# Turn thermostat down?

As I said, I'm aware of the physics, Don. But the questions, which have gone all around the barn, are about whether it's worth it. And the answer is that it's much more worthwhile if your periods of turning the furnace off (or the thermostat 'way down) are quite long.
DOE addressed the question accurately: if you have two periods of reduced heat per day -- the time you're at work and the time you're sleeping at night -- you'll typically save 10%. That's a figure that's been tested and reported for decades.
Most people think that shutting off their furnace for four hours should save four hours worth of fuel. It doesn't. If the temperature keeps dropping until you turn the furnace back on, you save the equivalent of roughly two hours of fuel, not four hours.
And another way to look at it is whether shutting off the furnace 12 times per day, for an hour each time, is equivalent in terms of savings to shutting it off one time for 12 hours. The answer is, no; it probably is on the order of 50% of the savings you get by shutting it off once for 12 hours.
The DOE's tests specifically addressed some common misconceptions. Both in terms of the misconceptions over the physics and in terms of misconceptions about actual savings, they were quite right.
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Ed Huntress

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On Fri, 30 Oct 2009 13:17:41 -0400, "Ed Huntress"

That's as good as any generalization. Savings vary a lot with how much the homeowner sets back and for what percentage of the time. The length of each period is only relevant if it is small or comparable to the time constant of the enclosure. If the period is short enough that the space can only cool 5 degrees, then lowering the setpoint more than that has no effect or benefit.
But the assertion about not saving anything until the house is stabilized at the lower temp is wrong, and the stuff about cycling thermal mass requiring net energy is also wrong.
When these studies were done, 10% savings was far from trivial. Houses are *much* more energy-efficient now than they were 30 years ago.
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wrote:

I should have said "anything significant." Somehow significance stuck in my mind throughout that discussion.

I don't know who said that, but it wasn't me. Or if I did, then I misspoke.
Just to make sure we agree here: If you read what DOE actually says, it's perfectly accurate. They don't get into thermal masses or hypothetical examples. They're talking about real savings, based both on theory and, more importantly, on real tests run over a period of decades.

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Ed Huntress

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On Fri, 30 Oct 2009 14:58:11 -0400, "Ed Huntress"

I don't know which DOE study you refer to here since there were several, but I have no quarrel with findings based on actual data. Honeywell did their own studies with similar findings.
Precis of findings: How much does a setback stat save? It depends. Are the savings significant? Maybe. 10% of not much is hardly any. 10% of a whole bunch is some. Buy a Honeywell Chronotherm. Your family deserves the best.
Some gov't studies are almost laughable when they wander off into the weeds trying to offer an impressive technical explanation of what the data says and why, which more often is really the writer's attempt to impress by verbose technobabble obfuscation. It's well known in contract research labs that the gov't wants big thick reports for their research buck. Hell, the proposals alone sometimes look like phonebooks. It's widely suspected that nobody actually reads these reports, they just weigh them and assign a grade.
Makes me think of a notorious DOH or OSHA study that spent 3/4 mil finding that pigshit is slick. (Safety issues in pig farms or something like that).
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wrote:

That probably was a report about Chicago's Grant Park in 1968. <g>
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Ed Huntress

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Ignoramus10802 wrote:

No, it isn't. It was documented on a well monitored high efficiency model home where the backup heat strips on the high efficiency heat pump were kicking in in order to provide a reasonable temp recovery time since the heat pump itself did not have the capacity. The electricity used during the temp recovery was more than would have been used on temp maintenance due to the switch to lower efficiency backup (100% vs. 300%+).
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But that only tells you that a lower-efficiency temperature-recovery system is...lower in efficiency. If you have that particular pair of heating systems, you have one situation. If you have a more-typical single heating system, you have quite another.
In the case you've described, you aren't dealing just with the thermodynamics of the situation. You're also adding the complexity of multiple heat sources that operate under different circumstances.
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Ed Huntress

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Ed Huntress wrote:

That isn't a "pair of heating systems", nearly all heat pumps include backup heat strips for times when the heat pump is not able to produce enough heat such as very cold weather / high demand.

