# Lost Electricity -2

Page 3 of 4
• posted on January 22, 2008, 7:54 pm

At which time the furnace will come on again.

No, you won't. Not unless you've discovered some new laws of thermodynamics.
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Regards,
Doug Miller (alphageek at milmac dot com)
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• posted on January 22, 2008, 8:03 pm
wrote:

it is called heat transfer. more heat in air at 80 will put more heat into the walls, getting them up to 70 faster. You won't save energy, you will save time. But, the heat loss out the windows will be greater with a temp up to 80.
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• posted on January 22, 2008, 8:34 pm

Any difference there might be would be far too small to be noticed. Were you imagining that heat transfer at 80F is 14% faster than at 70F? Doesn't work that way. Use absolute temperatures.
--
Regards,
Doug Miller (alphageek at milmac dot com)
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• posted on January 22, 2008, 8:57 pm

hey, this tread long ago drifted off into the theoretical natty gritty. Don't blame me! ;)
faster is faster. I think we agree on that. I think we also agree on "who cares - put on a sweater".
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• posted on January 24, 2008, 2:11 am
On Tue, 22 Jan 2008 20:34:01 GMT, snipped-for-privacy@milmac.com (Doug Miller) wrote:

That's the problem with using deg F, it bears no relation to reality :)
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• posted on January 22, 2008, 9:17 pm

Umm, my understanding is that for convection and conduction, heat transfer rate is proportional to the temperature difference. So there is a big change in heat transfer rate for 70F versus 80F air. [E.g. for 60F building materials, the temperature difference is double with 80F versus 70F air.]
Look at the original question this way--you want to get the average temperature of a building from 70F to 50F. Based on all the different materials and their heat capacities, this will require some number of BTUs; based on the furnace output rate the furnace will have to run some number of hours to put out the required heat. Say 2 hours.
Air temperature will be a leading indicator of average building temperature. So if you set your thermostat to 70, the furnace will run less than two hours, then it will cycle on and off for a while as the building catches up to the air temperature, until the total running time reaches two hours. Or you could set the thermostat artificially high for two hours and then reset it to 70 degrees. The two hours of furnace time required occurs all at once.
Clearly the latter strategy causes the building to reach equilibrium sooner. That's all I'm claiming, not that it is a good idea, will be more comfortable for the occupants, or that it is more efficient.
Cheers, Wayne
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• posted on January 23, 2008, 2:37 am
Doug Miller wrote:

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• posted on January 23, 2008, 2:35 am
Wayne Whitney wrote:

Well said.
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• posted on January 23, 2008, 2:45 am
CJT wrote:

Even if so (and I think it would take a pretty exceptional house design for it to make any discernible difference in any practical sense), it will certainly be more expensive and the time to reach the initial setpoint is still the same so at best it's a period after that initial warmup at most that can be affected at all.
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• posted on January 23, 2008, 3:02 am
dpb wrote:

I don't see why, unless you've got a heat pump with resistive backup.
First order, you need to put the same BTUs in either way to start the same mass at the same temp and end it at the same temp (although I will admit to the fact that the path can have a small effect due to the variation in delta-T's to the outside which affects losses along the way, but work with me here and assume that the heat needed to go from cold to warm is most of the energy used and that losses during the short time involved are second-order).
and the time to reach the initial

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• posted on January 23, 2008, 4:39 am
CJT wrote:

Because the input to reach the higher setpoint will also have higher proportional losses owing to the higher delta-T to the outside.
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• posted on January 23, 2008, 4:58 am
dpb wrote:

small relative to the heat required to heat up the house and everything in it.
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• posted on January 23, 2008, 2:57 pm
CJT wrote:

But, it's as real as the heat going into the material is and at higher dT the loss proportion goes up at the same rate as the gain portion so you can't win -- you might come close to breaking even, but you can't win. Well, actually I guess you could possibly break even in one specific instance but it would take really detailed measurements or calculations to come to that point--if you were to be able to find the time at which the exterior wall temperatures would first reach their steady-state temperature and cut the extra input at the time when the heat input on the inside surface would then be transferred to reach that exterior temperature, then it would be the break-even point. Once the interior temperature is higher than that, then the exterior temperature also would rise above its steady-state value and then the previous conclusion would also hold.
The point in my view is that the two paths are identical owing to the fixed input until the lower setpoint is reached so there's absolutely no advantage there. The only question is whether then raising the temperature above the end setpoint perhaps aids a little _from that point_ in "creature comfort" -- my opinion is that unless the house is one that is actually designed as a thermal mass rather than conventional likely to be essentially unchanged although it just might aid a little bit in "taking the chill" off in comfort level. But it can't help but be more energy-costly and can't help the initial recovery.
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• posted on January 22, 2008, 11:31 pm

Nit-pick: four state: off, refrigeration/heat, defrost (self nitpick: via controller) and supplementary heat.
The supplementary heat doesn't _have_ to be resistive. Ours (in a previous house) wasn't. If we go HP again, it won't be.

Ours (with gas backup) had two sensors. I don't think it did a lot of smarts with them, but it was remarkable how low the heating/gas bill was, even in the great white north.
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Chris Lewis,

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• posted on January 23, 2008, 2:27 am
Doug Miller wrote:

snipped-for-privacy@earthlink.net (Charles Bishop) wrote:

then set it back to your true desire and let the room "soak" you might reach equilibrium faster.
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• posted on January 23, 2008, 2:46 am
snipped-for-privacy@milmac.com (Doug Miller) wrote:

Not true for at least 1 version of mechanical and 1 (probably many) versions of electronic thermostats, which have means of compensating for overshoot, and therefore switch at differing points under differing conditions. Said adjustments are often out of whack if ignorant persons mess with a thermostat they don't understand the subtleties of, or install a new one without reading the directions. Thus the automatic adjustment that someone else mentioned for a newer electronic 'stat.
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• posted on January 26, 2008, 9:48 pm
(Charles Bishop) wrote:

Actually, the classic heating thermostat anticipator is a tiny heater that warms the sensing element a degree or two above the room's air temperature. The purpose is to cause the burner of the furnace to shut off just before the room reaches the desired temperature. This works because even after the burner shuts off, the heat-exchanger in the furnace and the blower continue to supply heat to the room for close to a minute longer. When properly adjusted, the burner will shut off just before the room air reaches the setpoint and the stored heat in the hot heat exchanger will continue and the room temperature will 'coast' up to the setpoint just as the blower shuts off.
This feature avoids an overshoot of the room temperature, but doesn't do anything to '...reduce the time needed to bring the room up to normal.'
daestrom
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• posted on January 27, 2008, 2:34 pm
On Sat, 26 Jan 2008 16:48:54 -0500, "daestrom"

Even if it has been turning the furnace off too soon, requiring the user to wait for another heat cycle?
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Mark Lloyd
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• posted on January 27, 2008, 4:15 pm
Mark Lloyd wrote:

Then you adjust the antisipator so it doesn't. It's relativly easy to do.
Duane
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• posted on January 27, 2008, 3:44 pm
On Sun, 27 Jan 2008 10:15:07 -0600, "Duane C. Johnson"

I didn't know about the heat anticipator 30 years ago, when I noticed the problem.
It was a case of the heat anticipator affecting 'the time needed to bring the room up to normal.'

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Mark Lloyd
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