With the Pi and Arduino all connected up and with the Arduino already knowing how to turn the boiler on and off, it didn't take long to get the Node server able to listen for boiler updates and signal changes of its own.
As the scheduler isn't (anywhere near) in yet, all we have for now is a greyed out flame icon next to the temperature read out. Clicking that flame icon will toggle the boiler to the alternate state:
All other connected clients will see the flame icon light up on their dashboards at the same time and the boiler fires up.
Just the scheduler to go and we've got the basics of a thermostat!