RE: [GBlist] question about turning heat down

["Ralph Bicknese" <ralph.bicknese@christnerinc.com>]

Anja:

As Corwyn suggested, basic physics suggests turning the temperature down
will save energy.  This becomes clearer as the length of time the
temperature is kept down increases from say 8 hours at a time to a week or a
month or a year at a time.  However, there can be mitigating circumstances
as both the duration of setback becomes relatively short (say 6 to 12 hours)
with a high mass building, or depending on the type of heating equipment
used (if heat pumps).

Thermal mass acts to both limit the highs and lows of temperature swings in
spaces.  The more mass (assuming it is the type of mass absorbs heat
relatively easy (water or concrete), the more it will moderate temperature
swings. If the set back allows all of the excess stored heat in the mass to
be exhausted (used to heat the space) the setback will save less energy than
it might have.  However, it will not cause more energy to be used than had a
setback not been used at all and in virtually all cases will still save
energy.

Energy will be saved in proportion to the less difference there is between
inside and outside temperature. So the more one sets back the more one
saves. This is true if a person sets the thermostat back 1 degree, 20
degrees, or 40 degrees.  The main limitations are that you do not want to
setback the thermostat so low that you risk freezing people or pipes, and
secondly that if temperatures are lowered too low mass will be cooled and
will take a while to warm back up potentially causing thermal discomfort.
Thermal comfort can be affected due to the radiant effect of warm bodies
loosing heat to cool bodies.  Warm people will loose body temperature to
cold walls and floors.  The bigger the temperature difference between the
two, the greater the rate of loss will be.

Setbacks in both a low mass and a high mass building can be used more
effectively if the temperature is allowed to float naturally, or if the
setback turn on time is set to come on after some initial warming of the air
and or mass occurs.  So, thermal mass and exposure to thermal mass can be
used to affect the timing of peak temperatures.  Preheating can occur if say
solar heat is used. Here is an example:  If windows face lightly east of
south a space can be "tuned" to warm up sooner than it would be if the
windows faced due south.  However, in a high mass building it will take more
than an hour of sunlight to heat the mass sufficiently to increase thermal
comfort in the morning and unless there is a lot of stored solar input
things will be cool for several hours in the morning unless supplemental
heat is used. In a low mass building the sun (or wood stove, or furnace)
will heat the space up faster but will have higher temperature highs and
lows without auxiliary heat. The result is often that when things get too
warm people open the windows and dump the heat this negating some energy
savings. Mass helps to minimize this kind of thermal dumping. Sizing and
placing thermal mass for desired affect can be an art and a science unto
itself.  Thermal modeling using computer simulations can remove some of the
mystery and can even model the effect of supplemental heating as would occur
using a setback thermostat.

I mentioned earlier the type of heating (and/or cooling) equipment used with
setback thermostats can cause a system to use more energy.  This seems to be
the case mostly with heat pump systems and has to do with the fact that a
heat pump's efficiency drops as outside temperature drops (especially below
about 40 degrees F).  If a setback calls for the heat to turn on at 6:00 am
when the outside temperature is at or near its daily low (before the sun
warms up the mass of the great outdoors) you would be asking it to provide a
great deal of makeup heat just at a time when the heat pump will operate
most inefficiently. Some heat pump manufacturers make special setback
thermostats for use with heat pumps to modify the way the setback function
operates and improve the system efficiency.

I hope this explanation provided more answers than it raises questions. As
Sacie mentioned, some of this has been discussed previously and further
discussion is available in the archives.

Cheers,
Ralph Bicknese

-----Original Message-----
From: Anja Kollmuss [mailto:anja.kollmuss@tufts.edu]
Sent: Monday, January 28, 2002 2:02 PM
To: Jan Fillinger
Cc: greenbuilding@crest.org
Subject: Re: [GBlist] question about turning heat down

Hi Jan,
Thank you for your note. That's what I had thought. But I run into the
argument (that turning off your heat over night can waste more energy
than just lowering it) so often that I started to think maybe there is
something to it I don't know.

Thanks again!
--Anja

Jan Fillinger wrote:

> The answer, as you may have gathered from the somewhat insulting
> comments, is that you will save energy if you don't heat the rooms
> while nobody is in the space.  Heating or lighting unoccupied spaces
> is wasteful.  There are of course more complex issues associated with
> this question, such as how long the space will be unoccupied.  Turning
> off the heat, air-conditioning, fluorescent lights, HID lights, or
> computers for only one hour of absence might not be economical for
> some of the equipment in the long run.  But for anything approaching
> two or more hours of off-time, the general consensus is that energy
> savings do will outweigh the wear and tear on the equipment.
>
> Jan Fillinger
>
>
> Rush Dougherty wrote:
>
>> I don't mean to flame or be insulting, but I find this a very
>> strange
>> question from someone who is teaching a class....
>>
>> Especially when I see a isp like tufts.edu which has a
>> http://www.tufts.edu/source/libraries.html and also a
>> http://ase.tufts.edu/its.asp
>>
>> Makes one think about the quality of education today...
>> Rush
>>
>> ----- Original Message -----
>> From: "Anja Kollmuss" <anja.kollmuss@tufts.edu>
>> To: <greenbuilding@crest.org>
>> Sent: Monday, January 28, 2002 8:51 AM
>> Subject: [GBlist] question about turning heat down
>>
>>
>>
>> > I teach environmental classes and I run into this question again
>> > and
>> > again:
>> >
>> > How much should one turn down the heat at night: It it true that it
>> > is
>> > better not to lower the heat by more than 10 degrees (eg. to 58
>> > from
>> > 68), because it would take more energy to heat the rooms back up
>> > (say
>> > fom 50 to 68)?
>> >
>> > Can someone explain this to me?
>> > Thanks so much!
>> > --Anja
>> >
>>
>>
>> ______________________________________________________________________
>> This greenbuilding dialogue is sponsored by REPP/CREST, creator of
>> Solstice http://www.crest.org, and BuildingGreen, Inc., publisher of
>> Environmental Building News and GreenSpec
>> http://www.BuildingGreen.com
>> ______________________________________________________________________
>>
>>
>>
> --
> Jan Fillinger, Architect
> 541-868-1400
> janfillinger@jb.com
>
>

--
Anja Kollmuss
Tufts Climate Initiative
Miller Hall
Tufts University
Medford, MA 02144

phone: 617-627-5517
fax:   617-627-6645



______________________________________________________________________
This greenbuilding dialogue is sponsored by REPP/CREST, creator of
Solstice http://www.crest.org, and BuildingGreen, Inc., publisher of
Environmental Building News and GreenSpec http://www.BuildingGreen.com
______________________________________________________________________


______________________________________________________________________
This greenbuilding dialogue is sponsored by REPP/CREST, creator of
Solstice http://www.crest.org, and BuildingGreen, Inc., publisher of
Environmental Building News and GreenSpec http://www.BuildingGreen.com
______________________________________________________________________