seifert@ihuxl.UUCP (D.A. Seifert) (02/02/84)
{ This should really be in net.mechanical_engineering,
or net.control_theory, but there isn't any, so... }
Question: why don't thermostats work?
I'm talking about the standard bi-metallic strip which opens
and closes electrical contacts to control a furnace (or ac)
and supposedly maintain a steady inside temperature dispite
a varying outside temperature. (fig 1) What usually happens
is that as the outside temp falls, the inside temp also falls,
(see fig 2) and you have to adjust the thermostat to maintain
a reasonable inside temp. With the current weather conditions
(30F below one day, 40F above the next), this is quite a pain.
inside | ideal thermostat set at 70F /
temp | (furnace only, furnace runs /
80 | out of BTUs at -10F) /
| /
70 | ________________________________/
| /
60 | /
| /
50 | /
| /
| /
|__________________________________________________
| | | | | |
-20F 0 20 40 60 80 outside temp
fig 1
inside | real thermostat set at 70F /
temp | (furnace only, furnace runs /
80 | out of BTUs at -10F) /
| /
70 | _________/
| _________/
60 | ______/
| _____/
50 | /
| /
| /
|__________________________________________________
| | | | | |
-20F 0 20 40 60 80 outside temp
fig 2
(the graph in fig 2 should be a smooth line, not a stair step,
but until we all get graphics terminals...)
Anyone know why this happens? The thermostat doesn't know what
temperature it is outside, it should only know "it's below 70
in here => turn furnace on". But somehow it's getting fooled
into thinking that the inside temp is 70 when it's really 60.
Sometimes it seems that a knob controlling the duty cycle
(with no feedback) would do just as well. -sigh-
--
_____
/_____\ from the flying doghouse of
/_______\ Snoopy
|___|
____|___|_____ ihnp4!ihuxl!seifertparnass@ihuxf.UUCP (Bob Parnass, AJ9S) (02/03/84)
Your thermostat and furnace form a control system. Control systems often have some amount of hysteresis designed in to prevent oscillation. If your system had no hysteresis, the furnace would be cycled off and on much more frequently, shortening the life of the components. -- ========================================================================== Bob Parnass, AT&T Bell Laboratories - ihnp4!ihuxf!parnass - (312)979-5760
jeff@heurikon.UUCP (02/04/84)
I'm not a thermostat expert, and it was a long time ago that I took
control theory, but I'll take a stab at answering your questions:
1) The bi-metal strip thermostats have something called an "anticipator"
in them. It's a little heater element which runs off the 250 ma or so
of current used by the furnace relay. When the thermostat is calling
for heat, the heater element warms up the strip, thus anticipating
the warming of the room. If this were not done, there would be wild
changes in temperature - it would get much warmer in the room before
the thermostat shut off. You'd be uncomfortable because of the wider
temperature swings. (Look inside your thermostat and you should find
a little calibration screw or lever, labeled in milliamps.)
(For you Californians who - through lack of use - don't know what a
thermostat is, it's that little box with levers under your doorbell.
Sorry, I just *couldn't resist that.)
2) A closed loop control system (such as a room-thermostat-furnace-room)
regulates the "process" by creating an error term. A thermostat
is a relatively dumb control unit. There must be an error in room
temperature in order for the thermostat to call for heat. The
thermostat does not know how much heat is leaving the room, it only
knows that there is an error in desired the temperature. The average
error will be proportional to the rate of heat loss in the room. So,
on colder days, the average room temperature will be lower.
Look at it this way: If a (dumb) thermostat *was* able to get the
room temperature *exact*, the error would be zero. If the error were
zero, the thermostat would not call for heat. But if it doesn't ask
for heat, the room temperature will fall. So the error *can't* be zero.
For the system to reduce the error to zero, there must be additional
intelligence. The system must be able to compute the heat loss. When
you manually increase the setting on a cold day, you are adding that
"intelligence" by adding a constant to the error term to compensate for
the extra rate of heat loss. There are some uproc based thermostats on
the market. I saw one (Heath?) which is smart enough to realize that
the error term isn't going to zero and increase its call for heat.
In control theory, the extra feedback term which is needed to completely
reduce the error to zero is one which integrates the error signal over
time. So, to regulate the error to zero, you need two feedback terms:
one which is proportional to the error itself, and the other which is
the integral of the residual error. Thermostats lack the latter.
Your idea of using a simple knob instead of a thermostat would allow you to
get the exact desired temperature as long as the room's heat loss rate was
a constant. Simply raising the thermostat setting does the same thing
and also compensates *some* for changes in the loss rate.
Now, is there a control theory expert out there who can confirm any of this?
