Controlling an ancient millivolt heater with a Nest

Chris Vale
9 min readApr 8, 2017

This is a story of empty victory, of disillusioned success. I have my Nest controlling an old heater now, and it works well, yet I don’t feel proud of the journey I took to bring it to life.

To spread my shame, thus lightening its’ load on my chest, I’m detailing my path to serve as a cheaters guide to other automation enthusiasts who want to eliminate risk from their lives more than they already have. And maybe I’ll learn something about myself along the way.

Looks simple right? It is :(

You see, like many others that live in San Francisco, I reside in a really old building. It looks impressive and is on a big hill, but it has window weights, non-insulated walls, and a gas heater from a bygone era. I didn’t know a thing about heaters, but wanted a “smart” thermostat to turn the heat on or off from the comfort of my bed and control usage while out of town.

So when my significant other wasn’t looking, I snuck a Nest onto our wedding registry, and a few friends bought it for us (thank you Tommie and Anthony!)

3rd Generation Nest Thermostat

Upon receipt, and against my strong habits to the contrary, I went to the Nest website to see how to install the darn thing before pulling my power tools out.

And found a little surprise. My heater was not supported.

I thought those fancy-pants Silicon Valley engineers could support anything!

I kept coming back and checking, but the answer never changed

Is my old heater really that special? Turns out it was. And as I learned more about it, I actually grew to appreciate its simplicity.

Our *millivolt* heater actually uses industrial revolution era boiler control, that doesn’t need any external power. The entire unit is powered from a hunk of metal that is perched directly in the gas flame. The hunk of metal is actually made out of different materials that generate a voltage when hot. And this small electrical difference was run via two wires all the way across the room to our hallway, where it connected to another foolproof system, a bi-metallic strip, that would open and close a circuit depending on how hot it was.

If the strip got too cold, it would move and close a circuit between those two wires, which would then turn the heater on.

So, I basically had the air-cooled Porsche 911 of heaters! And I wanted to put an autopilot on it. So now I had a really fun project on my plate of building a bridge between the old world and the new.

All I would need to do is give 24V AC power to the Nest, figure out how thermostats turn on heaters, and connect that signal to a relay that would turn on my vintage system. Time to dig out the old textbooks because I need to design a circuit!

However, the tragedy of this story struck when I started looking for 24V transformers and relays on the internet. In spending too much time researching components, I stumbled across the Game Genie for this application; a complete pre-engineered HVAC component made by Honeywell that has the perfect transformer, inputs and outputs for heater controls, AND a relay that not only works for millivolt control systems, it’s designed for it. (Shout out to Sabrina for possibly the most useful Amazon review ever…)

[EDIT (4/13/2017): my research failed to uncover that the UK version of the Nest comes with a very similar item, the Heat Link, that also has a transformer and relay built in. See Willem van der Velden’s response below.]

I’ll use BOLD to reference this wiring diagram below

I pulled up the wiring diagram and stared at my screen. This was my moment from the classics, the “central ordeal” of the hero’s journey where I could overcome temptation, reap my reward, and return to the homeland with newfound power.

But I was weak of mind and character at that moment, so I plucked the forbidden Honeywell R8845U from the tree of knowledge. Which in modern times equates to buying it from Amazon. ¯\_(ツ)_/¯

I started on my path of self-forgiveness when it arrived. Opening up this cheat-box, it actually looked pretty awesome. Big wide strips of silicon between circuits, replaceable relays, a test button and LED, crisp white labels on the board, and a widely available blade fuse for good measure. It all comes in a thumb-slicing stamped and bent sheet metal enclosure with conduit punch outs, which keeps everything nice and protected. And the wiring diagram is on the cover, which is how every piece of electronics would come if I were president. After I ban standing on the left side of the escalator of course.

I also bought a 50ft roll of Southwire 3-wire cable off Amazon when I bought the cheat box, and it came at the same time.

Digging into the Nest box next, they give you a jade egg-shaped screwdriver, two screws, and some wire labels to make your install easier. I meant to use the wire labels but forgot.

I keep a paint pen in my toolbox for marking, and it comes in handy all the time

Finally time to start breaking stuff.

Step one

After already spending hundreds of dollars I decided to confirm that the heater works the way the internet says it does. I took apart my old thermostat and here’s what it looked like opened up.

