Next Step: Natural Gas

As seen in my recent post, I've addressed our electric energy use with solar panels.  Excellent.  So it's on to the next thing - natural gas.  I haven't thought about natural gas much because, well, it's harder to do anything about it.  The gas we use goes toward either heating the air during the winter, or heating the water year round.  We have a water-efficient shower head (saving on hot water) and we don't use the heater in the winter unless it's really cold (We set the temperature to ~64 F).

It occurred to me that I'm pretty ignorant about the natural gas use of our home.  What is the carbon footprint relative to the electricity use?  What fraction of the gas goes to heating air, what fraction to water?  

So I decided to dig a little into the data.  My natural gas provider allows us to download a spreadsheet with our past usage, and a bunch of other relevant information. In particular, the average daily temperature of the billed month is tabulated along side the average daily use.  So I made a plot.  

Natural gas rate of use plotted as a function of average daily temperature. Use above ~62 F is purely water heating. 

Not surprisingly, as the weather got cold, our use went up dramatically as we ran the heat.  However, I was surprised by the shape of the curve at temperature above 62 F.  Above that temperature, we don't turn on the heat, yet our natural gas use is going up.  

To me, that meant that this could have meant a number of things:  1) the ground water is getting colder so it takes more heat to get it to the same desired temperature and/or 2) we take longer showers when it's cold in the house.  Both of these things are probably true at some level.  But is there more to it?