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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

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did the thermostat do to make that happen?

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Well, it did one of the three things a typical
thermostat can do.

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It commanded heat.

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It could have also told your forced-air furnace
to run the fan.

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Or, if we’re in the summer months, it can
tell it to run the fan and turn on the air conditioner.

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That’s it.

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The typical American thermostat has just 4
wires going to it.

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A common 24 volt AC feed, a return wire for
heat, another wire for cooling, and another

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wire for the fan.

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All the thermostat does is complete a circuit
on one of those three control wires and the

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heating and cooling system will respond accordingly.

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It will either heat.

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Or cool.

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Or circulate the air.

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Now, notice that it can’t tell the furnace
how much to heat.

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It can only tell it HEAT NOW.

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And then it can tell it to stop.

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That’s all the control it has.

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See, the furnace or air conditioner or whatever
you’ve got doesn’t typically have any

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sense of what’s actually going on.

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There are some more complicated systems, yes, but
the basic domestic HVAC systems you see all

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around the US are actually quite dumb.

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Really, most home appliances are dumber than
you think, but I digress.

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So, they rely on a thermostat as a sort of
life coach to tell them what to do.

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Now a lot of you are probably thinking “duh,
I know this, why is Technology Connections

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talking about thermostats?”.

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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.

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It just makes the heat run for a long time
as it works to reach that much-higher-than-right-now

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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.

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There is no medium blast.

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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.

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It boosts engagement!

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Anyway, the second reason I’m making a video
on thermostats has to do with the fact that

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they’re pretty neat.

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Now, new digital thermostats like this are
pretty boring.

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It’s basically a small battery-operated computer that closes

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the contacts of a relay when necessary.

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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

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sorts of amazing things can happen.

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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!

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So, unless your heating system is frightfully
ancient, there’s gonna be a control board

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in there somewhere with some relays, resistors,
and all that fun stuff, as well as a terminal

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block with a few wires attached to it.

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These wires are going to the thermostat which,
in this case, is upstairs in the living area.

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For now, ignore the blue one.

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We’ll get back to that.

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So, the control board is providing a 24 volt
A/C feed on this here terminal, labeled R

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for Red.

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These other terminals are inputs.

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The white wire is the input for heat.

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Yellow does A/C. And green runs the fan.

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Blue does nothing.

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Ignore the blue.

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To make the furnace heat, all we need to do
is jump the red wire to the white wire.

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Now, the logic board sees 24 volts on the
heat input, and it starts to heat.

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And when it no longer sees 24 volts on the
white wire, it shuts off.

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And what jumps those wires together?

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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

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(that’s roughly 34,000 BTUs per hour).

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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?

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It has no mechanism of its own to reduce its
heat output.

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You can only tell it to be on or off.

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Well that’s fine.

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I only need 30% of its total heat output,
so what I’ll do is I’ll have it run for

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three minutes, then be off for 7.

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In every 10 minute period it runs for three
minutes, which means it produces 30% of its

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rated heat, or 3 kilowatts.

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Simple.

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Yeah, it is pretty simple.

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Almost remarkably so.

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Thermostats are a reactive means of controlling
temperature output.

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If the current temperature is lower than the
thermostat’s set point, it will command

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heat from the furnace.

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After the house has warmed to a certain degree
beyond the set point,

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it will stop commanding heat.

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Even though it has only a binary state, either
on or off, it is able to regulate temperature

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by running the furnace periodically to stay
within a set temperature range.

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The most interesting thermostats are the super
basic cheap ones.

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These work using the most rudimentary of technologies,
and yet they are still an effective means

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of regulating temperature.

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This is as basic a thermostat as you can get.

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It just handles heat, and it’s very cheap
feeling and kinda ugly but hey.

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It works.

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Inside this you will find a pair of contacts,
one of which is on this groovy spring thing.

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That spring is a really long bimetallic strip
formed into a coil.

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A bimetallic strip will bend as its temperature
changes thanks to the slightly different thermal

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expansion properties of its two laminated
halves.

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When formed into a coil, a change in ambient
temperature will cause the coil to get tighter

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or to loosen.

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Since this end of the coil is free to move
about, the effect is that this end will move

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either left or right with a change in temperature.

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This exact sort of thing is what makes those
big ‘ol garden thermometers work, though

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in that case this end would be fixed and the
center would be free to rotate a pointer needle.

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Adjusting the set point of this thermostat
adjusts the tension on the coil through rotating

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its central attachment point.

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In this case, as the temperature falls, the
spring gets tighter which causes the free

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end to move to the right.

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As it gets colder and colder, the free contact
gets closer to the other,

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and once it gets close enough

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*click*

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The contacts are pulled
together with the help of this little magnet.

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And what are those contacts attached to, you
ask?

