WEBVTT
Kind: captions
Language: en

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When you go up to your thermostat to raise
the temperature, what really happens?

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Obviously the heating system comes on (if
we’re still in heating season) but what

00:00:10.150 --> 00:00:13.000
did the thermostat do to make that happen?

00:00:13.000 --> 00:00:17.010
Well, it did one of the three things a typical
thermostat can do.

00:00:17.010 --> 00:00:18.570
It commanded heat.

00:00:18.570 --> 00:00:21.640
It could have also told your forced-air furnace
to run the fan.

00:00:21.640 --> 00:00:26.960
Or, if we’re in the summer months, it can
tell it to run the fan and turn on the air conditioner.

00:00:26.960 --> 00:00:27.660
That’s it.

00:00:27.660 --> 00:00:31.640
The typical American thermostat has just 4
wires going to it.

00:00:31.640 --> 00:00:37.330
A common 24 volt AC feed, a return wire for
heat, another wire for cooling, and another

00:00:37.330 --> 00:00:38.830
wire for the fan.

00:00:38.830 --> 00:00:43.350
All the thermostat does is complete a circuit
on one of those three control wires and the

00:00:43.350 --> 00:00:45.960
heating and cooling system will respond accordingly.

00:00:45.960 --> 00:00:47.280
It will either heat.

00:00:47.280 --> 00:00:48.280
Or cool.

00:00:48.280 --> 00:00:49.290
Or circulate the air.

00:00:49.290 --> 00:00:52.280
Now, notice that it can’t tell the furnace
how much to heat.

00:00:52.280 --> 00:00:54.600
It can only tell it HEAT NOW.

00:00:54.600 --> 00:00:56.610
And then it can tell it to stop.

00:00:56.610 --> 00:00:58.370
That’s all the control it has.

00:00:58.370 --> 00:01:02.440
See, the furnace or air conditioner or whatever
you’ve got doesn’t typically have any

00:01:02.440 --> 00:01:05.540
sense of what’s actually going on.

00:01:05.540 --> 00:01:10.620
There are some more complicated systems, yes, but
the basic domestic HVAC systems you see all

00:01:10.620 --> 00:01:14.040
around the US are actually quite dumb.

00:01:14.040 --> 00:01:17.680
Really, most home appliances are dumber than
you think, but I digress.

00:01:17.680 --> 00:01:22.500
So, they rely on a thermostat as a sort of
life coach to tell them what to do.

00:01:22.500 --> 00:01:26.620
Now a lot of you are probably thinking “duh,
I know this, why is Technology Connections

00:01:26.630 --> 00:01:28.390
talking about thermostats?”.

00:01:28.390 --> 00:01:29.740
But there are reasons.

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First is that a number of people have asked
me to explain to someone in their life that

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setting the thermostat up to 80 degrees does
not make the heat go faster.

00:01:39.950 --> 00:01:44.979
It just makes the heat run for a long time
as it works to reach that much-higher-than-right-now

00:01:44.979 --> 00:01:46.150
temperature.

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If you want to set the thermostat higher,
just set it where you want it.

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The furnace will run on full blast until it
gets there, then it will shut off.

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It runs on full blast because that’s the
only degree of blast that a furnace can run.

00:01:58.640 --> 00:02:00.240
There is no medium blast.

00:02:00.240 --> 00:02:01.320
Only full.

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And off.

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I’m sure there are exceptions even in the domestic environment so be sure to comment about them.

00:02:06.500 --> 00:02:07.500
It boosts engagement!

00:02:07.620 --> 00:02:11.360
Anyway, the second reason I’m making a video
on thermostats has to do with the fact that

00:02:11.370 --> 00:02:13.330
they’re pretty neat.

00:02:13.330 --> 00:02:17.000
Now, new digital thermostats like this are
pretty boring.

00:02:17.000 --> 00:02:20.069
It’s basically a small battery-operated computer that closes

00:02:20.069 --> 00:02:22.500
the contacts of a relay when necessary.

00:02:22.500 --> 00:02:25.530
Add an LCD, some buttons, and boom.

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Programmable thermostat.