--
/"""\ Jeffrey Mattox, Heurikon Corp, Madison, WI
|O.O| {harpo, hao, philabs}!seismo!uwvax!heurikon!jeff (news & mail)
\_=_/ ihnp4!heurikon!jeff (mail - fast)peters@cubsvax.UUCP (02/07/84)
heurikon!jeff's commentary on this question is excellent... I just want
to amplify a bit. (By the way, I used to work for Owens/Corning Fiberglas,
and though I didn't work directly in energy conservation, some of my friends
were experts in "HVAC" [afficionados will know the acronym], and I picked
up some things informally... I also had to use PID controllers (see below)
to regulate processes.)
Jeff's comment about adding an integrating function to compensate residual
errors is correct. A proportional controller will in general settle down
or oscillate about a temperature different from the setpoint; the integrator
amounts to an automatic reset function. Even this, however, works well only in
steady-state conditions; in this case, that would mean constant outdoor
temperatures, essentially. By the way, such controllers are called "PI"
controllers -- for Proportional Integrating. For non-steady state systems,
such as when the temperature varies outdoors, or in a chemical process, a ton of
cold reactant is added to the kettle halfway through the process, a third
function is added, which responds to the rate of change of the error, and adds
an extra boost of heat if all of a sudden, say, someone opens the door on a cold
winter day. "PID" controllers, where the D stands for "Differentiating,"
incorporate this as well as the P & I functions, and let me tell you they are
a son-of-a-you-know-what to tune to a process!
Home thermostats are really only on-off sytems -- not even Proportional! --
and these tend to oscillate around the set point quite severely. What the
anticipator does is to heat up the bimetallic element while the couple is
calling for heat, to compensate for the time-lag involved with the room air
diffusing into the thermocouple box. A guy I worked with wrote his Ph. D.
thesis about modelling a home furnace/thermostat system. As earler articles
have pointed out, it's *very* complicated. I believe there were ten or
fifteen terms in his model.
Now, a few more comments about how to make it better. Industrial heating
systems work differently. In, say, an office building, in, say, the winter,
the periphery of the building (that means near the windows, for all you
hackers) is always being heated. The room air coming from the ceiling vents
is switched between heated and chilled air to either "buck" or augment the
peripheral heating system. Without that, it would always be very cold near
the windows, due to radiative heating (i. e., of the cold walls by warm bodies).
These systems, unfortunately, are also difficult to "tune," or "balance," and
maintenance people don't usually know enough to do it.
Eventually, thermostats will have a "learn" cycle, in which they record
temperature changes, etc., and adjust their own parameters, perhaps on the
fly. In additiion, if they have access to outside temperatures, together
with the information that heat flux is proportional to (T[in] -T[out]),
they should be able to do very well indeed.
{philabs,cmcl2!rocky2}!cubsvax!peters Peter S. Shenkin
Dept of Biol. Sci.; Columbia Univ.; New York, N. Y. 10027; 212-280-5517jeff@heurikon.UUCP (02/08/84)
I have found the information about the "smart" thermostat. It
is (or was) sold by JS&A, Northbrook, Il. (800)-323-6400. Are
they still in business? I'm looking at a 1982 catalog.
Anyway, it's called the "Love/Hate Thermostat". Named, I guess,
for its functionality and lack of, ah, beauty. It is microprocessor
controlled, of course, and allows a whole bunch of setbacks settings
per day. It even knows the difference between a weekday and the weekend.
Some interesting features (editted):
"You set most thermostats to the time you want the furnace to go on
in the morning. But what if one morning it's bitter cold outside
and the next morning it's much warmer? The Magic Stat senses and
computes the drop in temperature and the time it will take to get
your room to your exact wake up temperature. The system also
computes the ideal length the furnace should stay on to keep the
temperature within a range of plus or minus one and one-half degrees."
The thing also has a "learn" mode. You operate it manualy for a day
or two and it remembers your living pattern. Sounds like one of
those Detroit robots, doesn't it? Anyway, I think it represents
a perfect application of a microprocessor in a home. Price: $80.00,
ceaper than some "dumb" varieties.
--
/"""\ Jeffrey Mattox, Heurikon Corp, Madison, WI
|O.O| {harpo, hao, philabs}!seismo!uwvax!heurikon!jeff (news & mail)
\_=_/ ihnp4!heurikon!jeff (mail - fast)hogg@utcsrgv.UUCP (John Hogg) (02/08/84)
The response from the Great White North is that thermostats tend to be very
crude in design and poorly positioned. If a thermostat is placed above or
close to a radiator (the worst culprits are electric radiators with integral
thermostats) then it must be set high, since it thinks the room is warmer than
it actually is. Thus, as the outside temperature goes down and the heating
system must stay on longer to compensate, the thermostat lives in its own
warm little corner, and the rest of the house freezes. Or to rephrase, the
temperature gradient between the radiator and the outside wall increases, and
as the thermostat lives near the former, the actual living-space temperature
drops.
