More thoughts on Boiler Hacking

I've set up a 4 channel thermistor sensor adjacent to my boiler and have been logging the temperatures of flow, return, outside and inside temperature for the last few days. I've taken advantage of the pretty cold weather to get an overall assessment of how the boiler performs under various conditions.

At the weekend the house was cold so I measured the time taken for the boiler to get the house back up to temperature. Now that it's hovering around the set point on the thermostat, I'm monitoring how long the boiler cycle periods are, and see if I can reduce the cycling by direct manipulation of the water temperature control pot.

I've also discovered a subtlety about my Drayton Digistat wireless thermostat. In order to save battery life, it restricts the sending of messages to every 4 minutes. So if the thermistor in the thermostat reaches temperature, the boiler off command is only sent when the next message is scheduled. The timing of these messages is probably derived from a 1 second interrupt from a real time clock routine running of a 32kHz watch crystal. The effect of this is that the boiler appears to turn on and off, exactly synchronised to the seconds counter of my PC clock - i.e. all on and off activity appears to be when it's 32 seconds past the minute.

The Worcester Bosh Greenstar boiler, has a 2 minute start up sequence. If the boiler is cycling on and off every 20 minutes, it means that it goes through 3 start-ups per hour. If it is possible to reduce the number of start-ups and the cycling, then some of the wasteful start-up period would be removed and the heating would be more efficient.

It would also be advantageous to introduce some hysteresis into the thermostat control. Although the set point might be 19C, the user is not going to notice if you turn the boiler off at 20.0C and back on again at 18.0C. The boiler will have a longer runtime, and less time will be spent repeatedly reheating the pipes and radiators.

As far as controlling the boiler, it should be possible to use an RC servo to turn the pot on the front panel. The temperature control should be turned to maximum for a rapid warm-up, and then backed off once the property is up to temperature.

By way of some figures. Yesterday morning was about 1C outside. When the boiler came on at 4:30am it took 3hrs 30 mins to raise the room temperature by 3 degrees, and in the process used 46kWH of gas (£1.50 worth) and the room was still chilly. The boiler was restarted at 1pm and then only used a further 130kWh to keep the temperature maintained for the next 18 hours.