Programmable Thermostats - Do they really save anything?

[Kelemvor]

OK. This is something I've been mulling over for a long time. Maybe someone here can help straighten me out or say I'm right or something. The media constantly tells us how programmable thermostats save us so much money and this and that but I don?t buy it. Here?s my reasoning.

Please feel free to punch holes in this because it?s just what came to my mind and not necessarily true at all.

So here goes.

First off let?s assume that it?s 0 degrees outside. To MAINTAIN any particular temperature it takes XX amount of energy per hour. Whether I?m maintaining a temperature of 70 degrees or 60 degrees it takes the same amount of energy to MAINTAIN that temperate. If I?m maintain 70 degrees, my heat will fluctuate up to 71 and down to 69 or so and basically have a wave effect. If I?m maintaining 60 degrees, my heat will fluctuate up to 61 and down to 59 or so and will have the same wave effect.

Assuming the above paragraph is true, there?s no saving of energy whenever you are just MAINTAINing any particular temperature assuming it?s much colder outside than it is inside.

So this would mean that the only energy savings with the ?Set Back? thermostat method would come when the heat wasn?t needed for an extended period of time. Most Set Backs occur when you leave for work in the morning and when you go to bed at night. Since they are the same thing taking place we?ll just look at one.

If my morning temperature is 70 degrees and when I leave my thermostats goes down to 60 degrees, the amount of time it takes to cool down to 60 is time when I don?t have to spend the XX amount of energy to maintain the temperature so I?m saving energy. However, then when my heat comes back on and has to heat back up to 70, I?m spending much more energy than I would be to just maintain a set temperature. This excess spending of energy would pretty much negate the savings of energy I had during the Cool Down period.

So how does this save me anything? I save it in the morning and use more in the afternoon then save some when I go to bed and use more when it heats up in the morning. I?m just not seeing the savings anywhere.

I could post charts and such of a sample wave of energy usage which would show that the amount you spend heating back up would be the same you would have used to just maintain the initial temperature in the first place.

Where is my thinking wrong?

 

[notfred]

It doesn't cost the same to maintain any temperature. It'll be more difficult to maintain a temperature with a higher differential from the outside temperature.

Let's say it's 0 degrees out. You want to maintain 0 degrees. No energy used at all.

You want to maintain 70 degrees. Your heater comes on occasionally, using up a little bit of energy.

You want to maintain 90 degrees. Your heater stays on constantly, using much more energy than at 70 degrees.

By your logic, you could take a 70 degree soda can and submerge it in icewater, and another 70 degree soda can and submerge it it liquid nitrogen, and they'd both take the same time to cool to 35 degrees. That's not true. due to the much higher differential in temperatures in the liquid nitrogen tank, it will cool off much faster.

 

[Haircut]

Whether I?m maintaining a temperature of 70 degrees or 60 degrees it takes the same amount of energy to MAINTAIN that temperate.

This thinking is wrong, the rate of heat loss will probably be proportional to the temperature difference between outside and in.
Thus it takes more energy to mantain a higher temperature.

 

[Ryan]

I set my thermostat to turn off when I leave my house in the morning, and to turn on an hour before i come home. I have noticed about a 20% decrease in my electricity bill

 

[Kelemvor]

Well then maybe that's the part I wasn't picking up on.

I was assuming that the rate of cool down would be a constant but maybe it's really a variable. If that's the case, then using the same example...

If it's 0 outside and I have two identical places... One is 70 inside and one is 60 inside. If The heat is not on, after a given amoutn of time they would both NOT have decreased the same amount? So let's say in the time it took the 70 degree house to get down to 60 (lost 10 degrees), the 60 degree house might only be 51 (only lost 9 degrees)?

Does that seem more correct?

 

[Haircut]

That seems more like it.

Before, if you were assuming that the rate of cooling was a constant not dependant on the temperature difference then after a certain amount of time the room would reach absolute zero.

