Help settle an argument

[CardinalAndGold]

The girl and I were leaving for a short weekend getaway Saturday morning, set for a return late Sunday afternoon. We usually keep the air on about 71 degrees in the summer. Since we would be gone for more than a day, one of us wanted to turn the air up to about 78-79 degrees to save some money. Then, we would turn it back down to 71 when we got home the next day.

The other only wanted to turn it up to about 73, and said that it uses more electricity to turn the air back down when you get home if you have it turned up to 78-79, since it has to work so hard.

I'm not disclosing who argued what. So who is right? What is a more efficient strategy?

 

[LeSchmick]

I'm guessing you argued the latter.

 

[cyclonedave25]

Since you were only gone for about 36 hours, I would turn it up slightly, depending on how hot it was outside.
If you are gone for a longer time, 3+ nights, I would either turn it all the way off or turn it way up.

 

[Tre4ISU]

Just guessing but I would say turning it to 78-79 and trying to cool that much wouldn't be as efficient. I don't know the real answer but I probably would have gone with the 73 deal.

 

[CloneState1028]

I would've turned it up slightly.

 

[wxman1]

Never really thought about it, parents and others I know just do it...not always fun to come back to though.

 

[cyfan964]

I would have gone up to 78-79. My house can drop those 7-8 degrees in about an hour. I highly doubt it wastes that much more in that little of time.

 

[cyclonedave25]

Here's a little tidbit from this website: Ask Mr. Electricity: Saving money on air conditioning

It's a myth that leaving the AC on while you're away at work uses less energy than turning it on when you get home. Here's why:
Heat goes to where it's not. That's why heat from outside goes into your cooler home. With the AC off, at some point your house will be so hot it can't absorb any more heat. When you come home and turn the AC on, the AC removes all that heat.
But if the AC is on when you're gone, then you've turned your house into a heat magnet. But keeping it artificially cool, there's no limit to the amount of heat it can absorb. It can always absorb more heat. And your AC has to remove that heat constantly. Your AC kicks in and removes some of that heat, then the house is cooler so it sucks in more heat from outside, so your AC kicks in again and removes that heat, and so on.
This means that throughout the day, your house has absorbed way more than one houseful of heat. And your AC had to remove it all. By contrast, with the AC off all day, then it has to remove just one houseful of heat when you come home and turn it on.
Let's say you leave the AC off, and your house absorbs 20k BTU's of heat and then stops, because that's all it can absorb.
Now let's say that you have the AC running instead. The house absorbs 5k BTU's of heat, so the AC kicks in and removes it. Then it absorbs another 5k BTU's, and your AC kicks in and removes that. Repeat that process several times during the day.
The actual numbers will vary, and I haven't tested this to see exactly how much the penalty for leaving the AC on during the day is, but there is zero question that running the AC all the time uses more energy than turning it on when you get home. This is not a gray area, it's simple physics, and no person with any knowledge of this subject disputes it. Running the AC when you're not home wastes energy, period.

 

[HiltonMagic]

Here's a little tidbit from this website: Ask Mr. Electricity: Saving money on air conditioning

But will the plane take off from the treadmill?

 

[Together2813]

As an HVAC technician I can answer this one!

Leave the thermostat alone. Leave it at 71 degrees. If you set it up to 78 and come home and set it to 71 degrees its going to run a long time to get it back down to 71 degrees. I hate programmable thermostats in the summer. I suggest not using the programmable part of the thermostat in the summer because the humidity takes awhile to evacuate the home and makes your AC/Heat pump run awhile.

In the end, I suppose whomever said 73 degrees is the closest to victory.

 

[cyfan964]

As an HVAC technician I can answer this one!

Leave the thermostat alone. Leave it at 71 degrees. If you set it up to 78 and come home and set it to 71 degrees its going to run a long time to get it back down to 71 degrees. I hate programmable thermostats in the summer. I suggest not using the programmable part of the thermostat in the summer because the humidity takes awhile to evacuate the home and makes your AC/Heat pump run awhile.

In the end, I suppose whomever said 73 degrees is the closest to victory.

You're telling me it is cheaper to keep my house at 71 degrees for 1.5-2 days than it is to let it go up to 78-79 and then drop it back down when the temperature outside is around 80-90 degrees?

Sorry man, I'm not buyin' that.

 

[Bubbahotep]

I have mine set on 78 degrees wether I'm home or not. What's the matter with you people?

 

[cyclonedave25]

As an HVAC technician I can answer this one!

Leave the thermostat alone. Leave it at 71 degrees. If you set it up to 78 and come home and set it to 71 degrees its going to run a long time to get it back down to 71 degrees. I hate programmable thermostats in the summer. I suggest not using the programmable part of the thermostat in the summer because the humidity takes awhile to evacuate the home and makes your AC/Heat pump run awhile.

