OK, question me and my wife are currently arguing about.

Lets say the average temperature outside will be 80 degrees in the day and 65 in the evenings/nights, for the next 7 days.

Is it cheaper to leave the central air ON all the time, set at say 75 degrees... or is it cheaper to turn it off when you don't really need it?

When it's cooler outside than in the house, should I open the windows and leave the AC on, until the AC realizes it and turns itself off? Or should I set it to turn itself off first.

Basically, I posed this to my wife: "for example, if we leave the ac OFF for 8 months, and then turn it on and let it "get up" to our desired temperature, we SAVED a lot of money"

She told that's false, because the AC has to work that much harder to get back up to where we want it (even after 8 months of being off). Apparently it would use all the same electricity it would have used in 8 months, all in the same 2 hours to get the house cooled off.

So, what's the deal... I think my central air is about 2.5 tons. We have the upstairs door'd off and the vents upstairs closed, to force all the air to the downstairs vents...

I'm sick of the AC running all the time when its basically within 3-4 degrees of outside anyway.

I need something mathematical to explain to my wife why it's more cost effective to kill the AC when you don't really need it.

My bet is on you.
I can see her theory if you allow the house to get up to 80 or 90 degrees and have to get it back down to 60 or whatever you are comfortable with. But leaving an AC on constantly does run up the electricity.
Usually the windows open and a couple of fans should be enough when it is cooler.

Hopefully somebody here can break it down wattage/money wise.

i need something mathematical to explain to my wife why it's more cost effective to kill the AC when you don't really need it

Whe you find it post it because I can use calculations that can make both ways look good.

Quote:

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Originally Posted by yourcomputerguy

ok, question me and my wife are currently arguing about.

lets say the average temperature outside will be 80 degrees in the day and 65 in the evenings/nights, for the next 7 days.

is it cheaper to leave the central air ON all the time, set at say 75 degrees... or is it cheaper to turn it off when you dont really need it?

when it's cooler outside than in the house, should i open the windows and leave the AC on, until the AC realizes it and turns itself off? or should i set it to turn itself off first.

basically, i posed this to my wife: "for example, if we leave the ac OFF for 8 months, and then turn it on and let it "get up" to our desired temperature, we SAVED a lot of money"

she told that's false, because the AC has to work that much harder to get back up to where we want it (even after 8 months of being off). apparently it would use all the same electricity it would have used in 8 months, all in the same 2 hours to get the house cooled off.

so, what's the deal... i think my central air is about 2.5 tons. we have the upstairs door'd off and the vents upstairs closed, to force all the air to the downstairs vents...

i'm sick of the AC running all the time when its basically within 3-4 degrees of outside anyway.

i need something mathematical to explain to my wife why it's more cost effective to kill the AC when you don't really need it.

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Update:

Tested these things:

With ac on for 60 minutes straight, we used 3.0 kw hours on the electric box
With ac off for 60 minutes straight, everything else the same, we used 1.1 kw hours

There are a whole lot of circumstances that involve the answer to your question. Will you open the windows every time the temperature gets cooler outside? Will you be comfortable with the humidity the outside air will bring in when you open windows? Hmm... now for philosophy... "what is worth more to you....lower bills or comfort?" I leave my AC on unless there is a cooling trend in the near forcast... but for one or two days I don't touch it. And the thermostat controls the temperature anyway... so if everything is working correctly the AC won't come on when not needed... correct? If youwant to be comfortable you will do what it takes... either open a window or turn on the AC. Here's a suggestion; rather than turning it off, set it higher... maybe to 80. Then it will come on less, but be there when needed... and there won't be the drastic temperature changes your hvac will need to recover from.

Meet her 1/2 way,, you move into the basement, and let her be the boss for a month,, then she moves into the basement while you are the boss,, after 2 electric bills compare the paper work

Quote:

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Originally Posted by wmproop

meet her 1/2 way,,,,you move into the basement,,and let her be the boss for a month,,,then she moves into the basement while you are the boss,,,,,,after 2 electric bills compair the paper work

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Fabulous answer... might solve some other issues too :)

I'm going to throw a little wrench in the fan.

It's not temperature. It's the comfort chart and where each of you fall. Each of you is comfy in the summer when the temp is between 2 values and the humidity is between two values.

Allergies indoor and outdoor are also a factor. Let's assume none. But if ozone index is up, opt for AC,

If the temp/humidity outside is within this range, by all means open the windows.

The caveot is that during the day your house stores heat and when the sun goes down, that heat dumps into the house. So, you need some help getting the house cool. I use ceiling fans with the windows open pushing the air up to draw the air from outside.

A good night's rest is important. AC to sleep is always a good thing.

Next, closing off dampers CAN INCREASE electric consumption unless a bypass damper is installed. SO, you have to factor that variable. Simple example. Blow into a straw and now take another breath and blow into the room. Which one was easier? The room, right. When you close off too many intakes/exhausts the fan has to work harder moving the air unless a bypass damper is installed (Some Carrier systems exempt).

It might be time to invest in an ERV/HRV so that this can happen automagically. ERV is an Energy Recovery Ventillator. It will automatically use outside air when it can.

To remove humidity effectively, the AC must run a long time. Short cycling the AC will not remove humidity. Oversized AC units won't remove humidity. If will be cold and uncomfortable.

There are lots of variables here my friend. Invest in an ERV/HRV to save some dough. Your climate determines whether it's a good idea.

As for post #7, the cooling degree days have to be the same.

BTW: Off is always cheaper.

with ac on for 60 minutes straight, we used 3.0 kw hours on the electric box
with ac off for 60 minutes straight, everything else the same, we used 1.1 kw hours

This is only one part of the fromulation.

The other posts covered some of the others.