If we say that R, rate of cooling (dT/dt) = k * delta T where k is a constant and delta T is temp difference between outside and in then you can see that the greater the difference in temperature the greater the cooling.

If you plotted a graph of temperature against time you would see a curve with the inside temperature tending towards the outside temperature over time.

 

[Yossarian]

Heaters and insulation are pretty inefficient. Basically the less you have to use them, the more you save.

 

[ThumpR777]

After I figured out how to program the thing...I couldn't overclock it, upgrade the Ram, or put in a new video card, so I was not very interested in it.....then I set it to come on a half hour before I woke up, and a half hour before I came home from work. I never noticed the loss of tempurature heat wise, but I did notice that I woke up easier (a pavlov dog type of thing subconsiously) and the electric bill droped by about 25%. In California here, that works out to about $100 a month.

 

[Fausto]

Originally posted by: PipBoy
Heaters and insulation are pretty inefficient. Basically the less you have to use them, the more you save.

Yep. From what I understand, you actually save money if you allow the temp to rise or drop a fair amount while you're away during the day or asleep and then bring the temp back where you want it half an hour before you get home/wake up than if you just have it maintain the same temp all the time. I don't recall where I read this, but my heating/cooling bills bear this theory out.

 

[BillGates]

Originally posted by: PipBoy
Heaters and insulation are pretty inefficient. Basically the less you have to use them, the more you save.

Ummm, I don't really think that's true for insulation, at all....

 

[308nato]

What you want is a programmable 'stat with "Intelligent Recovery".

My Honeywell "learns" how long it takes to raise or lower the temp Xdegrees in X amount of time. If you set the 'stat to 50degrees and then to be 70degrees at 6pm, normally it will start the burner and run constant until the desired temp has been reached on a cheap programmable. This cuts into your savings by having these long-wide open recoveries.

Units like mine incrementally increase the burn time over the prior cycles so the desired temp is reached with a slow heat up rather than a 20 minute long Joan of Arc bonfire.

But, then again, my woodstove is usually roaring and the furnace is off anyway.

 

[NutBucket]

Some of the people in this thread need to take a class called Thermodynamics after having read the posts here.

 

[bizmark]

Originally posted by: NutBucket
Some of the people in this thread need to take a class called Thermodynamics after having read the posts here.

yeah, well, if we all had unlimited educations, none of us would ever have to ask anything, right?

 

[Kelemvor]

Well, I live in Wisconsin (it's 27 degrees as of right now) so we definitely use the heat in the winter. we try to keep it set low for normal to reduce costs (usually set at 67 degrees) and then we set it back at night and during the day (down to 63). We don't want to turn it down too far at night and wake up freezing in the middle of the night. Also we have a bunch of pets so don't really know how much the temperature affects tem during the day.

What temperatures do most of you use as the High and Low temperature? Could probably drop another couple degrees I guess since my cats and dog have fur that shoudl keep them warm...

 

[Ornery]

You want to keep warm and save some bucks? Do you have natural gas there? Are unvented gas appliances allowed in Wisconsin? If so, add these to your Christmas list:

• Vent-Free Gas Logs, FAQ
• Vent-Free Natural Gas Heater with Thermostat
• Robertshaw 9600 Programmable Thermostat Higher cost, higher quality
• "Nighthawk" Carbon Monoxide Detector

 

[teddymines]

Similar question: do you save more energy turning the thermostat DOWN or OFF when you're away?

I've always believed it is better to leave it off when you're away. Take the extreme case: if you leave your house for a week in the middle of winter, and turn the thermostat off, you will use NO energy. If you keep it at a lower temp like 50, you will use energy to maintain that temp. Sure, the unheated house will require more energy than the 50 degree house when you bring it up to 70, but that's a one-time hit.

 

[Kelemvor]

The more you turn it down the more you save, but if you turn it OFF and it gets too cold you run the risk of freezing water pipes and stuff like that so it's best to not go down that low.