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[read with intense apathy] 
Then put it on WiFi and connect it to the internet and all

00:02:29.660 --> 00:02:31.900
sorts of amazing things can happen.

00:02:31.900 --> 00:02:37.670
So, how can just 4 wires provide the necessary
control to regulate the temperature of your home?

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To show you, we need a furnace.

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Perfect!

00:02:40.440 --> 00:02:43.810
So, unless your heating system is frightfully
ancient, there’s gonna be a control board

00:02:43.810 --> 00:02:48.480
in there somewhere with some relays, resistors,
and all that fun stuff, as well as a terminal

00:02:48.480 --> 00:02:50.630
block with a few wires attached to it.

00:02:50.630 --> 00:02:55.110
These wires are going to the thermostat which,
in this case, is upstairs in the living area.

00:02:55.110 --> 00:02:57.230
For now, ignore the blue one.

00:02:57.230 --> 00:02:58.510
We’ll get back to that.

00:02:58.510 --> 00:03:03.599
So, the control board is providing a 24 volt
A/C feed on this here terminal, labeled R

00:03:03.599 --> 00:03:05.040
for Red.

00:03:05.040 --> 00:03:06.840
These other terminals are inputs.

00:03:06.840 --> 00:03:08.989
The white wire is the input for heat.

00:03:08.989 --> 00:03:11.849
Yellow does A/C. And green runs the fan.

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Blue does nothing.

00:03:13.100 --> 00:03:14.460
Ignore the blue.

00:03:14.470 --> 00:03:18.569
To make the furnace heat, all we need to do
is jump the red wire to the white wire.

00:03:18.569 --> 00:03:23.709
Now, the logic board sees 24 volts on the
heat input, and it starts to heat.

00:03:23.709 --> 00:03:27.770
And when it no longer sees 24 volts on the
white wire, it shuts off.

00:03:27.770 --> 00:03:29.410
And what jumps those wires together?

00:03:29.410 --> 00:03:31.460
Why, the thermostat, of course!

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Thermostats are among the simplest ways to
regulate temperature.

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And they do it with time.

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See, suppose you have a furnace that can output
10 kilowatts of heat

00:03:39.900 --> 00:03:43.000
(that’s roughly 34,000 BTUs per hour).

00:03:43.000 --> 00:03:48.099
There aren’t many days when you need that
much heat, but it’s there just in case.

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Suppose on a typical day, you only need an
average of 3 kilowatts of heat.

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Well, how can you make a dumb furnace do that?

00:03:55.390 --> 00:03:58.720
It has no mechanism of its own to reduce its
heat output.

00:03:58.720 --> 00:04:00.950
You can only tell it to be on or off.

00:04:00.950 --> 00:04:01.950
Well that’s fine.

00:04:01.950 --> 00:04:05.989
I only need 30% of its total heat output,
so what I’ll do is I’ll have it run for

00:04:05.989 --> 00:04:08.910
three minutes, then be off for 7.

00:04:08.910 --> 00:04:13.240
In every 10 minute period it runs for three
minutes, which means it produces 30% of its

00:04:13.240 --> 00:04:15.180
rated heat, or 3 kilowatts.

00:04:15.180 --> 00:04:16.120
Simple.

00:04:16.120 --> 00:04:17.700
Yeah, it is pretty simple.

00:04:17.709 --> 00:04:19.430
Almost remarkably so.

00:04:19.430 --> 00:04:22.849
Thermostats are a reactive means of controlling
temperature output.

00:04:22.849 --> 00:04:26.409
If the current temperature is lower than the
thermostat’s set point, it will command

00:04:26.409 --> 00:04:28.039
heat from the furnace.

00:04:28.039 --> 00:04:31.000
After the house has warmed to a certain degree
beyond the set point,

00:04:31.000 --> 00:04:33.180
it will stop commanding heat.

00:04:33.180 --> 00:04:37.719
Even though it has only a binary state, either
on or off, it is able to regulate temperature

00:04:37.719 --> 00:04:42.139
by running the furnace periodically to stay
within a set temperature range.

00:04:42.139 --> 00:04:45.940
The most interesting thermostats are the super
basic cheap ones.