It only had two wires running to it, and I confirmed that touching the red and white wires together would turn the heater on, so a relay that does this automatically will work just fine. Phew.

Step two

Give the cheat-box some power! I found an old laptop charger and performed a ritual sacrifice on the unneeded end. Trusting my life to the crowd wisdom of google images, I searched for “NEMA 5 wiring diagram” and labeled each wire by checking continuity between the blade of the plug and the stripped wire.

I connected the hot wire to L1 and neutral wire to L2 inside the wonderfully labeled cheat-box, and ran the ground wire to the metal case of the box itself.

If you squint your eyes you can make out the “HOT” and “NEU”

Step three

Connecting the relay outputs (X1 and X2 in the cheat-box) to the actual heater. I snipped off a five foot piece of the 3-wire HVAC cable, and only used the red and white wires. Using minimal critical-thinking skills, I wedged new red and white wires on top of the old red and white wires that ran from the wall.

Close-up of the wires running to my old heater. My new control wires are just run in parallel to the old ones.

I knew from my earlier test that they would turn the heater on if they touched together and close a circuit. So when X1 and X2 in my cheat-box close, the heater should turn on!

Step four

Wire the Nest to the inputs on the cheat-box. Rh and C on the Nest, which connect to R(T) and C, deliver 24V AC power to the thermostat from the big square yellow-ish transformer. (I have no idea why HVAC controls run at such a strange and unique voltage, but I hope the story includes monopolies and Nikola Tesla.) W1 is the actual signal that Nest uses to turn the heater on, and connects to W(T).

Time to put oven mitts on and plug this monstrosity into the wall.

It works!

Step five

Mounting, where I finally ran into a situation where I had to be slightly clever. The Nest (and probably every other smart thermostat on the market) is designed for a mounting situation where wires have already been run through the wall. Mine were originally stapled onto the wall=/ So getting a Nest to mount flush, with a wire sticking out of the bottom would be tricky.

Nest ships an ugly square mounting plate that I could have cut a slot into, but that ruins the beautiful simplicity of a turnable brushed steel knob. I’m already going to have an ugly wire running out of it.

So instead of trying to find an offsetting bracket the exact same thickness of my 3-wire cable, I just used the 3-wire cable itself! I think it worked pretty well.

I hot-glued a piece of the cable to the outside edge of the Nest mount, and left a gap at the bottom for the actual signal cable. (Also, you can see I left the white and red signal wires that worked with the old thermostat. That way I can easily remove the Nest if we move, or I get sick of it.)

Step six

Button it up. To try to make myself feel macho, I bought actual conduit cable clamps to run the cables into the cheat-box. I pryed the punch-outs off the bottom of the box with a flat-head screwdriver without cutting myself and screwed in the clamps.

That way if someone accidentally trips over a wire, the clamp should take the force and it won’t yank the wires out of the more fragile screw contacts inside the box.

So here it is, all wired up and ready to go.

I put the protective cover on, plugged it into the wall, and hid the box out of sight in the corner near the heater.

Next, I ran the cable hanging out of the Nest mount down along my baseboards, hammering it into place with cable ties, and smushed the Nest onto the mount until it popped into place. It immediately powered up, but how do we fool this smart device into controlling our cheat-box? The old millivolt heater doesn’t have any fans on it, and just passively heats the corner it is installed in, so I told Nest that it was controlling a radiator, which acts the same.

After it had my Wi-Fi password and everything else it wanted, I spun the dial to ‘hot’. I heard a faint click from the relay in the cheat-box, and then a whoosh as the flames ignited in the heater. Great success!

The heater has been working great, and it’s very nice to wake up in the morning with the heat already on. Rationalization is a powerful drug, and in the months of Nest use, I have come to terms with taking the easy way out.

But that’s what progress is, right? Standing on the shoulders of giants has allowed humans to get to the moon, and just now has allowed me to push my confessions to the web without touching a smidgen of actual HTML.

Taking it to the extreme, should I really feel ashamed of riding in an airplane that I didn’t design myself? A better analogy is probably being ashamed to get a glass of water delivered on Postmates, but my story is now told, and I want to close with Frost:

I shall be telling this with a sigh

Somewhere ages and ages hence:

Two roads diverged in a wood, and I —

I, well, I took the clearly marked trail that had lights, hand rails, and a 4.5 star rating with over 1,000 reviews on Yelp,

And I think of the Honeywell engineers who paved that trail for me with deference.