discussion has gone to the Toilet with no clear answer

Thanks for sharing everyone!

Dave Carpentier

OttoPilot said:

Therefore the answer is... When your house temperature is above the thermostat setting (for any reason) it requires less energy to cool than when it is at the thermostat setting.

It would certainly require less energy to hold it at a new (higher) temperature, ie stop the heat increase. With a smaller delta across the "wall" , it would have less work to do.
But if its a question of driving the interior back down to the desired point, does all of the thermal mass (that has now warmed up some) not require extra effort ?

To continue the toilet talk-- when we built this place we were living in a small travel trailer using a rented portapotty. In December... -10 to -20f .. quite a few outhouse speed records were broken.
No bugs though.