That complexity exists everywhere and that was my point - you have to do the actual analysis of the home in question to get the correct answer - you can't rely on blanket statements / myths.
An additional complication is occupancy, since for folks who are retired or work from home, or a stay at home spouse, you loose half or more of your theoretical savings period with the occupants not being away during the day.
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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.

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Thu, 29 Oct 2009 13:30:44 -0500 did write/type or cause to appear in rec.crafts.metalworking the following:

OTOH., we can turn the heat down in the rest of the house, warm just the office, and double up the blankets at night. Just don't let the pipes freeze. B-) (that means no coffee.)
pyotr - pyotr filipivich We will drink no whiskey before its nine. It's eight fifty eight. Close enough!
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pyotr filipivich wrote:

Well, ideally you should be zoning and not heating long unoccupied rooms beyond maintenance level anyway. Certainly I have the dampers mostly closed on my spare bedroom and dining room pretty much all the time, only opening them on the rare occasion those rooms will be occupied.
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Thu, 29 Oct 2009 23:04:40 -0500 did write/type or cause to appear in rec.crafts.metalworking the following:

My problem is that the 'office' is the one room with the worst heat. So I just turn the thermostat down for the house, and use a space heater in the office, and in the bathroom. And dress warm. - pyotr filipivich We will drink no whiskey before its nine. It's eight fifty eight. Close enough!
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When I am working the house cools off considerably at night and the next day and I have to fire the wood stove hotter than its efficient range to make up in the evening. When I am home all day I keep it warmer with about the same amount of fuel.
OTOH if I fire it normally to recover there is an obvious and considerable savings from allowing the temperature to drop for a day or two, as during holiday trips or when measuring the cooling and recovery rates.
Exact numbers are difficult, outdoor temperature changes constantly and internally the living space and basement (where the stove is) act like two loosely coupled thermal masses that cool at different rates. I think the house loses between 2% and 3% of the in-out difference per hour, substantially through infiltration which I don't want to reduce below what it is now.
jsw
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Pete C. wrote:

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
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:Pete C. wrote: : :> No, it isn't. It was documented on a well monitored high efficiency :> model home where the backup heat strips on the high efficiency heat pump :> were kicking in in order to provide a reasonable temp recovery time :> since the heat pump itself did not have the capacity. The electricity :> used during the temp recovery was more than would have been used on temp :> maintenance due to the switch to lower efficiency backup (100% vs. :> 300%+). : :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.
My gas furnace is 2-stage, and not particularly exotic (a common Trane model, 80,000 BTU). Presumably it's more efficient when operating on low-heat, as otherwise there's little point in the added complexity of a 2-stage burner and control. Except in really extreme weather, the only time it runs on high-heat is during recovery from night time setback.
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Bob Nichols AT comcast.net I am "RNichols42"

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Echoing what Robert Nichols said:
I'm in the midst of optimizing a brand new, Bryant 80% forced air, 2 stage, 110k Btu furnace. It sucks every BTU out of things when running in the low fire mode. On high fire it is just trying to get you comfortable enough to let it get back to the economy mode. I doubt that the difference is more than a few percent but I suspect it is measurable. If nothing else, the high fire mode requires the high speed fan and attendant increase in electrical use.
Jon Elson wrote:

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80% are hardly known for "sucking every BTU".
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Christopher A. Young
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Granted that the 80% is less than the 92% units. But the **WEIGHTED** average stack temperature is much less than the older versions. Every short fire of the gas starts with the heat exchanger at ambient plus no more than a couple of degrees. Stack doesn't even get warm to the touch for half of the burn time in moderate weather. Hard to argue with that.
This also argues to the point that a temperature setback may not produce the fuel savings one might expect.
Stormin Mormon wrote:

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Ignoramus10802 wrote: i

No, there is quite an element of truth! You have to compare the thermal mass of the house and the heat loss. If the house had enormous thermal mass, like lots of stone floors and massive stone fireplaces (some people build houses intentionally to have very high thermal mass) then although the furnace could warm the air quickly to make you comfortable, it would still run for hours to warm up all that mass. If you have a lot of thermal mass and low heat loss (good insulation) then turning the thermostat down for a couple hors gives no benefit. If you have low thermal mass and lots of heat loss, then turning it down for even a couple hours will give significant benefit.
Jon
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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