00:04:45.940 --> 00:04:50.740
These work using the most rudimentary of technologies,
and yet they are still an effective means

00:04:50.740 --> 00:04:52.789
of regulating temperature.

00:04:52.789 --> 00:04:55.380
This is as basic a thermostat as you can get.

00:04:55.380 --> 00:04:59.610
It just handles heat, and it’s very cheap
feeling and kinda ugly but hey.

00:04:59.610 --> 00:05:01.050
It works.

00:05:01.050 --> 00:05:05.979
Inside this you will find a pair of contacts,
one of which is on this groovy spring thing.

00:05:05.979 --> 00:05:09.830
That spring is a really long bimetallic strip
formed into a coil.

00:05:09.830 --> 00:05:14.300
A bimetallic strip will bend as its temperature
changes thanks to the slightly different thermal

00:05:14.300 --> 00:05:17.689
expansion properties of its two laminated
halves.

00:05:17.689 --> 00:05:22.259
When formed into a coil, a change in ambient
temperature will cause the coil to get tighter

00:05:22.259 --> 00:05:23.389
or to loosen.

00:05:23.389 --> 00:05:27.520
Since this end of the coil is free to move
about, the effect is that this end will move

00:05:27.520 --> 00:05:30.789
either left or right with a change in temperature.

00:05:30.789 --> 00:05:34.639
This exact sort of thing is what makes those
big ‘ol garden thermometers work, though

00:05:34.639 --> 00:05:39.879
in that case this end would be fixed and the
center would be free to rotate a pointer needle.

00:05:39.879 --> 00:05:44.089
Adjusting the set point of this thermostat
adjusts the tension on the coil through rotating

00:05:44.089 --> 00:05:46.169
its central attachment point.

00:05:46.169 --> 00:05:50.479
In this case, as the temperature falls, the
spring gets tighter which causes the free

00:05:50.479 --> 00:05:52.330
end to move to the right.

00:05:52.330 --> 00:05:56.260
As it gets colder and colder, the free contact
gets closer to the other,

00:05:56.260 --> 00:05:58.160
and once it gets close enough

00:05:58.160 --> 00:05:59.020
*click*

00:05:59.020 --> 00:06:02.939
The contacts are pulled
together with the help of this little magnet.

00:06:02.939 --> 00:06:05.389
And what are those contacts attached to, you
ask?

00:06:05.389 --> 00:06:09.509
Why, the red and white wires going to the
board on your furnace.

00:06:09.509 --> 00:06:14.279
So long as these contacts are touching, it
commands heat from the furnace.

00:06:14.279 --> 00:06:19.490
Once the ambient air has heated up enough
to cause these contacts to break apart, then

00:06:19.490 --> 00:06:23.990
the furnace no longer sees 24 volts on the
white wire, so it stops heating.

00:06:23.990 --> 00:06:25.379
It’s that simple.

00:06:25.379 --> 00:06:29.939
The reason there’s a magnet is to create
clear start and stop points in order to prevent

00:06:29.939 --> 00:06:33.599
the furnace from running very frequent, very
short cycles.

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Without it, imagine what would happen when
the contacts touched.

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The furnace would switch on, but after heating
just a teeny tiny bit, the contacts would

00:06:41.479 --> 00:06:42.539
move apart again.

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Then of course, they’d almost immediately
touch when it cooled just an itty bitty bit.

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This is no bueno.

00:06:49.610 --> 00:06:54.399
The magnet prevents this by forcing the contacts
to stay together until the ambient air temperature

00:06:54.399 --> 00:06:57.419
has risen by at least a degree or so.

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And as a bonus, once the spring can finally
overcome the magnet’s attraction and the

00:07:02.099 --> 00:07:06.280
contacts snap apart, it’s now so far to
the left, that it needs to get at least a

00:07:06.280 --> 00:07:10.210
degree or so colder before they touch again
and command heat.

00:07:10.210 --> 00:07:11.360
Clever.

00:07:11.360 --> 00:07:15.710
Additionally, it helps to prevent arcing on
the contacts by ensuring they connect and

00:07:15.710 --> 00:07:17.659
disconnect as quickly as possible.