You can get the reverse effect by putting the thermostat on an outside
wall or in a drafty hall. As the outside temperature falls, the thermostat
gets hit by cold blasts and over-responds, cooking the rest of the house.
Problems of this sort will be amplified by poor insulation. If your
heating system can't keep the house at a reasonable temperature when it's only
-10F outside, then either its main component is a large candle or you should
seriously investigate the price of weatherstripping...
--
{allegra,cornell,decvax,ihnp4,linus,utzoo}!utcsrgv!hoggrpw3@fortune.UUCP (02/09/84)
#R:ihuxl:-87700:fortune:6700029:000:1129
fortune!rpw3 Feb 9 03:13:00 1984
In the good electric blankets there are TWO thermostats. One is
"distributed" through the blanket (in lumps, usually) and measures
how warm the blankets (~~you) are, and the other is in the control
housing and measures how cold the room is. Some have split (he/she)
controls (and therefore split blanket thermostats).
DON'T put the control under the blanket. You will freeze! But also don't
put it on the floor, near the central heat, next to a window, etc. Put it
somewhere that approximates the air environment around you. (My parents
used to pull out one of the drawers and lay it on the folded shirts.)
Even so, it only works well if the heat loss from the blanket to the
room approximates what the manufacturer thought is was going to be
(e.g., they thought you would use the electric blanket under a single
light spread, and you use it (a) on top, or (b) under a comforter!)
(I don't like them, myself, even under the best of conditions. Oh well...)
Rob Warnock
UUCP: {sri-unix,amd70,hpda,harpo,ihnp4,allegra}!fortune!rpw3
DDD: (415)595-8444
USPS: Fortune Systems Corp, 101 Twin Dolphins Drive, Redwood City, CA 94065peters@cubsvax.UUCP (02/10/84)
Yes, industrial heating systems are inefficient, especially in the summer.
In the winter, one bucks warm peripheral air with chilled air that is free;
that is, it can be taken from outside, and usually has to be pre-heated a
little in cold weather before being used. This isn't so bad. In the summer,
however, one cools the periphery with expensive conditioned air, and then
bucks this cooling effect with air which has been pre-heated at additional
energy expense! (Conditioning of the warm air is necessary to get rid of
the humidity.)
Unfortunately, without this, there would be great temperature gradients as
between an outside wall and the interior of the building.
There are usually trade-offs between efficiency and performance, and this
is an example.
The inefficiency is compensated by the fact that heat-loss is proportional
to surface area; the lower surface-to-volume ratios of large industrial
buildings -- which is, after all, what causes the problem which the more
complicated heating/cooling systems are supposed to fix --
also makes them inherently more energy efficient than small
structures like houses. I believe (though I
haven't seen data) that the heating bill for these large structures will be
lower per square foot of floor space than for, say, a house, other things being
equal.
{philabs,cmcl2!rocky2}!cubsvax!peters Peter S. Shenkin
Dept of Biol. Sci.; Columbia Univ.; New York, N. Y. 10027; 212-280-5517jeff@heurikon.UUCP (02/10/84)
To Phil, RE: Electric blankets.
The thermostats on electric blankets regulate by sensing the
room temperature, not *your* temperature. (Wow! what a straight
line that is!) Anyway, a blanket thermostat has a little heater in
it which adds heat to the thermostat whenever the blanket is adding
heat to your bed. I guess the theory is that the rate of heat loss
by the bed will be similar to the rate of heat loss by the thermostat.
Mine seems to work pretty well, but I don't know how accurate they are.
What should *really* be regulated is not the bed temperature, but your
*feeling* about what the temperature is. During the night, as your
metabolism changes, I'm sure most people would sense a different
temperature even if the actual bed temperature didn't change.
Now, somebody could probably revolutionize the electric blanket
thermostat industry by designing some sort of sensor for *that*.
--
/"""\ Jeffrey Mattox, Heurikon Corp, Madison, WI
|O.O| {harpo, hao, philabs}!seismo!uwvax!heurikon!jeff (news & mail)
\_=_/ ihnp4!heurikon!jeff (mail - fast)jeff@heurikon.UUCP (02/10/84)
> Some electric blankets have thermostats spread thoughout the blanket. I always thought the little sensors *inside* the blanket were not thermostats but rather emergency cut offs in case of over temperature. Fire protectors. I don't like them; one always seems to be rubbing my ribcage. -- /"""\ Jeffrey Mattox, Heurikon Corp, Madison, WI |O.O| {harpo, hao, philabs}!seismo!uwvax!heurikon!jeff (news & mail) \_=_/ ihnp4!heurikon!jeff (mail - fast)
phil@amd70.UUCP (Phil Ngai) (02/15/84)
I have a couple of questions in response to Peter Shenkin's article.
1) You mean they heat the periphery rooms and then put chilled air in?
Is this inefficient?
