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; 212-280-5517KING%KESTREL@sri-unix.UUCP (02/07/84)
From: Richard M. King <KING@KESTREL>
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.
I think this is incorrect. It would be correct if the thermostat
were "proportionally controlled", which means that it can set the furnace to
"partially on" as well as on or off. In a proportionally controlled unit,
the heat generated (or, more generally, the equilibrium restoration effort)
is proportional to the error term. The human body's temerature control
mechanism is proportionally controlled; you don't go from maximum shivering
to maximum sweating several times per day. Indeed, as Jeff writes, there
are additional inputs besides body temperature. Humans hypothalmi have skin
temperature inputs, and I recall in a Scientific American article a few
years ago that dogs have exercise transducers (but humans don't).
However, a house thermostat has only an on/off setting. Because of
this it has to have a hysteresis, or a range of temperatures over which it
will not change state. Otherwise it would change state rapidly, reducing
contact life and furnace life. If you set the thermostat to 20C, and it has
a hysteresis of 1C (a reasonably realistic number), it will start the
furnace at 19.5C and turn it off when the temperature has risen to 20.5C.
The anticipator is an attempt to account for the heat stored in the
radiator or whatever.
Assuming that the difference between the indoor and outdoor
temperatures is large compared to the width of the hysteresis band, we
observe that the temperature will fall at a constand rate within that band
when the furnace is off, following which it will rise at a constant rate
when the furnace is on. The average temperature is always halfway between
the ends of the hysteresis band.
So why does the room feel colder when the outside is colder? It has
been shown that a major reason is that the walls are colder, and a human
loses heat by radiation. The thermostat is comparitively insensitive to
radiation for two reasons; the active elements do not give off heat when the
furnace is off, so they radiate a lot less than a person, and they are
usually enclosed in a little metal box which prevents them from "seeing" the
walls. I've always thought that thermostats should be mounted on exterior
walls with just the right amount of insulation behind them so they would
take (a linear approximation of) the proper amount of notice of this effect.
Minor note: if your home is heated by a "heat pump" (a central air
conditioning unit that reverses in the winter to bring heat INTO the house
rather than take heat OUT) the thermostat has not two but three positions;
no heat, heat pump only, and heat pump plus auxiliary electric resistance
heaters. In this case, there are two hysteresis bands; the resistance
heater might turn off at 20C and on at 19.5C, and the heat pump might turn
off at 20.5C and on at 20C. In this case, on a cool day the heat pump might
cycle on and off and the resistance heat might never run, making the average
room temperature (20+20.5)/2=20.25, and on a frigid day the heat pump might
run continuously and the resistance heater cycle, making the average
temperature (19.5+20)/2=19.75. This form of heat is relatively rare. Note
that it is "closer" to proportional control than the usual thermostat.
Dick
-------jeff@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)STEINBERG@RUTGERS.ARPA (02/08/84)
From: Lou <STEINBERG@RUTGERS.ARPA> > Richard M King: > Assuming that the difference between the indoor and outdoor >temperatures is large compared to the width of the hysteresis band, we >observe that the temperature will fall at a constand rate within that band >when the furnace is off, following which it will rise at a constant rate >when the furnace is on. Yes BUT: The rates of rise and fall depend on the outside temperature. When it is colder, the house cools faster and warms slower. Thus when it is colder outside the average temperature is indeed lower. Also, since there is some delay between the time the thermostat turns on and the time the heat effectively reaches the room (due, e.g., to the heat capacity of radiators), the bottom of the hysteresis band will be lower when it's colder. However, these effects may be less significant than the other effects that have been mentioned, such as radiation and evaporation. -------
KING%KESTREL@sri-unix.UUCP (02/08/84)
From: Richard M. King <KING@KESTREL>
Return-path: <Mailer@KESTREL>
Received: from RUTGERS by KESTREL at 8-Feb-84 1019-PST
Date: 8 Feb 84 13:19:24 EST
From: Lou <STEINBERG@RUTGERS.ARPA>
Subject: Re: Why don't thermostats work?
To: KING@KESTREL.ARPA, physics@SRI-UNIX.ARPA
cc: STEINBERG@RUTGERS.ARPA
In-Reply-To: Message from "Richard M King <KING@Kestrel>"
of 7 Feb 84 12:18:00 EST
> Richard M King:
> Assuming that the difference between the indoor and outdoor
>temperatures is large compared to the width of the hysteresis band, we
>observe that the temperature will fall at a constand rate within that band
>when the furnace is off, following which it will rise at a constant rate
>when the furnace is on.
Yes BUT: The rates of rise and fall depend on the outside temperature.
When it is colder, the house cools faster and warms slower. Thus when it
is colder outside the average temperature is indeed lower. Also, since
there is some delay between the time the thermostat turns on and the time
the heat effectively reaches the room (due, e.g., to the heat capacity of
radiators), the bottom of the hysteresis band will be lower when it's
colder. However, these effects may be less significant than the other
effects that have been mentioned, such as radiation and evaporation.
-------
Lou, you caught me napping! Yes, there is this first-order effect that
causes the minimum temperature and the average temperature in a house to be
lower on colder days.
I will concede incorrectness, but I will offer the following
interesting second-order effect that goes the other way.
The rate of temperature rise and fall are NOT constant. If the
temperature is just barely cold enough to require occasional heat, the
temperature will drop faster when it is near the top of the hysteresis band
than when it is near the bottom. This will depress the average temperature.
The temperature will RISE at a steady rate (consider the relative distances
of the top and the bottom of the hysteresis band from the asymtotic
temperature with the heater locked "on"). The average temperature, ignoring
Lou's effect, is below the midpoint.
Now take a frigid day, one where the heater is only slightly more
than adequate to heat the house. This means that the asymptotic temperature
with heater on is slightly above the top of the H. band, so the temperature
will rise more quickly at the bottom of the band than at the top. It will
fall approximately linearly when the furnace finally succeeds in shutting
off the thermostat. Ignoring Lou's observation, the average temperature is
ABOVE the band midpoint.
Cases where the heater actually has to run ALL the time are not of
interest because the thermostat has no role.
Dick
-------peters@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)Schauble@MIT-MULTICS.ARPA (02/14/84)
I haven't ever really studied control heory, but I have a lot of
practical experience with heating system. Unfortunately, the Usenet time
warp has delivered this reply without delivering the original message,
but I'll try answering anyway.
1) Most home thermostats of whatever type do contain an anticipator that
works as jeff describes. But not for that reason. The problem is that
the heating system has a considerable thermal capacity that has to be
warmed before it begins to delivery heat to the room. After the furnace
is shut off, this heat storage capacity causes the system to continue
delivering heat. This stored heat would make the room too warm. The
anticipator warms the thermostat and causes it to shut the furnace off
early, thus compensating.
2) The description of the "error term" that the thermostat generates is
accurate. But this effect is small, perhaps a degree for a good
thermostat. Too small to be really noticed. The perceived coldness of
the room is a subjective effect that does not depend on room
temperature. This is because the room walls are colder, increasing body
heat loss to radiation, and because the indoor air is dryer, thus
increasing heat loss to skin evaporation. If you live in a cold climate,
both of these are correctable, by wall hangings and by humidification.
Paulphil@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!phildmmartindale@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)
KING%KESTREL@sri-unix.UUCP (02/29/84)
From: Richard M. King <KING@KESTREL> Those lumpy things in the blanket are safety devices. Covering an electric blanket with another blanket does not affect its duty cycle. I have performed this experiment (with one specific brand of blanket, in my youth). Dick -------
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