Thermostats: Cooler than you think!


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Published: 1 week ago
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Some additional information!

Firstly, two-stage heating is sorta common so the notion that "there is no medium blast" is probably true but if you have a fancy pants HVAC system, then maybe cranking the heat up might make it go faster. No idea what percentage of homes have a two stage heating system, but I'd guess it's in the 10-20% range.

In re: the thermostat wire question. Apparently, to this day, the current going through the contactors that actually turn on the high current loads such as your A/C or an electric furnace's heating elements is carried through the thermostat. So, it's more than just color coding; they are designed to carry some current, and thus need to be a minimum wire gauge. So while Cat5 cable or similar would probably work for most HVAC systems, it might not be able to carry enough current for others (and could, potentially, pose a fire risk under the right circumstances).

On that note, and I'm mad at myself for not bringing this up, you might have noticed that in the mechanical thermostat, a separate insulated wire connects the R terminal to the contact on the bimetallic strip. This prevents the current from being carried through the bimetallic strip, which could inadvertently heat it up and prematurely stop the furnace. Neat!

1 week ago

Few things you'll probably not read but here goes:


Y2, W2 and X2 are for two stage systems. Generally for systems with a "medium" heat. While auxiliary heat is used for a kind of ultra heat mode. However the distinction often gets blurred between W2, E and X. While Y2 pretty much always means a compressor with two stages and will also have a W2 if its a heat pump. Note that the numbered designations can keep going up. Many systems have 2 compressor stages Y and Y2 and three heat stages W, W2 and W3. Their smart boards interpret these stages to actually enable different parts of the system.



There is a big difference between the E (emergency) and X (auxiliary heat). The former should be used instead of the heat pump while X is used along side the heat pump. This distinction is specific to heat pumps and on furnaces often E and X are the same. X is often labeled emergency heat although it specifically calls heat unlike E which only enables a mode on the HVAC.


O and B are used to enable reversing valves and heating mode. O for cooling will enable the cooling reversing valve and enable the condenser plus its fan. B is used to reverse into heat pump mode or it can used to enable a furnace if its not connected via auxiliary heat. Most actual HVACs will do this control indirectly with a control board. The signals of O and B are reversed and always inverse of each other. Many HVACs combined these two.



The common wire exists in several forms. For example emergency heat its often not called directly by the thermostat its a mode of the HVAC. That means a thermostat can leave X on to mean "emergency heat can be used when needed" and this also acts as a common. Further some thermostats are powered by a closed relay loop by limiting the current below the relays tripping current.


Some systems have separate 24v signals for heating and cooling because of the different current characteristics of the two systems. However most thermostats have R meaning either system and Rc for cooling. Which means if you had an Rh wire you'd use the R wire on your thermostat. Whereas if you had Rc and Rh you should jumper them together for a system thats commonly signaled between the two and keep them separate for systems with separate signals.


U1, U2, U3, Un are used for humidity control (which is usually just controlled via the fan). But the U wires provide granular control. Typically these systems are called IAQ systems and might have entirely separate air circulation from the HVAC. They will filter air and maintain humidity.

2 days ago

+v2joecr There are a few more brands then Rheem and Ruud that use "B" for the reversing valve to energize in heat Bard, Weatherking(owned by Rheem) Ameristar(Owned by Trane) and Bocsh

2 days ago

+v2joecr That's not true if there is rollout happening most of the time you have a cracked heat exchanger flames should never rollout on a gas furnace

2 days ago

You are an engineer? Because you putting nice class-like information here.

3 days ago

Technology Connections I would like to see a video of Hi 8 tapes and that analog video format

3 days ago

american thermostats suck, in south africa we ACTULLY HAVE THERMOSTATS that have temperature measurements, and adjusting the temps makes it more or less sensitive towards cold or heat, making the aircon turn on and off more or less regularly.

27 minutes ago

I have a simens thermostat which allows you to turn on the heat without changing the temperature. It has an overdrive button.

3 hours ago

It's pretty cool that there are mechanical schmit triggers in old school thermostats.

Makes sense I suppose.

15 hours ago

Engaging comment for an informative video. 🥂

17 hours ago

we had an old mercury thermostat when I was growing up

18 hours ago

Here in Israel we don't have these at all...
We just have AC units that acts as a both way heat pump.
You set it via the remote and that's it.
I always thought it was stupid that in most models the thermostat of these was in the base unit instead of in the remote, but i guess if it wasn't effective and stupid it wouldn't be like this in most AC units.

I also hate these stupid IR remotes with rubber buttons that stops working after a few months and makes you break a finger trying to squeeze these mushy buttons.
After owning a 50$ Xiaomi MI TV BOX with a nice clicky bluetooth remote that actually has feedback of what the device does and does not I hate these!!!!
this makes all of our ACs that costs considerably more obsolete and really annoying to use.

How hard is it to just make an AC with a nice UI? why isn't it everywhere?

22 hours ago

One time, I tricked the boiler.

1 day ago

All this complexity...
Meanwhile, here in Iceland, the most sophisticated heating system (why would you want to cool your house down? Ever heard of windows? /s) relies on hot water running (or not running) through iron tubes stuck to our walls..