00:07:17.659 --> 00:07:19.309
But what about the green wire?

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And the yellow one?

00:07:20.929 --> 00:07:24.300
You might have already spotted that there’s
a suspiciously similar indentation in the

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plastic to the left of the heating contact.

00:07:27.189 --> 00:07:32.340
If there were another contact placed there,
then we’d also be able to control an air conditioner.

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Since the contact moves to the right as it
gets colder, it would work in exactly the

00:07:36.249 --> 00:07:41.029
same fashion, though in reverse--in this case,
when it gets too warm, the contact snaps to

00:07:41.029 --> 00:07:46.159
the left, which would put 24 volts on the
yellow wire and start the air conditioner.

00:07:46.159 --> 00:07:50.759
And of course, once it’s cool enough, it
snaps back to the right and breaks the connection,

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turning the air conditioner off.

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Honeywell didn’t really try to hide the
fact that they just sold you a de-featured

00:07:55.709 --> 00:07:56.709
thermostat.

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There’s a rather clear Y label for what
would be your air conditioning terminal.

00:08:01.319 --> 00:08:03.599
Too cheap to make a different mold, eh?

00:08:03.599 --> 00:08:08.029
When equipped for air conditioning, too, there
would be a mode switch coming out the bottom

00:08:08.029 --> 00:08:10.439
to disable heating or cooling commands.

00:08:10.439 --> 00:08:15.339
All that it would do is prevent a cooling
call from happening in heat mode, and vice versa.

00:08:15.339 --> 00:08:19.840
This prevents a scenario where it cools so
much that it snaps to the right, which turns

00:08:19.840 --> 00:08:25.300
on the heat, which snaps it to the left, which
turns on the A/C, which snaps it to the right,

00:08:25.300 --> 00:08:29.090
which turns on the heat, which snaps it to
the left, which turns on the A/C, which snaps

00:08:29.090 --> 00:08:32.099
it to the right, which turns on the heat,
which snaps it the left...

00:08:32.099 --> 00:08:35.880
Now that you know that thermostats are really
just shunting a couple of wires together to

00:08:35.880 --> 00:08:40.180
command the furnace to do something, you’ll
understand why newer digital thermostats

00:08:40.180 --> 00:08:41.720
need batteries.

00:08:41.720 --> 00:08:45.730
In the case of this basic digital thermostat,
this relay performs the task of connecting

00:08:45.730 --> 00:08:47.240
the wires together.

00:08:47.240 --> 00:08:51.010
The mode switch probably changes the path
of the connection, allowing for one relay

00:08:51.010 --> 00:08:53.120
to do both heating and cooling.

00:08:53.120 --> 00:08:57.830
It uses a thermocouple to determine the current
temperature and compares it to the set point,

00:08:57.830 --> 00:09:01.780
and when it determines that it needs to command
heat, you hear a click.

00:09:01.780 --> 00:09:07.270
Then, and only then, is there an actual complete
circuit going through the thermostat.

00:09:07.270 --> 00:09:11.970
While there is 24 volts potential on the red
wire, the only way to complete a circuit and

00:09:11.970 --> 00:09:16.680
thus be able to use some of that power is
to tell the furnace to do something.

00:09:16.680 --> 00:09:23.020
So, unless you want the fan to run 24/7, you
need the thermostat to have its own power source.

00:09:23.020 --> 00:09:27.380
Unless, of course, you’re either tricky
or you have what’s called a C wire.

00:09:27.380 --> 00:09:29.720
That’s the blue one.

00:09:29.720 --> 00:09:33.850
Anyone who has installed a smart thermostat,
or even considered installing one, knows what

00:09:33.850 --> 00:09:35.230
the C wire is.

00:09:35.230 --> 00:09:41.440
Essentially it’s just a return path to the
furnace’s logic board that doesn’t command anything.

00:09:41.440 --> 00:09:46.190
This way, a thermostat can be powered by the
furnace itself by using the 24 volts potential

00:09:46.190 --> 00:09:49.070
across the red and blue wires all the time.

00:09:49.070 --> 00:09:52.890
To command either heat, cooling, or the fan,
the thermostat will shunt the red wire to

00:09:52.890 --> 00:09:56.180
either the white, green, or yellow just like
any other thermostat.