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Why, the red and white wires going to the
board on your furnace.

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So long as these contacts are touching, it
commands heat from the furnace.

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Once the ambient air has heated up enough
to cause these contacts to break apart, then

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the furnace no longer sees 24 volts on the
white wire, so it stops heating.

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It’s that simple.

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The reason there’s a magnet is to create
clear start and stop points in order to prevent

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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

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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.

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The magnet prevents this by forcing the contacts
to stay together until the ambient air temperature

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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

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contacts snap apart, it’s now so far to
the left, that it needs to get at least a

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degree or so colder before they touch again
and command heat.

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Clever.

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Additionally, it helps to prevent arcing on
the contacts by ensuring they connect and

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disconnect as quickly as possible.

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But what about the green wire?

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And the yellow one?

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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.

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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

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same fashion, though in reverse--in this case,
when it gets too warm, the contact snaps to

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the left, which would put 24 volts on the
yellow wire and start the air conditioner.

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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

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thermostat.

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

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Too cheap to make a different mold, eh?

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When equipped for air conditioning, too, there
would be a mode switch coming out the bottom

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to disable heating or cooling commands.

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All that it would do is prevent a cooling
call from happening in heat mode, and vice versa.

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This prevents a scenario where it cools so
much that it snaps to the right, which turns

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on the heat, which snaps it to the left, which
turns on the A/C, which snaps it to the right,

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which turns on the heat, which snaps it to
the left, which turns on the A/C, which snaps

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it to the right, which turns on the heat,
which snaps it the left...

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Now that you know that thermostats are really
just shunting a couple of wires together to

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command the furnace to do something, you’ll
understand why newer digital thermostats

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need batteries.

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In the case of this basic digital thermostat,
this relay performs the task of connecting

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the wires together.

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The mode switch probably changes the path
of the connection, allowing for one relay

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to do both heating and cooling.

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It uses a thermocouple to determine the current
temperature and compares it to the set point,

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and when it determines that it needs to command
heat, you hear a click.

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Then, and only then, is there an actual complete
circuit going through the thermostat.

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While there is 24 volts potential on the red
wire, the only way to complete a circuit and

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thus be able to use some of that power is
to tell the furnace to do something.

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So, unless you want the fan to run 24/7, you
need the thermostat to have its own power source.

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Unless, of course, you’re either tricky
or you have what’s called a C wire.

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That’s the blue one.

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Anyone who has installed a smart thermostat,
or even considered installing one, knows what

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the C wire is.

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Essentially it’s just a return path to the
furnace’s logic board that doesn’t command anything.

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This way, a thermostat can be powered by the
furnace itself by using the 24 volts potential

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across the red and blue wires all the time.

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To command either heat, cooling, or the fan,
the thermostat will shunt the red wire to

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either the white, green, or yellow just like
any other thermostat.

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But, because it can keep an active connection
across red and blue at all times, it doesn’t

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need its own power source.

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Some smart thermostats, like the Nest, have
a small rechargeable battery inside them that

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*usually*

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allows you to get away without a
C wire.

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That’s the being tricky option.

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What these do is charge the battery whenever
it’s commanding something, because remember

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there is a complete circuit across red and
one of these three so long as the furnace

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is actually supposed to be doing something.

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But, depending on your specific furnace or
heating system, this might not work out so

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well because the thermostat is of course putting
a load on that circuit that isn’t normally

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there, so the furnace might freak out and
not behave correctly at all.

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So that’s neat.

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Now there are certainly more complicated heating
and cooling systems out there that need more

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than 4 wires to work.

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For example many heat pump systems will have
a backup heat source,

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sometimes referred to as emergency heat,

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that the thermostat will command on if
the heat pump isn’t providing enough heat

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on its own.

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It’s for systems like these and other more
complicated ones that you sometimes find ridiculous

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terminal blocks inside some relatively basic
thermostats.

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While most systems just need heat/cool/fan/common,
some are a little more elaborate.

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Alright, and for my last demonstration of
the power of thermostats,

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we’ll need a toaster oven.

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And not just any toaster oven!

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One of those with the glowy quartz heating
elements.

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I promise this isn’t about toast!

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This is about the oven part.

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One of my favorite things is when I discover a

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connection between two seemingly distant technologies.

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[pregnant pause]

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And one day, I realized that
thermostats are kind of like a really slow

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precursor to pulse width modulation.

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I don’t want to get too into PWM right now

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(‘cause we’ll save that for another video!)

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but what PWM does is modulate the power output
of something by rapidly turning it on and

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off, and adjusting the percentage of time
that it’s on.

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And that’s exactly what thermostats do!

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Just much more slowly.

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They don’t regulate the actual amount of heat coming from the furnace or heating elements or whatever.

