Previously I stated that for an older property, there was the heat required to warm the house up, plus the heat to maintain a given set point temperature. Over this weekend which has been bitterly cold I have had the opportunity to do some datalogging and put some figures on to this.
First, here's the warmup from nominal 17C to nominal 19C on Sunday morning. This took 4 hours and used 50kWh of gas. In each of the following plots the x-axis is the time in minutes.
At the same time the outside temperature was rising from about -4 to about -1C - but more importantly to the controller, was the difference between inside and outside temperature - the Delta temp. As you can see the delta temp was between 20 and 21.5 C for all of the warm up period.
By 10am the room had come up to temperature and the controller enters the second phase - main room temperature at a comfortable 19C +/_ 0.2C.
The next set of 3 plots shows the detail from 10am Sunday to 10am Monday. This was the coldest night so far with temperatures down to -9.3C! Note how the delta reaches a maximum of 29. In real terms this means that the heating has to work about 50% harder, than if delta is about 20 - and thus use a lot more gas.
Below is the plot of the room temperature, once the system had stabilised, holding the room at 19C +/- 0.2C. Each little sawtooth is the effect of the boiler coming on at 18.8C and heating the room up to 19.2C - this maintaining an average temperature of 19C.