00:09:56.180 --> 00:10:01.460
But, because it can keep an active connection
across red and blue at all times, it doesn’t

00:10:01.460 --> 00:10:03.190
need its own power source.

00:10:03.190 --> 00:10:07.920
Some smart thermostats, like the Nest, have
a small rechargeable battery inside them that

00:10:07.920 --> 00:10:08.980
*usually*

00:10:08.980 --> 00:10:11.780
allows you to get away without a
C wire.

00:10:11.780 --> 00:10:14.090
That’s the being tricky option.

00:10:14.090 --> 00:10:17.850
What these do is charge the battery whenever
it’s commanding something, because remember

00:10:17.850 --> 00:10:21.970
there is a complete circuit across red and
one of these three so long as the furnace

00:10:21.970 --> 00:10:24.120
is actually supposed to be doing something.

00:10:24.120 --> 00:10:29.390
But, depending on your specific furnace or
heating system, this might not work out so

00:10:29.390 --> 00:10:35.340
well because the thermostat is of course putting
a load on that circuit that isn’t normally

00:10:35.340 --> 00:10:39.570
there, so the furnace might freak out and
not behave correctly at all.

00:10:39.570 --> 00:10:40.700
So that’s neat.

00:10:40.700 --> 00:10:44.340
Now there are certainly more complicated heating
and cooling systems out there that need more

00:10:44.340 --> 00:10:46.280
than 4 wires to work.

00:10:46.280 --> 00:10:50.040
For example many heat pump systems will have
a backup heat source,

00:10:50.040 --> 00:10:52.240
sometimes referred to as emergency heat,

00:10:52.240 --> 00:10:56.280
that the thermostat will command on if
the heat pump isn’t providing enough heat

00:10:56.280 --> 00:10:57.280
on its own.

00:10:57.280 --> 00:11:02.950
It’s for systems like these and other more
complicated ones that you sometimes find ridiculous

00:11:02.950 --> 00:11:06.760
terminal blocks inside some relatively basic
thermostats.

00:11:06.760 --> 00:11:12.030
While most systems just need heat/cool/fan/common,
some are a little more elaborate.

00:11:12.030 --> 00:11:15.800
Alright, and for my last demonstration of
the power of thermostats,

00:11:15.800 --> 00:11:17.660
we’ll need a toaster oven.

00:11:17.660 --> 00:11:19.560
And not just any toaster oven!

00:11:19.560 --> 00:11:22.020
One of those with the glowy quartz heating
elements.

00:11:22.020 --> 00:11:24.200
I promise this isn’t about toast!

00:11:24.200 --> 00:11:25.560
This is about the oven part.

00:11:25.560 --> 00:11:27.640
One of my favorite things is when I discover a

00:11:27.640 --> 00:11:30.600
connection between two seemingly distant technologies.

00:11:30.600 --> 00:11:31.600
[pregnant pause]

00:11:31.600 --> 00:11:36.240
And one day, I realized that
thermostats are kind of like a really slow

00:11:36.240 --> 00:11:39.240
precursor to pulse width modulation.

00:11:39.240 --> 00:11:41.900
I don’t want to get too into PWM right now

00:11:41.900 --> 00:11:44.420
(‘cause we’ll save that for another video!)

00:11:44.420 --> 00:11:49.290
but what PWM does is modulate the power output
of something by rapidly turning it on and

00:11:49.290 --> 00:11:53.150
off, and adjusting the percentage of time
that it’s on.

00:11:53.150 --> 00:11:55.680
And that’s exactly what thermostats do!

00:11:55.680 --> 00:11:57.340
Just much more slowly.

00:11:57.350 --> 00:12:02.720
They don’t regulate the actual amount of heat coming from the furnace or heating elements or whatever.

00:12:02.720 --> 00:12:07.770
They instead regulate the duty cycle of the
heat source; how long it runs over time expressed

00:12:07.770 --> 00:12:09.340
as a percentage.

