Heating water from afar

Please do not take anything you read in any of my posts (but especially not this post) as engineering advice.

My parents’ house has an on-demand water heater for energy efficiency reasons. This has a small drawback: you have to press a button to prime the water heater and then wait 2-5 minutes before showering.

This turns into a somewhat bigger drawback for the one room for which it’s just Not Possible to wire up a button. The water heater company hypothetically sells a plug-and-play wireless solution for this sort of thing, but that is seemingly incompatible with my parents’ walls (???).

Thankfully, I have a have a playbook for this.

There’s just one problem: how do I make a Raspberry Pi talk to the water heater? I investigated a couple of different approaches:

• analyze and replicate the water heater company’s proprietary Bluetooth protocol
• install a physical button presser, inspired by the Recurse Center’s elevator-related shenanigans

but after a couple of discussions with my dad and a support tech from the company, we determined that we should instead emulate a button press. To find out what that means, let’s take a look at a very sketchy wiring diagram:

This means we would have to add a new “button” and have it briefly connect two wires. Because the last time I actually touched some wires was over 10 years ago in robotics and I don’t want to start any fires, I reach out to the usual suspects: Tom and Logan. They inform me that the thing I am looking for is called a relay and that companies sell pre-built relay hats for the Pi. Super.

I ended up buying:

• A Pi Zero 2W and accoutrements
• A relay hat with headers and stabilization posts
• Telephone wire from the hardware store, which already has bundled red/black wire

At some point I sit down to build the thing and realize that I don’t actually know how relays work. The relay I bought had three ports: NC, NO, COM. After some searching, I figure out that I want one wire in NO (“normally open”) and one in COM (“common”). This means that the relay, when activated, will close the circuit.

I downloaded the sample code from the company that sells the relay hats and realized that it is an extremely thin (~10 LOC) wrapper over the existing Python GPIO library provided and pre-installed by Raspberry Pi, so I just manually inlined it:

#!/usr/bin/env python3
import RPi.GPIO as GPIO
import time

GPIO.setmode(GPIO.BOARD)

class relay:
    relay_pins = {"R1":31,"R2":33,"R3":35,"R4":37}

    def __init__(self, pins):
        self.pin = self.relay_pins[pins]
        self.pins = pins
        GPIO.setup(self.pin,GPIO.OUT)
        GPIO.output(self.pin, GPIO.LOW)

    def on(self):
        GPIO.output(self.pin,GPIO.HIGH)

    def off(self):
        GPIO.output(self.pin,GPIO.LOW)

print("Starting water heater...")
r2 = relay("R2")
# time is in seconds, so turn on once for 4ms and then turn off again
TIME_IN_MS=4
MS_PER_SEC=1000
r2.on()
time.sleep(TIME_IN_MS/MS_PER_SEC)
r2.off()
print("...signal sent. Please wait 5 minutes before showering.")

If you read my previous post (linked above), you will know that is is, of course, a CGI script that is triggered on a website button press:

All of the rest of the software is the same as in the previous post. Very boring stuff: httpd, systemd. Hopefully nothing goes wrong. But if it does and I need to administer this device from afar, I also set up Tailscale (no, this is not an ad; just happy).

The total bill for this came to ~$40 or so, which isn’t half bad. It could probably be done for 35 cents using an old microcontroller and a paperclip or something but I wanted an exceptionally boring (to me) approach. That’s all for now. Thanks for reading!