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They instead regulate the duty cycle of the
heat source; how long it runs over time expressed

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as a percentage.

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It’s a little different because it’s reactive,
being tripped by the actual temperature change,

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but the effect is the same as PWM.

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And that’s why we have the toaster oven.

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And now, a brief explanation into why the
role of Toaster Oven will be performed by

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a stunt double.

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[The opening theme to Auntie Mame plays]

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Here it is running in the oven mode at three
distinct temperatures.

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On the left, it’s set to 250 degrees.

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In the middle, 350, and on the right 450.

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If we speed this footage way way up, we’ll
see that the elements run for a longer period

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of time and more frequently as the temperature
increases.

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It spends a greater percentage of time outputting
power as the requested temperature goes up.

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This happens naturally because the thermostat
acts to keep the temperature within a

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set range, but if we worked out what the duty
cycle was, we could actually rig up a PWM

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circuit to control the oven.

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That would be a bad idea because we want a
reactive thermostat in the case of cooking--both

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to actually get it up to the right temperature
quickly, and because ambient temperature and

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the contents of the oven will change the required
duty cycle--but it would technically work.

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See, thermostats are in a sense a natural
duty cycle determination device.

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They don’t know that’s what they’re
doing, even though I’m fairly sure they

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might be sentient, but that is kind of what
they do.

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Yeah, really it’s just “it’s too cold,
run the heat” followed by

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“it’s warm now, shut if off” but if you take a step
back and look over time,

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it’s really just slow pulse width modulation.

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With a bit of imagination, anyway.

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So to recap--thermostats can’t affect how
much heating or cooling comes out of your

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furnace when it runs.

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They only affect how often it runs and for
how long.

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So cranking the heat up does not make it go
faster--it just makes it go longer.

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Same goes for A/C (usually).

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Because it’s a regulation of heating or
cooling output based upon time, it is kinda

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like pulse width modulation.

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Just a very slow, very old, not quite so exact
implementation of it.

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And before I go, I asked on Twitter for your
thermostat-related questions!

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Let’s go through those while these fine
folks who support the channel on Patreon start

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scrolling up your screen.

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Sneaky (The N1) asks, “Why is the sensor
of house thermostats in the controlling monitor

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itself instead of little devices placed in
certain areas of the house other than because

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it's cheaper?

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Does putting it in the controlling monitor
give some sort of advantage I'm not entirely

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seeing?”

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It’s not that there’s no advantage, it’s
that there’s no disadvantage.

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With a central heating and cooling system
where you can’t influence the amount of

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heat any individual room gets, it really doesn’t
matter which room the thermostat is monitoring.

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They’re all going to get the same proportional
amount of heating and cooling regardless.

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More sensors could give you a more accurate
average, but it wouldn’t give you any actionable

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difference without motorized baffles in the
ductwork or some other zoning system.

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I’m A Mason Now asks, “Thermostat wire.

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Why does it exist?

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Why not just use Romex or phone cable or even
Ethernet?”

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Well I think it’s probably just down to
the color coding being pretty standardized

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now.

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In theory you could use any of those.

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Admittedly, at least one thermostat installation
instruction manual tells you to ignore the

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colors so…

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Visionary asks “Are you covering thermostatic
valves on radiators too?”

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No.

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Colin Cogle asks “Why did old thermostats
have a bulb full or mercury?

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I thought the bimetallic strip did all the
work!”

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In fact the bimetallic strip did do all the
sensing work, but back in the day thermostats

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used mercury tilt switches to control the
furnace.

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A little vial of mercury with a couple of
contacts poking in on one side would sit atop

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the bimetallic strip, and if it leaned to
the left--the mercury pooled to the left and

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no circuit.

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If it leaned to the right--now the mercury
pooled around the contacts and yes circuit.

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The weight of the mercury created the same
resistance that the magnet does in the modern

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thermostat, ensuring it cycled for a sufficient
period of time.

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Were it not for mercury’s rather toxic nature,
I’d wager this switch would still be preferred

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due to its potentially unlimited lifespan
and completely silent operation.

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Oh, but one downside was that the thermostat
had to be mounted exactly perfectly level

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in order for it to work correctly.

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And CantComeUpWithUsernames asks “is there
a thermostat that allows you to turn on the

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heat or AC for just one “cycle” without
actually adjusting the desired temperature?”

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Well, I don’t know, but that sounds like
a great idea!

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Say you just came in from mowing the lawn,
and want to cool off.

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Press a button and run the A/C for a half
hour, but then go back to the normal set temperature

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afterwards.

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I could totally get down with that.

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Nest.

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Update your thermostats to allow this, please.

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And if anyone knows of a thermostat that does
this now, let me know!

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♫ temperately smooth jazz ♫