00:12:09.340 --> 00:12:14.540
It’s a little different because it’s reactive,
being tripped by the actual temperature change,

00:12:14.540 --> 00:12:17.320
but the effect is the same as PWM.

00:12:17.320 --> 00:12:20.000
And that’s why we have the toaster oven.

00:12:20.000 --> 00:12:24.880
And now, a brief explanation into why the
role of Toaster Oven will be performed by

00:12:24.880 --> 00:12:25.950
a stunt double.

00:12:26.360 --> 00:12:29.260
[The opening theme to Auntie Mame plays]

00:12:29.900 --> 00:12:33.500
Here it is running in the oven mode at three
distinct temperatures.

00:12:33.500 --> 00:12:36.080
On the left, it’s set to 250 degrees.

00:12:36.080 --> 00:12:39.780
In the middle, 350, and on the right 450.

00:12:39.790 --> 00:12:44.240
If we speed this footage way way up, we’ll
see that the elements run for a longer period

00:12:44.240 --> 00:12:48.120
of time and more frequently as the temperature
increases.

00:12:48.120 --> 00:12:53.620
It spends a greater percentage of time outputting
power as the requested temperature goes up.

00:12:53.620 --> 00:12:57.480
This happens naturally because the thermostat
acts to keep the temperature within a

00:12:57.480 --> 00:13:03.100
set range, but if we worked out what the duty
cycle was, we could actually rig up a PWM

00:13:03.100 --> 00:13:05.380
circuit to control the oven.

00:13:05.380 --> 00:13:10.580
That would be a bad idea because we want a
reactive thermostat in the case of cooking--both

00:13:10.580 --> 00:13:14.690
to actually get it up to the right temperature
quickly, and because ambient temperature and

00:13:14.690 --> 00:13:20.130
the contents of the oven will change the required
duty cycle--but it would technically work.

00:13:20.130 --> 00:13:24.590
See, thermostats are in a sense a natural
duty cycle determination device.

00:13:24.590 --> 00:13:27.171
They don’t know that’s what they’re
doing, even though I’m fairly sure they

00:13:27.171 --> 00:13:30.600
might be sentient, but that is kind of what
they do.

00:13:30.600 --> 00:13:34.140
Yeah, really it’s just “it’s too cold,
run the heat” followed by

00:13:34.140 --> 00:13:38.400
“it’s warm now, shut if off” but if you take a step
back and look over time,

00:13:38.400 --> 00:13:41.710
it’s really just slow pulse width modulation.

00:13:41.710 --> 00:13:43.500
With a bit of imagination, anyway.

00:13:43.500 --> 00:13:47.550
So to recap--thermostats can’t affect how
much heating or cooling comes out of your

00:13:47.550 --> 00:13:49.250
furnace when it runs.

00:13:49.250 --> 00:13:52.950
They only affect how often it runs and for
how long.

00:13:52.950 --> 00:13:58.310
So cranking the heat up does not make it go
faster--it just makes it go longer.

00:13:58.310 --> 00:14:00.870
Same goes for A/C (usually).

00:14:00.870 --> 00:14:05.720
Because it’s a regulation of heating or
cooling output based upon time, it is kinda

00:14:05.720 --> 00:14:07.820
like pulse width modulation.

00:14:07.820 --> 00:14:12.440
Just a very slow, very old, not quite so exact
implementation of it.

00:14:12.440 --> 00:14:15.910
And before I go, I asked on Twitter for your
thermostat-related questions!

00:14:15.910 --> 00:14:19.600
Let’s go through those while these fine
folks who support the channel on Patreon start

00:14:19.600 --> 00:14:21.110
scrolling up your screen.

00:14:21.110 --> 00:14:25.500
Sneaky (The N1) asks, “Why is the sensor
of house thermostats in the controlling monitor

00:14:25.500 --> 00:14:30.070
itself instead of little devices placed in
certain areas of the house other than because

00:14:30.070 --> 00:14:31.300
it's cheaper?

00:14:31.300 --> 00:14:34.920
Does putting it in the controlling monitor
give some sort of advantage I'm not entirely

00:14:34.920 --> 00:14:35.920
seeing?”

00:14:35.920 --> 00:14:39.430
It’s not that there’s no advantage, it’s
that there’s no disadvantage.

