So here is a final report of this season - 2010/2011.
Last year we started pretty early with the heating, in August, due to the birth of our little daughter, and the fact that the end of August was pretty cold already, at the end, with the last days averaging less than 15 degrees.
After that, the boiler didn't stop working, during a pretty long and cold winter, with snow for about a month, and an average December temperature of -1.1C, when it is normally 4.0C.
Last year we got a new boiler installed, a new condensing boiler, so I was very curious to be able to compare the performance of this boiler to the previous old one with pilot light. Since that one spend about 0.7m3 a day just on the pilot light, I guess I would have about 21m3 a month less than usual, which, for the 7 months of heating translates to about 150m3 less, and about 90 euro of gas (about 60 cents per m3).
But, I could not do a proper comparison, since, with the arrival of the baby our habits changed a lot. First, the overall set point was raised by 1C, from 18C to 19C; second, my wife stayed at home for most of the winter on maternity leave, so the house was warmed the whole day instead of the usual night only; third, and similar to the previous, we spend most (if not all) days/nights at home, whereas before we were going out quite often for dinner, movies, parties...
With all these to take into account, I knew that it would not be easy to compare. But anyway, here are the values, and a small analysis (sorry for the bad formatting, but couldn't figure it out...):
I did not include April although for almost half of the month the heating was still on.
This leads to a grand total of 2105.18m3 spent, against a predicted (formula based on the first year of occupancy of the house) 3059.46m3. So we spent 69% of the predicted gas consumption.
The other years this value was around 55%, so there was a large increase in our consumption, although, the comfort levels were increased and the time the house was occupied also increased significantly. Maybe it would be a nice idea to make some statistics on this, since I have all the data available.
I cannot however conclude much about the improvement, since the usage conditions have changed so dramatically. I am nevertheless convinced that if I had kept the other boiler I would have spent more than the prediction.
So overall I am quite happy with the result. Next project is still to get a PID controller going based on opentherm (maybe starting with opentherm lite) to try to get the boiler to operate in its ideal range. Now it is still on/off control, which isn't the most efficient. I do try to run the boiler efficiently by limiting the temperature of the water to 55C whenever the outside temperature allows it.