openHAB and Our Fireplace Part 1

When moving into our place nearly six years ago, my hacker eye noticed the gas fireplace and my attention was drawn to the fact that it has a remote that has manual/auto and scheduling. It’s the first place I’ve lived in with a gas fireplace, so my first thought was the RF signal was prime for hacking. I never got around to trying anything and it is probably more difficult than the outdoor temperature sensor I did earlier this year. Recently, with the adventures in openHAB and with all of my Z-Wave research it dawned on me: I probably can just bypass the remote and interface directly. One simple test would answer my question. I took a jumper and followed the receivers two wires to the gas controller. Sure enough jumping them turns it on. The local manual switch and the remote receiver are in parallel. My question was then, where is the power coming from to detect the closed circuit? I measured about 500mV. I didn’t know much about fireplaces/heaters before I started this adventure. A little research later…
How gas fireplaces work
How to test thermopile

Diagram from fireplace manual

Fireplace manual for reference.

Now I know that the pilot light produces enough heat to generate < 1V. Just enough to run the trigger and provide safety. The small current keeps the gas valve open but when the pilot light turns off, the drop in voltage will close the valve. Genius! The voltage is generated from the thermopile and thermocouple. It’s called a millivolt system and our heater works the same way.

There are two ways I can go about this.

1. Design my own microcontroller and relay to control the fireplace. Probably would use a esp8266.

2. Buy a Z-Wave relay (dry contact)

I did some research on possible Z-Wave relays. This would cost a bit more but less time which I am in short supply of lately. I figured it would be easier and more reliable to use off the shelf parts. This is part of the greatness of openHAB. I can mix my own creations with off the shelf devices.

GoControl
Evolve
MimoLite

I narrowed it down to a few choices. My determining requirement is the ability to preserve the existing functionality while staying in sync with openHAB. The problem was that all of the relays that have switch input were meant for AC powered devices (in that it shares its power source with the source of what you are switching). Many bytes later, I found the MimoLite dry contact relay. Sweet! It even has a DC adapter. A bit pricey, but fits the bill and if/when we move, I am sure I will find plenty of uses for it. Almost a shame to just use it for the fireplace since it can do some cool stuff.

Receiver on the bottom, gas controller top middle.

Armed with an ADC/pulse counter you can do much more than just accept a switched input. Seems like a lot of people use it for their garage door since it can also do momentary latch. Awesome device. The idea is I can connect the receiver to the relay and the relay to the fireplace. The local switch would be in parallel with the receiver which is via the ADC input. The input also has a pull-up resistor so you can use it with a switch such as this or a garage door safety sensor.

Stay tuned for part two in which I go over installation!