When you go up to your thermostat to raise the temperature, what really happens?
Obviously the heating system comes on (if we’re still in heating season) but what
did the thermostat do to make that happen?
Well, it did one of the three things a typical thermostat can do.
It commanded heat.
It could have also told your forced-air furnace to run the fan.
Or, if we’re in the summer months, it can tell it to run the fan and turn on the air conditioner.
That’s it.
The typical American thermostat has just 4 wires going to it.
A common 24 volt AC feed, a return wire for heat, another wire for cooling, and another
wire for the fan.
All the thermostat does is complete a circuit on one of those three control wires and the
heating and cooling system will respond accordingly.
It will either heat.
Or cool.
Or circulate the air.
Now, notice that it can’t tell the furnace how much to heat.
It can only tell it HEAT NOW.
And then it can tell it to stop.
That’s all the control it has.
See, the furnace or air conditioner or whatever you’ve got doesn’t typically have any
sense of what’s actually going on.
There are some more complicated systems, yes, but the basic domestic HVAC systems you see all
around the US are actually quite dumb.
Really, most home appliances are dumber than you think, but I digress.
So, they rely on a thermostat as a sort of life coach to tell them what to do.
Now a lot of you are probably thinking “duh, I know this, why is Technology Connections
talking about thermostats?”.
But there are reasons.
First is that a number of people have asked me to explain to someone in their life that
setting the thermostat up to 80 degrees does not make the heat go faster.
It just makes the heat run for a long time as it works to reach that much-higher-than-right-now
temperature.
If you want to set the thermostat higher, just set it where you want it.
The furnace will run on full blast until it gets there, then it will shut off.
It runs on full blast because that’s the only degree of blast that a furnace can run.
There is no medium blast.
Only full.
And off.
I’m sure there are exceptions even in the domestic environment so be sure to comment about them.
It boosts engagement!
Anyway, the second reason I’m making a video on thermostats has to do with the fact that
they’re pretty neat.
Now, new digital thermostats like this are pretty boring.
It’s basically a small battery-operated computer that closes
the contacts of a relay when necessary.
Add an LCD, some buttons, and boom.
Programmable thermostat.
[read with intense apathy] Then put it on WiFi and connect it to the internet and all
sorts of amazing things can happen.
So, how can just 4 wires provide the necessary control to regulate the temperature of your home?
To show you, we need a furnace.
Perfect!
So, unless your heating system is frightfully ancient, there’s gonna be a control board
in there somewhere with some relays, resistors, and all that fun stuff, as well as a terminal
block with a few wires attached to it.
These wires are going to the thermostat which, in this case, is upstairs in the living area.
For now, ignore the blue one.
We’ll get back to that.
So, the control board is providing a 24 volt A/C feed on this here terminal, labeled R
for Red.
These other terminals are inputs.
The white wire is the input for heat.
Yellow does A/C. And green runs the fan.
Blue does nothing.
Ignore the blue.
To make the furnace heat, all we need to do is jump the red wire to the white wire.
Now, the logic board sees 24 volts on the heat input, and it starts to heat.
And when it no longer sees 24 volts on the white wire, it shuts off.
And what jumps those wires together?
Why, the thermostat, of course!
Thermostats are among the simplest ways to regulate temperature.
And they do it with time.
See, suppose you have a furnace that can output 10 kilowatts of heat
(that’s roughly 34,000 BTUs per hour).
There aren’t many days when you need that much heat, but it’s there just in case.
Suppose on a typical day, you only need an average of 3 kilowatts of heat.
Well, how can you make a dumb furnace do that?
It has no mechanism of its own to reduce its heat output.
You can only tell it to be on or off.
Well that’s fine.
I only need 30% of its total heat output, so what I’ll do is I’ll have it run for
three minutes, then be off for 7.
In every 10 minute period it runs for three minutes, which means it produces 30% of its
rated heat, or 3 kilowatts.
Simple.
Yeah, it is pretty simple.
Almost remarkably so.
Thermostats are a reactive means of controlling temperature output.
If the current temperature is lower than the thermostat’s set point, it will command
heat from the furnace.
After the house has warmed to a certain degree beyond the set point,
it will stop commanding heat.
Even though it has only a binary state, either on or off, it is able to regulate temperature
by running the furnace periodically to stay within a set temperature range.
The most interesting thermostats are the super basic cheap ones.
These work using the most rudimentary of technologies, and yet they are still an effective means
of regulating temperature.
This is as basic a thermostat as you can get.
It just handles heat, and it’s very cheap feeling and kinda ugly but hey.
It works.
Inside this you will find a pair of contacts, one of which is on this groovy spring thing.
That spring is a really long bimetallic strip formed into a coil.
A bimetallic strip will bend as its temperature changes thanks to the slightly different thermal
expansion properties of its two laminated halves.
When formed into a coil, a change in ambient temperature will cause the coil to get tighter
or to loosen.
Since this end of the coil is free to move about, the effect is that this end will move
either left or right with a change in temperature.
