Again, you're talking about 8% of homes, at most.
No, it doesn't exist everywhere. It only exists where you have multiple heating systems operating at very different levels of efficiency -- 8% of homes, at most.
OK, this is a killer example, where the furnace efficiency goes down the tubes when it needs to raise the temp suddenly. It might be possible to stage the temp rise to avoid that with a suitable thermostat.
But, many other heating systems have no such penalty for a rise in temp setting, such as a traditional gas forced-air furnace.
Jon
So what?
Jon, with all respect, I must say that you are mistaken here.
From the law of conservation of energy,
heat input into the house == heat loss from the house
The lower is house temperature, the lower is the heat loss.
i
Either that, or you live in a cave.
You could judge how much of it is insulation effectiveness and how much is high thermal mass if you had a few thermometers, a clock, a calculator, and more time on your hands than anyone should have. d8-)
This is more equations than I care to handle, but a house with very good insulation and high thermal mass is not going to benefit noticeably until the temperature has dropped and stabilized. Otherwise, all you're doing is cycling the cooling and warming of the thermal mass.
Savings begins the instant inside temperature is reduced, whether or not it is stable at a lower setpoint. This is true regardless of how well the house is insulated, what the thermal mass might be, who the president is, which party controls congress or whether DOE likes it or not. Rate of heat loss at any (and every) instant depends upon temperature gradient from inside to outside.
Whether or not the savings on the energy bill is noticable is another question depending on how much the setback was for how long.
I wonder how much the DOE spent on a study to address what would be an easy test question in thermodynamics 101.
OK, Stormin, let's have your point of view now.
Poll question:
How many guys here have wives who mistakenly think that when warming up a cooled down house the rate of temperature change produced by a typical home heating system will be faster if they shove the thermostat setting all the way up to 90F than if they just move it to the appropriate setpoint.
Then of course, they forget to reset it when the place reaches a comfortable temperature which some time later causes the man of the house to snarl, "Why the hell is it so damn hot in here?"
And visa versa for A/C of course.
It can't just only happen to me.
Jeff
PS, I realize there may be some HVAC systems (and some wives too of course) which don't conform to the above scenario, but they sure aren't in the majority around here.
One of my tests for a potential girlfriend is to have here explain how a thermostat operates. Few pass this test of course, but it sure saves a lot of time and money.
The point is that what you're talking about is insignificant in most homes. That's what the DOE statement is all about: cycling of the heat mass overwhelms the effect of the insulation, until the temperature is reduced and stabilized for a while. Otherwise, all you're doing is cycling the heat retention of the thermal mass, with relatively much less actual savings as the temperature drops in the house.
Probably enough to know that you learned your thermodynamics with a calculator and a vacuum jar, rather than a house.
I've seen those studies for years, Don, starting with a book I read in the '70s, titled something like _Low-cost, Energy-efficient Shelter_. What the DOE says is widely known.
[raises hand] [raises sweaty hand]
[raises blue, shaking hand]
Very well put.
That study might have been a part of economic stimulus.
i
Huge savings on flowers and condoms, too.
i
But it has little to do with the question of whether you get any significant energy savings from turning down the thermostat for relatively short periods of time.
Again, if you shut it down for eight hours and the temperature drops, say,
12 - 15 degrees F (typical for my house), you will spend hours waiting for the temperature to climb back up -- and (you do the calculus, not me ) the benefit you get from it is LESS than the theoretical savings you would have if you lived in a vacuum bottle and the temperature had dropped only 6 or 7 degrees for all of that time. Meanwhile, you're freezing your butt off, part of the time at close to 12 degrees lower than your regular setting.Are we together on this, Dr. Algebra?
You guys had better figure in thermal mass and look up some values for thermal mass versus R-values in a typical house. The calculation is not as simple as you make it out to be. For a short time, as DOE says, the saving is trivial.
My test was whether she could double-clutch a racing crash box transmission -- or if she was teachable and wanted to learn.
Add me to that list.
She's also convinced that the toaster will toast faster if you set it at it's darkest setting (insert sound of smoke detector going off).
-Brian
That statement may not be entirely correct, Ed.
Remembering waaaay back to my childhood playing around with control line model aircraft. before the glow plug engine was developed we had but two choices, conventional spark plug engines and diesel engines.
The diesels were never very popular, but they are still being made for thos who want to add a bit of "authenticity" to replicas of vintage model aircraft. Those are surely far below 300 cc, but the fuel they burn may not be quite the same as what cars and trucks use.
How the heck I just remembered that the tube used in those receivers was an RK61, a gas filled triode, I'll never understand.
Jeff
Sure. Savings would be there, t possibly too small to overcome inconvenience. The solution to this has been well known and it is thermostats with a time schedule function.
Yes, but it is still there, which is the point.
i
The furnace is supplying an average of so many BTU when it is running (assuming the usual on/off type of furnace).
The house is losing so many BTU when it is at temperature T0 inside, and the losses will always Be LESS when the temperaure is closer to the outside temperature (assumed to be lower, of course).
When the house is cooling, the thermal mass is supplying part of the heat to the outside. When the house is heating up, the furnace is supplying heat to warm the thermal mass, PLUS the heat which is being lost to the outside.
The heat lost to the outside during the cooling *that is supplied by the thermal mass* is made up for that part of the heat supplied by the furnace to heat the house back to T0, so that part IS a wash.
What's missing is that for for every second the house is at a lower temperature than T0, the heat loss to the outside is less, so even if it cools down and immediately is heated up there will be an energy saving.
If it was linear (which it isn't, but bear with me) then say the outside temperature was 25F and the internal temperature was 75F. The loss is k*(75-25) = k*50, where k is a constant. Now allow the house to cool to 50F over an hour, then heat it back up again to 75F over another hour. The heat loss to the outside during that time is k*(62.5-25) = 37.5*k, which is 25% less (62.5F is the *average* inside temperature over those two hours). Chances are the heat loss is actually worse than linear (convection and radiation are worse, and conduction is linear), so this should be conservative. Now if it's 25% of 2/24 of a day, that's not very much (about 2% saving) but it is going to be a saving.
Yes, that *part* of it is a wash.
What the DOE says is widely known, but incorrect. Don has explained the problem correctly. Thermal mass and insulation determine the thermal time constant, which affects how much saving results. But has no effect on whether there is a savings.
As Pete C. points out there are some heating systems that change efficiency depending on the demand. Heat pumps are one case. Another case is modulating furnaces. These will be less efficient at higher loads. But the common furnace located in a non heated area, will be somewhat more efficient as the furnace will run for a longer time before shutting off and loosing heat to the unheated area.
Dan
One minute of this video
As for the poll, *my* wife understands how most thermostats operate, and she had a good laugh when we watched that episode of Peep Show. Watch the whole episode if you want to see a really good poem about GWB.
Wayne
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