One of my roomates always turns off the thermostat in the winter --he’s cheap. The others say “turning it on and off all the time uses more energy”. I think not, since all the thermostat does is turn the thing on and off all day. The less it is blowing, the less power consumed. There’s no “warm-up” period, right? (This is electric heat BTW). That’s one point of contention. The other is the end result of all this in terms of ambient temperature. Anyone with thermodynamics knowledge?
Paulo, This is straightforward thermo, or follow the dancing BTU. All the electric energy in the heater is converted to heat, which is absorbed by the building and the air in it. The heat transfer rate from the building to the environment is a direct function of the temperature differential. The lower you set the thermostat, the less electricity you use. The line about a cycling heater using more energy than one constantly on is one of those old wives’ tales that just won’t die. My favorite is “Hot water freezes faster than cold water.” Ever heard it?
BTW, welcome to SDMB.
I like that one too. You’ll never believe where I first heard it. From my 9th grade physical science teacher. She also tried to convince me that a tray of ice cubes at 30F had less heat than a single ice cube at the same temperature… “because there is more “cold” in the tray”. Bwaaahahahaha!
Things are random only insofar as we don’t understand them.
Stevo, Good one! I’ve often wished I could go to the store and buy a bag of cold without having to mess with all that clunky ice just to carry it around.
Your science teacher sounds about as bad as my H.S. biology teacher. She was a creationist. Once, in all seriousness, she told us about a woman who ate a whole lot of vanilla wafers while she was pregnant, and sure enough, that baby was born an albino!
Similar one on energy myths is the one where people think that leaving a flourescant lights on uses less power than turning them off.
I reckon in the past some electrician was trying to explain about power factor correction in electricity supplies and maybe used an example badly
Concerning
This is what Cecil says.
I couldn’t find a cite for the start current in a fluorescent bulb but cycling a fluorescent bulb will cause it to wear out faster. A fluorescent bulb that is never turned off will last ~25000 hours. If you turn it on and off every 5 minutes it may not last 1000 hours. The published lifetime for fluorescent bulbs assumes that it will be lighted for 3 hours for every time it is turned on.
Hey, Oblio, what about incandescent?
Rarely in my experience does an incandescent bulb blow out while it’s turned on. Rather, it happens when it’s turned on.
I’ve always wondered if the “hours” at which incandescant bulbs are rated is based on continuous usage (never turning them off), or some formula based on the average time and number of times it’s turned on.
Mjollnir.
It doesn’t matter too much about the frequency of turning the light on off because the cost of the power supplied far exceeds the cost of the replacement both for incandescant and flourescant.
You are right to wonder about the frequency of switching on/off - lamp specifications are based on a common duty cycle- but deviate from this and things can change.
Hey, bro, welcome to the madhouse!
The cost of the power doesn’t always “far exceed” the cost of replacement. In my area, residential electricity is less than 5 cents per kilowatt-hour, and a standard four-foot, 40 Watt flourescent tube is about $2. If Oblio’s numbers are correct, then 1000 hours of lifetime = 0.050.041000 = $2 of electricity, which is equal to the cost of the tube. Of course, that assumes you turn it on and off every 5 minutes, but with that extreme the cost of the tube is significant.
Arjuna34
Lets take a more use of a light typical in an office or workplace.
Our light is on 10 hours a day its illuminated life is say 10000 hrs so that means it is useful for 1000 days.
Our test light is on 24 hours a day its illuminated life is 25000hrs .
In the first 1000 days the test light will have consumed more than double the power of the 10 hour light but we will need a new lamp to replace the 10hr one.
I believe you will find that the saving in electricity will more than pay for replacement lights.
BTW most lights in that type of environment will probably be 80W or sometimes 100W.
Of course if I wished to leave a light on permanently I might use a differant type of lamp such as a high pressure sodium thereby reducing the energy consmption.
Horses for courses as they say.
My source, Practical Electrical Wiring 12 Ed. - Richter & Schwan, McGraw Hill only mentions that they use the average life on incandescent bulbs, no mention of thermal cycles in the testing.