Eternal Vigilance is the Price of Efficiency, part 3 – gas

In my last two posts I talked a little about our utility usage for electricity and for water; last up is natural gas usage.  (I don’t think I’ll ever start weighing my garbage, but who knows).

Oddly, gas was a little tougher to get a handle on – there is a lot more variability month-to month, as well as year-to-year depending on the weather.  Our therms used per billing period has ranged from around 10 to almost 200 in the last 3 years.  I’ve also done a lot of work that should help things (note, some amazon affiliate links follow):

And on the increased-use side, we turned the basement into a zoned, heated space last year.

So it’s a ton of variables all working together.   How does it look, are there any trends?  My best attempt to analyzing that question is to look at therms used per heating degree day during the heating months.

A heating degree day is”a measurement designed to reflect the demand for energy needed to heat a home or business. It is derived from measurements of outside air temperature” according to Wikipedia.  I gathered degree days per month from www.degreedays.net (which didn’t perfectly overlap with billing periods, but close enough), subtracted about ten therms from each period (a rough approximation of our water heating use, probably a little low), and graphed the remaining therms per degree day:

We were on a fairly steady decline until about March/April 2011… which is when we put the 2nd heating zone in the basement.  Oops!  This is what actually prompted these posts; I hadn’t looked for a few months, and when I did, I realized that something had apparently gone a bit wrong in our gas conservation efforts.  I’ve since decided to let the basement stay cold unless we are down there; the baseboard radiators heat up in just a couple minutes, there’s no need to keep it warm down there all day long.

Here is the graph of therms per HDD (without normalizing for water heating use) and therms per day as a 12 month rolling average.  Around March 2011 looks again like a reversal of the downward trend:I’m not sure if this is really enough data to see a trend – and I’m not quite sure how to properly normalize for weather & separate out hot water from space heat use.  This is the best effort so far; I’m open to suggestions!

See also:

9 thoughts on “Eternal Vigilance is the Price of Efficiency, part 3 – gas

  1. In some sense there should be a correlation between outside temperature and the setting on your thermostat, so when the outside is cold you’d expect more demand for heating. So given your trying to account for that by heading degree days, are you taking that as a fixed for the whole day thermostat setting with an average for the whole day outside temperature? That would be like saying you have the heating switched on all day triggered by a thermostat – if you do that would be an interesting result, as I suspect you might find it better to only have it switched on when you need it (your house isn’t that well sealed from energy flowing out). If you do have it switched on only when needed then your ‘model’ is not going to fit that precisely – day time tends to be warmer than night time for instance.

    I’d expect there to be some lag in the time correlation especially if you have a well sealed and insulated house :-) Additionally you have an asymmetric relationship, i.e. when it is actually warmer outside then your heating demands you should have ‘zero’ heating needs (other than water), but assuming no cooling you’d have a net gain in inside temperature on days when it is warmer outside than some magical unknown threshold, this may then mean the next ‘colder’ day is offset by the previous ‘warmer’ day. In essence it is like you’d need to be modeling the flow of ‘heat’.

    You also have other sources of heat in the house, in particular human bodies, computers, monitors, TVs and other electronics which may effectively offset the base temperature at which your heating turn on, i.e. your other sources of heating act as a kind of base level of heat generation.

    Obviously the converse should occur with cooling, all be it that cooling tends to actually increase energy consumption overall due to the efficiency/heating effect of motors, etc, increasing the asymmetry.

    • I haven’t really gotten into the day-by-day granularity (although that might be neat!) I think that as long as I add up all heating degree days in a year, and add up all therms used in that year, it gives me a reasonable notion of whether I am doing better or worse on overall conservation and efficiency. It doesn’t tell me, though, if differences were due to physical changes in the house, or behavioral changes (i.e. changed thermostat programs or overrides, guests, increased exhaust fan use, etc)

      As for cooling; we almost never run AC (usually about 1 week in the summer) so I haven’t worried about tracking that yet. :)

  2. Take care with the insulation. There was an explosion nearby-ish to Atlanta this summerr blamed on LP gas. A *slow* gas leak built up over a long time because the house was insulated to the point of rooms being nearly sealed. Spray insulation’s on our to-do list, but I’ll also be getting at least one detector.

    • Well – it sounds like the problem was the gas leak, not the insulation. :) But yep, the mantra should be “build it tight and ventilate right!”

      I did a lot of this stuff as part of a utility rebate program – when all the work was done, they came back in and used flue gas detectors to make sure we had no combustion gas backflow, etc – sealed up the house, turned on both bathroom fans, dryer, and anything else that would draw a vacuum, then started up the gas appliances and made sure there was no backdrafting. Not quite the same as your gas leak, but another thing to consider.

      Still, I would sum it up as “be smart about it” rather than “don’t insulate your home, lest it explode!” :)

      • Oh, definitely. I was more thinking of how to monitor which is wise regardless. We don’t have a sufficient rebate program around here (as far as I know), so we’re going about this one little step at a time. Unfortunately, the first step will be removing a big old tree that both provides summer shade and has a fungus indicative of root rot. sigh.

        • Check out http://dsireusa.org/ for local rebates and incentives. We have some local programs that are amazing – 30year, 0 payment 0 interest loans for insulation & boiler replacement, finances half of the improvements, or 100% for people that meet certain income criteria. Those came into effect after I did my work, though.

          If the old tree is on the south side, maybe that opens up the possibility of solar PV? :)

  3. Pingback: Therms per Heating Degree Day | Eric's Blog

  4. Eric,
    In response to weighing your garbage – I think Seattle’s waste management program does that (or at least they used to) and bills households directly by weight.

    Our family has taken a different approach – we discontinued our garbage pick up service. So we are not weighing it, but we are much more aware of the waste we produce and now things like composting the kleenex doesn’t seem like such a crazy idea. I will tell you more about our family’s waste management plan next time I see you (if you are interested)

    Christi

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