1 day ago

HEAT NOW.... or maybe don’t

1 day ago

Interesting fact that might not be present in USA but yes in EU.
Nest heating in OpenTherm, reads the outside temperature using Internet, temperature difference between inside setpoint and inside current temperature, then elaborates its algorithm and sends the information to the boiler (furnace in UK) to run a specific temperature in the radiators. The higher the gap between the setpoint and current temperature, the higher the temperature in the radiators. As it gets closer, it lowers the flow temperature up to the sweet balance between the energy the house is loosing and the one provided by the radiators.
This will leave the heating running longer at very low temperatures (great for condensing boilers) and avoiding cycling, which has been proven to waste energy and be less efficient.
This protocol powers the thermostat and send/receive communication using only two wires, similar to BUS.
Different brands use different protocols but all do pretty much the same.
Smart thermostats using ON/OFF use a thermal inertia algorithm. They measure the time the house takes to increase the temperature and it stops the heating before it reaches the set point (inertia will do the rest); at the same time it will measure how long it takes to loose the temperature and it'll start the heating before the setpoint is lost by too much.
Fact: by-metal thermostats are banned to be sold in the UK due its high hysteresis. New installations must have electronics capable of measure decimals of Celcius.

1 day ago

European hot water radiator heating here. My gas fired Bosch Boiler has 8 burner modes (off-100%) and adjusts the water temperature for the radiators based on outdoor temperature (colder outside = hotter radiators). Indoor temperature is remarkably more constant compared to the old binary on-off boiler and gas usage is much lower since gas boilers are a lot more efficient at lower temperatures.

1 day ago

I do not believe the spring coil is laminated, only bimetal strips are.

1 day ago

I’m surprised that control board is using through-hole components and not surface mount.

1 day ago

I appreciate the captions! Thanks!

1 day ago

Im not a fun sucker. That was a good gag

1 day ago

I had a furnace sales man try and sell me a furnace and a/c unit that never "turns off" it had a variable speed motor in it, and a very complex heating element, according to him it would just slowly change how much fuel it was burning and how fast the fan was blowing. According to the sales media this design worked better with humans and overall saved energy by only burning the fuel required blah blah. I read that after I saw the price tag so I don't really remember clearly.
The fact it would cost more then a luxury car to install was an issue for me.. so I cannot report on it.

1 day ago

After PWM, I'd love a video on PID controllers

2 days ago

Great video can't wait for the PWM full vid

2 days ago

Our Bryant HVAC does have the ability to "HOLD" the set temperature for a certain time and revert to normal or scheduled. I can choose 15 or 30 or 1 hr so on....

2 days ago

I’m intrigued by the complex circuit board shown in your furnace. I remember when all there was on the furnace side was: on/off; hi/low limit and a few other basic functions (which I don’t remember.) What the heck happened to furnace tech?

2 days ago

I am not a fun sucker, but there is a speed thingy in the options menu on youtube, in which you can go as slow as a quarter of normal playing speed.
Also, the '<' and '>' key on a keyboard will advance or reverse the video frame by frame respectively.

your welcome

2 days ago

I'm convinced that not only have my furnace and air conditioner developed the ability to sense what's going on , they are conspiring with the other appliances to force me to report me to the authorities for crimes I have not committed.

2 days ago

I'd be really happy if you made a video about barcodes. How do they work? Checksums? How does the scanner know what product it is? Is it a global or local database? How are the codes generated, so they don't create collisions? Etc.

2 days ago

Hold on if you have a natural gas furnace, does not that furnace have a valve to regulate gas flow, in theory if that valve had some for of electtrical contrll you (in this case the thermostat) shuld be able to reduce the gas flow and thus the heat output ( ok thing might not be liniar but yoy get the point). Ot am i missing something?

2 days ago

13:25 who leaves their house at -56°?

2 days ago

Modern European heating systems, known as boilers, use heated water with radiators and they sometimes support PWM for the hot water temperature via the OpenTherm protocol. If the connected thermostat is compatible, the temperature of the water going through the radiators can be adjusted in increments between full blast and semi-blast. This helps maintain a nice warm heat without searing hot radiators, while still allowing full blast for those really cold days. Generally setting the temperature way up doesn't allow you to heat faster (http://www.youtubesmov.com/mov/Iq22buOImXl/vid.html?), secondary heating systems aside.


While most thermostats don't control their duty cycle by anything more than current indoor temperature, the Nest Learning Thermostat can also take into account outdoor temperature, weather forecasting, hourly energy prices, occupancy modeling, learned thermal properties of the home, remote temperature sensing, and more. Still, even the smartest thermostats are "just a switch" (if you check the bottom of the legal screen on a NLT you'll occasionally see an easter-egg that admits just that). Source: Nearly 6 squirrel-free years of working on the NLT.

2 days ago

Jayne Tucek Haha, no OpenTHerm in US, we are still living in 20th century.
4 wires for 4 bits of information :)

1 day ago

I never knew about < and > to advance frames... That was a damned game-changer
I'm glad I took the time to read the whole thing

3 days ago

16:29 A timer, like you suggest in a couple seconds would be awesome. However a "cycle" is completely dependent on all the variables in the house in question, including structural as well as environmental conditions. If you leave for a short vacation and return to your Southern home in July during the afternoon, a "cycle" could be hours long. If you live in a studio apartment in Omaha during the Fall and return home from grabbing fast food, a "cycle" could be 5 minutes.

3 days ago

As a qualified HVAC technician I can assure you your home thermostat is NOT like a volume for temperature. Try this at home: put a thermometer near a wall register and near the return vent. The return is where you put the filter. While the system is running check the temperatures. If the AC is on then subtract the register temp from the return temp. You should see a 15-20 degree F difference. If you see something higher or lower you've got problems. If you're running heat then switch your math around accordingly. You'll notice a trend. The difference while cooling should always be a 15-20F difference. This is the optimal differential and is a good indicator of problems. If you've been setting the AC to 50F and notice only warm air coming out you've probably frozen your evaporator coil! But fear not a service call! Try running only the fan for a few hours and you'll notice something almost magical, the AC starts cooling again! Maybe not as strong but it starts getting better. After a couple hours you can run the AC again, but PLEASE set it to 70F. Your checking account will thank you.
Thank you for explaining thermostats!

3 days ago