The gas consumption at JeeLabs is enormous these days – some 15..20 m3 per day right now. One reason for this is that our house is well-insulated but very open. All the warm air tends to move 2..3 flights up, even though we try to keep all the doors upstairs tightly shut.
The trip to Germany a few days ago provided an interesting opportunity to get a better insight in how all this heating works.
Our thermostat is set up to heat the house from roughly 6:00 (6AM) to 23:00 (11PM). It’s a fairly advanced unit with some predictive logic to attain those settings, which is why it actually starts about an hourly early:
The above graph shows two superimposed heating cycles, with the current one still in progress (it was around 13:00 when I took that snapshot). As you can see, the heater is almost flat out, with some extra peaks during hot water use.
Here’s the gas consumption over the past 7 days:
The gray bands are sunset/sunrise, ie. day/night periods.
What I did was turn down the heating on Friday morning, when we left for our trip. The normal setpoint is 19..20°C, but the thermostat has a “vacation mode” which changes that into a permanent 14°C.
As you can see, the house took almost a day to cool off. Not bad, knowing that it was permanently freezing outside at that time.
On Saturday, the heating starts up a bit again, and then stays on at a reduced level most of the time, i.e. day and night, until we got back late Monday evening. Which is when I restored the normal cycle.
The interesting bit is the end effect of getting back to normal. Here are the same readings, now as totals over the entire day:
Same pattern as before, of course: 17th and 18th almost nothing, then slightly lower consumption rates to keep the house at 14°C, and finally on the 21st a big push to get back to our normal comfy levels.
Here are the same values, numerically:
- 15th – 15.71 m3
- 16th – 16.83 m3
- 17th – 3.52 m3
- 18th – 5.79 m3
- 19th – 12.40 m3
- 20th – 14.02 m3
- 21st – 28.16 m3
I should add that outside temperatures were a bit lower on the 19th and 20th, so these consumption levels cannot be compared 100% accurately.
But what stands out is that heating up the house back to 19..20° takes almost as much energy as what was saved on the days before. In other words: you can try to save all you like by turning the heater low when leaving the house – if you come back and want to get it back to the original level again, you basically have to add almost as much energy back in as if you hadn’t turned the heater down in the first place!
Heating is not a matter of “on = comfy, off = energy saving”, but one of keeping a whole pile of stones and concrete at a certain temperature. And this holds true even in very cold times. Apparently, the amount of stored energy is substantial compared to the amount of energy loss, and having a slightly cooler house doesn’t affect the rate of energy loss all that much.
This probably also explains why our gas consumption can still be 25% lower than average in this neighbourhood, despite the fact that many people are away and work elsewhere – while I keep the house heated all day long… (just a bit more sparingly than most, I guess).