Computing stuff tied to the physical world

Gas consumption

In Musings on Dec 24, 2010 at 00:01

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:

Screen Shot 2010 12 22 at 12.53.21

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:

Screen Shot 2010 12 22 at 12.51.45

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:

Screen Shot 2010 12 22 at 13.21.47

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).

  1. That’s quite a lot of gas.. for December, there are 3 days where gas consumption exceeds 11 m3 per day over here (that’s including hot water usage).

    But you triggered me to do a similar test myself :-) Our setback schedule from Monday to Friday is as follows: 23:00-06:00: 18.0 degrees 06:00-07:45: 20.0 degrees 07:45-13:00: 18 degrees 13:00-23:00: 20.5 degrees

    At night, the temp in the living room drops to a minimum 19 degrees just before 06:00. I’ve set my thermostat to a continuous setback of 20.5 degrees for this day, just to see what happens to our gas consumption and how much difference it makes.

  2. I have heard before that it is better to leave the heating running. It does make sense as the whole building will start to cool down.

    Obviously at some duration there will be a cross over point where it does work out best to turn the heating off… Sounds like you’re going to have to take a selection of holidays of varying lengths JC!

  3. Are you still using the same sensors for power and gas you talked about in your “better mousetrap” post from Dec 2008? I’m also curious what software you used to make the pretty graphs.

    • Yup. Graphs are from an old version of JeeMon with the Flot JavaScript package. I’ve been running that for 2 years now…

  4. The heat lost from the house is always proportional to the temperature difference (just like voltage drop across a resistor) so if the house is colder inside (and the weather outside unchanged) then you will loose less heat. So I think if you add up all the numbers, you will always save at least some amount of energy by turning down the thermostat when not needed. Maybe not a lot, but some. This would only be untrue if your heater is significantly less efficient when running at above-normal levels, and I’m not sure why that would be true.

    • This whole discussion is very interesting…

      Indeed, and contrary to what you think, the heater is much less efficient when running at above normal levels… The efficiency of a modern boiler is proportional to the return temperature, and it is amazing how fast it goes from nearly 100% at low return temperatures, to less than 80% when the return temperature is above 55C. I have a graphic somewhere, which I can try to find back and put here, of the efficiency vs return temperature. Such things can have a huge influence on the consumption…

    • Found it again! here it is:

      after all it is not below 80%, but below 90%

      I think most thermostats (even modulating ones) do not take any of this stuff into consideration, like heating slower at the expense of comfort in order to achieve higher efficiency… Hence my interest for OpenTherm and boiler control!

  5. What model of thermostat are you using ? I-ve seen some fancy units in the USA with ethernet conectivity and some sort of ‘learning’.

  6. You should look at degree-days as a way to normalize your consumption data.

  7. I agree with JBeale and would like to add the following. Your vacation was to short to make the profit out of heating. You told the house took 1 day to cool down, so you could now make simple FACTOR calculation. It also takes let’s say a day with furnace full-on to get back on 20 degrees Celsius. The overshoot made no profit for 4 day vacation or just slight. The drawback was you came into a cold house. From all this you could conclude that to make a significant profit, you must turn down the temperature to 14 when you are away for at least 5+ days or so. I also made a check for my house. I have 15 windows each 1.5-3m2. If I pull down the shutters at night during outside temperature below the house temperature I profit 10-15%. That is the base for my home automation of shutters. And I only turn down thermostat if we are away for at least 7 days (15cm house isolation + 40cm thick walls). The units for outside measurement and shutter control are done with RF12 and m328, and the code is fully tested. So compatible with Jeenode.

  8. Oops… it’s 10:00 am and gas consumption is 6.44 m3; total gas consumption for yesterday was 6.37 m3… It seems the delta-T rule for the amount of energy loss does still apply ;-) Check

  9. Hello to everybody,

    You can compare your gas usage by introducing “graaddagen” the formula for graaddagen is 18 degrees Celcius minus the average outsite temperature. For my house and way of living gas usage = 0.41*graaddagen + 2.6 the offset depends on your hot water usage and the slope on the type of house, your thermostat settings etc. To get the average outsite temperature I use this site

  10. We could use an easy-electrons kind of analogy here: a bucket of water with a leaking hole near the bottom. With a water faucet we try to maintain a certain water level in the bucket. The amount of water in the bucket is like the ‘heat buffer’ in your house.

    An important difference though is that the water flow is proportional to the sqrt() of the water level (height) whereas the heat flow is linearly proportional to the temperature difference, but that doesn’t matter much for the mental model.

  11. Hm! Maybe I am misunderstanding your conclusions – but I seem to read the figures slightly differently from the rest of you here.

    My conclusion is that turning down the temperature for your weekend away was a success, and that you saved at least 20% on your gas consumption, unless I am missing something.

    This is based on the following reasoning :

    • On the only 2 “normal” days (Wednesday and Thursday) the average use of gas was 16.3 m3.
    • On Friday the house consumed almost zero gas after you left the house (according to the 7 day graph almost all the gas was used before you turned the thermostat down)?
    • On the day after you came back you used 28.6 m3.

    I assume (as we have no further datapoints) that the house was back to normal temperature after the end of the graphs.

    So let’s for arguments sake say that what you saved on the first day is equal to what you used on the day after you came home.

    Based on this almost all savings are done on Saturday and Saturday. If you look at those 2 days in isolation the gas usage was in total 18.2 m3, where the average would have been 32.5 m3, a substantial saving.

    This was all based on slightly “skewed” data, as a substantial part of the gas usage on the 16th and 20th was done before you left and after you came home according to the 7 day graph.

    In other words for your house you really need to go away for more than 2 days to see substantial saving by turning the temperature down to 14 degrees Celsius.

    Calculations : (if the software does not mess the table up)

    Date - Usage - Calculation
    15  15.71
    16  16.83   16.27   Average gas usage
    17  3.52    12.75   Saving vs. average
    18  5.79    10.48   Saving vs. average
    19  12.4    3.87    Saving vs. average
    20  14.02   2.25    Saving vs. average
    21  28.16   -11.89  Excess vs average

                17.46   Total saving

    Or have I missed something here?

    • Hm!

      Hm, indeed. What you’re saying sounds plausible. I didn’t do such a detailed analysis. The savings might even be slightly more, given that the outside temperature was lower in the last days, so average consumption would probably have been higher.

      One day, I’d like to go some more into this, and put it next to the recorded outside temperature averages. Might be possible to predict the daily use, and then try to beat it… also would let me understand how much savings we’d get from using the fireplace, i.e. burning some wood.

      Thanks for working out the proper calculations!

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