00:14:39.430 --> 00:14:42.940
With a central heating and cooling system
where you can’t influence the amount of

00:14:42.940 --> 00:14:48.670
heat any individual room gets, it really doesn’t
matter which room the thermostat is monitoring.

00:14:48.670 --> 00:14:53.490
They’re all going to get the same proportional
amount of heating and cooling regardless.

00:14:53.490 --> 00:14:58.010
More sensors could give you a more accurate
average, but it wouldn’t give you any actionable

00:14:58.010 --> 00:15:02.520
difference without motorized baffles in the
ductwork or some other zoning system.

00:15:02.520 --> 00:15:05.450
I’m A Mason Now asks, “Thermostat wire.

00:15:05.450 --> 00:15:06.470
Why does it exist?

00:15:06.470 --> 00:15:09.540
Why not just use Romex or phone cable or even
Ethernet?”

00:15:09.540 --> 00:15:13.360
Well I think it’s probably just down to
the color coding being pretty standardized

00:15:13.360 --> 00:15:14.360
now.

00:15:14.360 --> 00:15:16.290
In theory you could use any of those.

00:15:16.290 --> 00:15:20.260
Admittedly, at least one thermostat installation
instruction manual tells you to ignore the

00:15:20.260 --> 00:15:21.980
colors so…

00:15:21.980 --> 00:15:26.010
Visionary asks “Are you covering thermostatic
valves on radiators too?”

00:15:26.010 --> 00:15:27.010
No.

00:15:27.010 --> 00:15:30.890
Colin Cogle asks “Why did old thermostats
have a bulb full or mercury?

00:15:30.890 --> 00:15:33.310
I thought the bimetallic strip did all the
work!”

00:15:33.310 --> 00:15:37.710
In fact the bimetallic strip did do all the
sensing work, but back in the day thermostats

00:15:37.710 --> 00:15:40.650
used mercury tilt switches to control the
furnace.

00:15:40.650 --> 00:15:45.180
A little vial of mercury with a couple of
contacts poking in on one side would sit atop

00:15:45.180 --> 00:15:50.650
the bimetallic strip, and if it leaned to
the left--the mercury pooled to the left and

00:15:50.650 --> 00:15:51.800
no circuit.

00:15:51.800 --> 00:15:57.870
If it leaned to the right--now the mercury
pooled around the contacts and yes circuit.

00:15:57.870 --> 00:16:02.670
The weight of the mercury created the same
resistance that the magnet does in the modern

00:16:02.670 --> 00:16:06.320
thermostat, ensuring it cycled for a sufficient
period of time.

00:16:06.320 --> 00:16:12.170
Were it not for mercury’s rather toxic nature,
I’d wager this switch would still be preferred

00:16:12.170 --> 00:16:17.380
due to its potentially unlimited lifespan
and completely silent operation.

00:16:17.380 --> 00:16:22.760
Oh, but one downside was that the thermostat
had to be mounted exactly perfectly level

00:16:22.760 --> 00:16:24.700
in order for it to work correctly.

00:16:24.700 --> 00:16:29.100
And CantComeUpWithUsernames asks “is there
a thermostat that allows you to turn on the

00:16:29.110 --> 00:16:33.500
heat or AC for just one “cycle” without
actually adjusting the desired temperature?”

00:16:33.500 --> 00:16:37.460
Well, I don’t know, but that sounds like
a great idea!

00:16:37.460 --> 00:16:40.830
Say you just came in from mowing the lawn,
and want to cool off.

00:16:40.830 --> 00:16:45.330
Press a button and run the A/C for a half
hour, but then go back to the normal set temperature

00:16:45.330 --> 00:16:46.330
afterwards.

00:16:46.330 --> 00:16:48.580
I could totally get down with that.

00:16:48.580 --> 00:16:49.580
Nest.

00:16:49.580 --> 00:16:52.090
Update your thermostats to allow this, please.

00:16:52.090 --> 00:16:55.160
And if anyone knows of a thermostat that does
this now, let me know!

00:16:56.420 --> 00:16:59.180
♫ temperately smooth jazz ♫