In the end, I suppose whomever said 73 degrees is the closest to victory.

Is there an explanation for this doing it this way? I'm not saying I believe you or not, I just want some type of scientifical explanation to help me understand this.

 

[Together2813]

Few more tips for those of you who care.

1. Don't shut registers i rooms that aren't being used. Leave them open! Your furnace/AC is measured for your house and the number of registers are installed for the size of your equipment. You close off registers it is hard on your furnace. Your blower motors job is to push air throughout your house. If you close registers it is restricting it and this can hurt your blower motor. Even in your basement where it is cooler than your very top floor, keep those registers open. Ah but there is a solution!!!

Once you open up all your registers, go to your thermostat and flip your Fan mode from auto to "on". This will act as a whole house fan and will even the temperatures in your house in every room. Your upstairs will cool down by a few degrees and your basement will no longer be absolutely freezing. Your blower motor will take that cold air in your basement and distribute it out too the whole house. You will potentially see your utility bills lower (in some cases). How??? Here is how!

1. Your outdoor unit won't run as often if your house can find other ways too cool it's self. Your outdoor unit uses 240 volts to energize your fan and compressor. If you can use your blower motor (115 volts) and reduce the use of the 240 volts, your energy bill can be lowered.

Give this technique a 2 week trial and you will notice your upstairs bedroom considerably more comfortable!

 

[Cyclone42]

So I take it I am the only one here who would have no hesitation to turning the dang thing completely OFF when I am not at home? This is what I do all the time. And these same electricity-mongering idiots are some of the very same guys getting on thie VERY tall horses to preach against the idea of some guy trying to get out of a credit-card debt. The depth of hypocrisy in our society knows no bounds.

 

[Clonehomer]

Turn if off. Here's why:

q=(1/R)*(Tin-Tout)

So the greater difference in temperature results in greater amount of heat transfer in to your home. In addition, your air conditioner may run for a while, but it is still removing less heat from your house than it would have throughout the time you are gone.

 

[heyguy85]

So I take it I am the only one here who would have no hesitation to turning the dang thing completely OFF when I am not at home? This is what I do all the time. And these same electricity-mongering idiots are some of the very same guys getting on thie VERY tall horses to preach against the idea of some guy trying to get out of a credit-card debt. The depth of hypocrisy in our society knows no bounds.

I turn mine off but no longer will I do that thanks to Together. Makes me nervous about running my fan constantly for that long but I'm willing to try anything to lower my electric bill.

 

[Together2813]

You're telling me it is cheaper to keep my house at 71 degrees for 1.5-2 days than it is to let it go up to 78-79 and then drop it back down when the temperature outside is around 80-90 degrees?

Sorry man, I'm not buyin' that.

Okay here is how I will explain this:

You set your thermostat at 71 degrees at all times. Your thermostat kicks the AC on when your thermostat hits 72 degrees to cool it back to 71 degrees. The AC runs for 7 minutes too cool it back to 71 degrees.

Now.... You have your thermostat set at 78 degrees. It goes up to 79 degrees and your AC kicks on. It will take aprox. 7 minutes too cool it back to 78 degrees.

Your AC will kick on a bit more at 71 degrees then it would 78 degrees because your thermostat is going to be more prompt at keeping the humidity out of your house. But.... here is the kicker.....

If you have a programmable t-stat for wake, afternoon, evening, sleep and you move that temperature up and down, you are asking for a high electric bill.

EXAMPLE: Wake 72 Afternoon 78 Evening 72 Sleep 68.

When evening comes every Monday-Friday, you are asking your A/C system to run for aprox 30 minutes straight (maybe more depending on the age of your equipment). If you leave your system at one temperature instead of raising your temperature by big amounts, your AC will do short run times rather than long periods. Then you factor in the humidity that your AC has too pull out of your house.

By experience, most houses at 78 degrees has a humidity level of 65 wet bulb. Houses at 72 is around 59 wet bulb. So your AC has too pull an average of around 6 degrees of humidity out of your house every night. That = big electric bills.

 

[ia8manfan]

When I had my one bedroom apartment when I first moved to Dallas, I set the thermostat around 74 and left it at that 24/7. My first month's electric bill was over $200. I went and bought a programmable thermostat, put it at 74 while I'm home, and 84 when I'm at work. Cut my bill in half. I tell everyone I know to do a programmable thermostat.

 

[clones26]

turn it up to 78 then back down when you return. That Iowa smart home program on between kcci stories just showed this as the right thing to do a month ago or so.