This exact sort of thing is what makes those big ‘ol garden thermometers work, though
in that case this end would be fixed and the center would be free to rotate a pointer needle.
Adjusting the set point of this thermostat adjusts the tension on the coil through rotating
its central attachment point.
In this case, as the temperature falls, the spring gets tighter which causes the free
end to move to the right.
As it gets colder and colder, the free contact gets closer to the other,
and once it gets close enough
*click*
The contacts are pulled together with the help of this little magnet.
And what are those contacts attached to, you ask?
Why, the red and white wires going to the board on your furnace.
So long as these contacts are touching, it commands heat from the furnace.
Once the ambient air has heated up enough to cause these contacts to break apart, then
the furnace no longer sees 24 volts on the white wire, so it stops heating.
It’s that simple.
The reason there’s a magnet is to create clear start and stop points in order to prevent
the furnace from running very frequent, very short cycles.
Without it, imagine what would happen when the contacts touched.
The furnace would switch on, but after heating just a teeny tiny bit, the contacts would
move apart again.
Then of course, they’d almost immediately touch when it cooled just an itty bitty bit.
This is no bueno.
The magnet prevents this by forcing the contacts to stay together until the ambient air temperature
has risen by at least a degree or so.
And as a bonus, once the spring can finally overcome the magnet’s attraction and the
contacts snap apart, it’s now so far to the left, that it needs to get at least a
degree or so colder before they touch again and command heat.
Clever.
Additionally, it helps to prevent arcing on the contacts by ensuring they connect and
disconnect as quickly as possible.
But what about the green wire?
And the yellow one?
You might have already spotted that there’s a suspiciously similar indentation in the
plastic to the left of the heating contact.
If there were another contact placed there, then we’d also be able to control an air conditioner.
Since the contact moves to the right as it gets colder, it would work in exactly the
same fashion, though in reverse--in this case, when it gets too warm, the contact snaps to
the left, which would put 24 volts on the yellow wire and start the air conditioner.
And of course, once it’s cool enough, it snaps back to the right and breaks the connection,
turning the air conditioner off.
Honeywell didn’t really try to hide the fact that they just sold you a de-featured
thermostat.
There’s a rather clear Y label for what would be your air conditioning terminal.
Too cheap to make a different mold, eh?
When equipped for air conditioning, too, there would be a mode switch coming out the bottom
to disable heating or cooling commands.
All that it would do is prevent a cooling call from happening in heat mode, and vice versa.
This prevents a scenario where it cools so much that it snaps to the right, which turns
on the heat, which snaps it to the left, which turns on the A/C, which snaps it to the right,
which turns on the heat, which snaps it to the left, which turns on the A/C, which snaps
it to the right, which turns on the heat, which snaps it the left...
Now that you know that thermostats are really just shunting a couple of wires together to
command the furnace to do something, you’ll understand why newer digital thermostats
need batteries.
In the case of this basic digital thermostat, this relay performs the task of connecting
the wires together.
The mode switch probably changes the path of the connection, allowing for one relay
to do both heating and cooling.
It uses a thermocouple to determine the current temperature and compares it to the set point,
and when it determines that it needs to command heat, you hear a click.
Then, and only then, is there an actual complete circuit going through the thermostat.
While there is 24 volts potential on the red wire, the only way to complete a circuit and
thus be able to use some of that power is to tell the furnace to do something.
So, unless you want the fan to run 24/7, you need the thermostat to have its own power source.
Unless, of course, you’re either tricky or you have what’s called a C wire.
That’s the blue one.
Anyone who has installed a smart thermostat, or even considered installing one, knows what
the C wire is.
Essentially it’s just a return path to the furnace’s logic board that doesn’t command anything.
This way, a thermostat can be powered by the furnace itself by using the 24 volts potential
across the red and blue wires all the time.
To command either heat, cooling, or the fan, the thermostat will shunt the red wire to
either the white, green, or yellow just like any other thermostat.
But, because it can keep an active connection across red and blue at all times, it doesn’t
need its own power source.
Some smart thermostats, like the Nest, have a small rechargeable battery inside them that
*usually*
allows you to get away without a C wire.
That’s the being tricky option.
What these do is charge the battery whenever it’s commanding something, because remember
there is a complete circuit across red and one of these three so long as the furnace
is actually supposed to be doing something.
But, depending on your specific furnace or heating system, this might not work out so
well because the thermostat is of course putting a load on that circuit that isn’t normally
there, so the furnace might freak out and not behave correctly at all.
So that’s neat.
Now there are certainly more complicated heating and cooling systems out there that need more
than 4 wires to work.
For example many heat pump systems will have a backup heat source,
sometimes referred to as emergency heat,
that the thermostat will command on if the heat pump isn’t providing enough heat
on its own.
It’s for systems like these and other more complicated ones that you sometimes find ridiculous
terminal blocks inside some relatively basic thermostats.
While most systems just need heat/cool/fan/common, some are a little more elaborate.
Alright, and for my last demonstration of the power of thermostats,
we’ll need a toaster oven.