2) Does anyone else find electric blankets uncomfortable? How can they
regulate the heat if the thermostat doesn't know what the blanket's
temperature is? Or are you supposed to put the thing *under* the blanket?
--
Phil Ngai (408) 988-7777 {ucbvax,decwrl,ihnp4,allegra,intelca}!amd70!philpreece@uicsl.UUCP (02/17/84)
#R:heurikon:-20500:uicsl:7500047:000:300 uicsl!preece Feb 16 11:34:00 1984 I think the principal function of the thermostats in electric blankets is protection against fires caused by overheating. There are a bunch of thermostats in an electric blanket and their number and placement is regulated (check the recent Consumer Reports review of electric blankets for details).
dmmartindale@watcgl.UUCP (Dave Martindale) (02/25/84)
I disagree. Electric blankets have thermostats in the blanket which will shut off the flow of current if that particular area of the blanket gets very hot. These are not adjustable. The thermostat which controls the blanket temperature normally is in the control box. It will respond to room temperature changes, but it is really maintaining a constant temperature in the control box (there is a little heater in there which is on whenever the blanket is on) and thus the under-blanket temperatures will not necessarily remain constant if the room temperature changes.
jeff@heurikon.UUCP (02/26/84)
> Electric blankest have their thermostats in the blanket, not in > the control box. Thus they do actually measure the temperature > under the blanket. > Ed Gould ucbvax!mtxinu!ed Ed: Then how come when I take my control box apart I find a bi-metal strip with a little heater coil wrapped around it? And how come my control box clicks on and off all night? And how come when the control box clicks "on" my blanket gets warmer and when it clicks "off" the blanket gets colder? And how come if I put the control box *under* the blanket things cool off? And how come the temperature control knob is on the control box? And how come... Ah, I know! You California types don't have what we call "electric blanket savvy". Either that or they aren't making blankets like they used to. -- /"""\ Jeffrey Mattox, Heurikon Corp, Madison, WI |O.O| {harpo, hao, philabs}!seismo!uwvax!heurikon!jeff (news & mail) \_=_/ ihnp4!heurikon!jeff (mail - fast)
phil@amd70.UUCP (Phil Ngai) (02/29/84)
I sure find it hard to believe the thermostat is IN the blanket.
I think it's in the box.
--
Phil Ngai (408) 988-7777 {ucbvax,decwrl,ihnp4,allegra,intelca}!amd70!philcwb@cbneb.UUCP (Bill Brown) (03/02/84)
I always figured there was some sort of temperature sensing device in my electric blanket since there are three wires between the box and the blanket. I know the box has a switch because I hear it click. My guess is that a thermister or some such device controls the current in the third conductor in such a way as to influence a bimetal switch in the box. Why doesn't somebody tear one apart. I would, but I can never get these kinds of things back together.
rfg@hound.UUCP (R.GRANTGES) (03/02/84)
Fortunately, I think, I have missed 100 of the last 102 items on this subject. However, if you must know about electric blankets: everyone is right. There are temperature sensing devices (thermostats?) in <both> the blanket itself <and> the control box. The one in the control box is the one that controls the amount of current to the blanket, hence, its warmth. As the room gets colder, this control heats up the blanket more. There is an adjustment for how hot do you want to be anyhow, sometimes two adjustments: one for each half of the bed. The temperature sensors <in> the blanket itself are there to <hopefully> keep you from burning up if something goes wrong and the blanket overheats (say you have covered part of it with a heavy blanket or stuck the heating wire under the mattress). Think of these sensors as fuses. Dick Grantges hound!rfg
olney@fortune.UUCP (John Olney) (03/06/84)
Here's an analysis of how an electric blanket works that I haven't seen on the net yet. (Of course, I've only been on the net about a week....) The temperature of the blanket is actually regulated in the control box; it works on the same principle as modern electric stoves and u-wave ovens. I don't know what the technical term is, but it's like a switching power supply, switching at a very low frequency. (It's also like a D-class audio amplifier, using pulse width modulation.) If you want your bed to be very warm, the control box keeps the heating wires going most of the time. If you want to be a little bit less warm, the box keeps the wires going a little bit less of the time. If you want to be relatively cool, the heating wires are turned on every once in a while. It's a duty-cycle kind of thing: the more time that the control box lets current flow through the wires, the warmer the blanket gets. The actual current stays constant; it's the time span that counts. The person who dissected his control box and found a heating coil surrounding a bimetal strip was looking at the on/off timer. The thermostats inside the blanket are for safety. If any portion of the blanket gets too hot, the internal thermostats will cut it off. I think this is mandated by the Federal Government. Regards to those in netland. -- jho
ed@unisoft.UUCP (03/09/84)
Electric blankest have their thermostats in the blanket, not in the control box. Thus they do actually measure the temperature under the blanket. -- Ed Gould ucbvax!mtxinu!ed