How I Wired My Humidifier Into My Nest Thermostat

A little while back, I wrote about my first year using a Nest thermostat and since then several people have asked me for details on how I wired up my whole-house humidifier. I hesitated for a long time to post much detail, since it’d be really easy to completely fubar your HVAC, humidifier and Nest all in one fell swoop. But, after much mental consternation, I’ve decided to provide some detail, but with this really big disclaimer:


I am not an electrician, an HVAC specialist, a trained Nest consultant or frankly anyone you should trust. I provide this information solely as a reference implementation and provide no warranty that it’s accurate or even safe. You should consult an electrician or specialist before doing anything you read here. Incorrect wiring can cause massive damage to your HVAC system, or even result in fire.

In determining how to wire things up, I relied on advise from the good folks at The Thermostat Forums, and you can also look to the DIY StackExchange site.

What’s a Humistat?

From my original post, you may recall:

A “Humidistat” is the thermostat equivalent for a whole-home humidifier. It generally has an external sensor that reads the current humidity levels of the outside air, as well as a built-in sensor for the inside air. When the humidity inside is too low, such as in the winter when the heater is drying out the air, it will evaporate water into the hot air leaving the furnace and into the house.

My Wiring

Again, from my original post:

Now, connecting the Nest to the humidifier was not a straight-forward process. Humidistats come in two types, which basically comes down to 1-wire or 2-wire controls. If yours only uses 1 wire, good news, you can easily and quickly hook it up to the Nest. But most utilize 2 wires, much like a light switch, where a low voltage signal travels up one wire, the humidistat acts as a switch, closing the circuit for the voltage to travel down the second wire back to the humidifier to signal it should turn on. So in order to use the Nest, which only has 1 wire to control the humidifier, you must install a relay circuit, which, if done wrong, can damage your HVAC main board, your Nest and your humidifier — so Nest asks that you have a professional do the work. However, I am comfortable doing that kind of wiring and was able to install the relay and get the Nest hooked up on my own.

So, here’s what the original wiring looked like:

You’ll notice on the HVAC control board, there are five terminals, labeled R, C, W, Y and G. My understand, which is confirmed by this eHow Guide and this Thermostat Wiring Explained article, the terminals are:

  • R: Power (usually has a red wire)
  • C: Common
  • W: Heating (usually a white wire)
  • Y: Cooling (usually a yellow wire)
  • G: Fan

I used a 6AZU2 relay, which seems to be a commonly used relay for people doing exactly this (see the comments on the Amazon page). I used crimp-on spade connectors on the end of the wires to connect them to the relay terminals.

Here is what the new wiring looks like:

So when the Nest sends the signal on the “*” wire, the relay closes the circuit between the two lines on the humidifier, engaging it.

You can see someone else describe the same scenario, with a wiring diagram, on this site


Everyday Usability Design: The Good and Bad – All At Once

Very often, particularly while helping someone non-technical with a technical problem, I think to myself “this would have been so much simpler/less confusing if the designer would have put just a bit more thought into the user experience.” Similarly, I occasionally come across something where I say “man, why don’t more people do this.”

Yesterday, I had each of those reactions — to the same piece of technology! So I had to share. What was this magical piece of technology that mixed both the good and bad design experiences? Why, a gas pump, of course.

Take a look at this portion of the pump user interface. On the top, you have a credit card reader, and on the bottom, a keypad.

The Good:

The credit card magnetic stripe reader. They are everywhere: gas pumps, ATMs, grocery stores — pretty much every brick-and-mortar retail outlet you can find has some form of this device. And it seems like every time I use one, I swipe my card, only to realize I had the mag stripe on the wrong side and have to flip it and swipe again.

So what’s so good about this one? Well, take a look at the little helper pictures on the device. Did you see it? You can swipe the card with the mag stripe on… wait for it… either side. There is basically no way the user can do it wrong. The manufacturer has created a pit of success for me to fall into by adding a second receiving device on the other side. I LOVE IT!

This was the first thing I noticed on the gas pump, and I rejoiced. It put me in a better mood.

The Bad:

Immediately after swiping my card, the pump asked me to enter my zip code for credit card verification, followed by the dreaded “Do you want a car wash too?” prompt. Entering the zip code for credit card verification was straight forward — I had a set of what is clearly a numbered keypad to use. So I tapped it in. No problem.

Now I get the “Do you want a car wash?” prompt. In my head, as I recall this little episode playing out, I hear this 80s era computer voice asking “Shall we play a game?”

Ok, I can do this — “just press ‘No'” I tell myself. So I reach down to the keypad and… um.. where’s the ‘No’ key?

Now, it’s hard to tell from the picture, but the 0-9 keys, plus the yellow “Clear” and green “Enter” are all raised, like an early 90s telephone. So this is a very familiar interface to me. The yellow and green colored buttons stand out to me — clearly the designer gave them color to draw my eye to them. Then there are these stickers on the left and right side, which are also of various colors.

Here’s how it played out in my head:

Ok, so, now, how to say “no”? Let’s see, what are my options? There’s no clear “No” key, but there is a “No” sticker next to the “Clear” key and they are both yellow — so maybe that’s it. But wait, there’s a red “Cancel” label next to the green “Enter” key, and red usually means No — but green means confirm and the key itself is green. Better go with the yellow “Clear” key.

I hit the key — the machine beeps. The screen refreshed: “Do you want a car wash?”. Ok, maybe it didn’t register… so I pushed it again. And back comes the car wash prompt. (or, in my head “How about Global Thermonuclear War?”….) Clearly, this is not the right button.

To make a short story less long, here’s where I landed: Turns out, those “stickers” on the side aren’t just labels — they’re actual buttons. I probably should have taken the picture from a slight angle to make it more clear how much the buttons stick out — but to me, it was not at all obvious that these were interactive components, especially since they are juxtaposed immediately with what are clearly interactive components. So I eventually hit the yellow “No” sticker and went on my way.

There is so much going on with this little 3″ x 3″ square of user interface.

  • keys that are buttons
  • stickers that are buttons
  • stickers in yellow, red, green and white (what do those colors mean???)
  • keys in black, yellow and green

As a user, I don’t know what this all means, which means I’m likely to get it wrong, at least once. Had the buttons all been the same physical design, I would have immediately known which button to push.

It makes me wonder if the two parts of this user interface were designed by the same people.

Multi-Monitor Support For Remote Desktop Sessions

At the office, I have two nice, big monitors so I can spread out my work. I have become so accustomed to this much real estate that when I work from home, not having two monitors becomes a noticeable hindrance to my productivity.

While I do have a second monitor at home to attach to my laptop, I primarily remote desktop into my machine at the office, and that meant going back to a single monitor — until now!

With Windows 8 (or maybe it was 8.1), the remote desktop client allows you to utilize all your local monitors. It’s really easy to use, too. Just check the checkbox!

Update: I’m told this works in Windows 7 as well — seems this was a well-kept secret.