And not just any toaster oven!
One of those with the glowy quartz heating elements.
I promise this isn’t about toast!
This is about the oven part.
One of my favorite things is when I discover a
connection between two seemingly distant technologies.
[pregnant pause]
And one day, I realized that thermostats are kind of like a really slow
precursor to pulse width modulation.
I don’t want to get too into PWM right now
(‘cause we’ll save that for another video!)
but what PWM does is modulate the power output of something by rapidly turning it on and
off, and adjusting the percentage of time that it’s on.
And that’s exactly what thermostats do!
Just much more slowly.
They don’t regulate the actual amount of heat coming from the furnace or heating elements or whatever.
They instead regulate the duty cycle of the heat source; how long it runs over time expressed
as a percentage.
It’s a little different because it’s reactive, being tripped by the actual temperature change,
but the effect is the same as PWM.
And that’s why we have the toaster oven.
And now, a brief explanation into why the role of Toaster Oven will be performed by
a stunt double.
[The opening theme to Auntie Mame plays]
Here it is running in the oven mode at three distinct temperatures.
On the left, it’s set to 250 degrees.
In the middle, 350, and on the right 450.
If we speed this footage way way up, we’ll see that the elements run for a longer period
of time and more frequently as the temperature increases.
It spends a greater percentage of time outputting power as the requested temperature goes up.
This happens naturally because the thermostat acts to keep the temperature within a
set range, but if we worked out what the duty cycle was, we could actually rig up a PWM
circuit to control the oven.
That would be a bad idea because we want a reactive thermostat in the case of cooking--both
to actually get it up to the right temperature quickly, and because ambient temperature and
the contents of the oven will change the required duty cycle--but it would technically work.
See, thermostats are in a sense a natural duty cycle determination device.
They don’t know that’s what they’re doing, even though I’m fairly sure they
might be sentient, but that is kind of what they do.
Yeah, really it’s just “it’s too cold, run the heat” followed by
“it’s warm now, shut if off” but if you take a step back and look over time,
it’s really just slow pulse width modulation.
With a bit of imagination, anyway.
So to recap--thermostats can’t affect how much heating or cooling comes out of your
furnace when it runs.
They only affect how often it runs and for how long.
So cranking the heat up does not make it go faster--it just makes it go longer.
Same goes for A/C (usually).
Because it’s a regulation of heating or cooling output based upon time, it is kinda
like pulse width modulation.
Just a very slow, very old, not quite so exact implementation of it.
And before I go, I asked on Twitter for your thermostat-related questions!
Let’s go through those while these fine folks who support the channel on Patreon start
scrolling up your screen.
Sneaky (The N1) asks, “Why is the sensor of house thermostats in the controlling monitor
itself instead of little devices placed in certain areas of the house other than because
it's cheaper?
Does putting it in the controlling monitor give some sort of advantage I'm not entirely
seeing?”
It’s not that there’s no advantage, it’s that there’s no disadvantage.
With a central heating and cooling system where you can’t influence the amount of
heat any individual room gets, it really doesn’t matter which room the thermostat is monitoring.
They’re all going to get the same proportional amount of heating and cooling regardless.
More sensors could give you a more accurate average, but it wouldn’t give you any actionable
difference without motorized baffles in the ductwork or some other zoning system.
I’m A Mason Now asks, “Thermostat wire.
Why does it exist?
Why not just use Romex or phone cable or even Ethernet?”
Well I think it’s probably just down to the color coding being pretty standardized
now.
In theory you could use any of those.
Admittedly, at least one thermostat installation instruction manual tells you to ignore the
colors so…
Visionary asks “Are you covering thermostatic valves on radiators too?”
No.
Colin Cogle asks “Why did old thermostats have a bulb full or mercury?
I thought the bimetallic strip did all the work!”
In fact the bimetallic strip did do all the sensing work, but back in the day thermostats
used mercury tilt switches to control the furnace.
A little vial of mercury with a couple of contacts poking in on one side would sit atop
the bimetallic strip, and if it leaned to the left--the mercury pooled to the left and
no circuit.
If it leaned to the right--now the mercury pooled around the contacts and yes circuit.
The weight of the mercury created the same resistance that the magnet does in the modern
thermostat, ensuring it cycled for a sufficient period of time.
Were it not for mercury’s rather toxic nature, I’d wager this switch would still be preferred
due to its potentially unlimited lifespan and completely silent operation.
Oh, but one downside was that the thermostat had to be mounted exactly perfectly level
in order for it to work correctly.
And CantComeUpWithUsernames asks “is there a thermostat that allows you to turn on the
heat or AC for just one “cycle” without actually adjusting the desired temperature?”
Well, I don’t know, but that sounds like a great idea!
Say you just came in from mowing the lawn, and want to cool off.
Press a button and run the A/C for a half hour, but then go back to the normal set temperature
afterwards.
I could totally get down with that.
Nest.
Update your thermostats to allow this, please.
And if anyone knows of a thermostat that does this now, let me know!
♫ temperately smooth jazz ♫