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?

A friend mentioned to me that it could also mean larger heat losses from my hot water heater because of the larger temperature difference between the water and the outside air (my hot water heater is outside).  Indeed, we would expect a similar shape to actual data in that there is a linear relation between the temperature difference and the heat losses (resulting in a straight line on this plot).  However, if heat losses were the primary culprit, the slope of the curve should be such that extrapolation to 130 F (the approximate temperature of the water) should remain above the x-axis.  That is, at 130 F there should be no losses due to heat and there should still be some energy needed to heat the water in the first place.  However, the extrapolation of that slope shows that it crosses the x-axis at about 100 F.  That means the slope is way too steep to be attributed to heat losses alone (though it still might contribute significantly).  

Ok, that is interesting.  I now know that heat loss from my water tank is not the primary reason for the increase in natural gas toward lower temperatures.  But what is the loss?  I looked at my hot water heater and saw that it had an r-value rating of thermal resistance of 19.2 (btw, the units on that are h·ft²·°F/Btu, seriously). That's a reasonably efficient value.  The higher the value the harder it is for the heat to escape the hot water tank.  With this R-value, the surface area of the hot water heater (30.5 sq. ft.), and the average temperature difference between the hot water and the outside air (~50-60 F), we can calculate the rate of heat loss from the tank.  It's about 0.02 therms/day.  So that's an interesting number because it's only about 1/7th the amount of gas I use in reasonably warm (70 F) days.  That means that the vast majority of the gas must be used to first heat the water, not to maintain heat against losses.  

So let's check that.  If that's true, that would mean that simply heating the water should account for the rest of the gas use.  Assuming the temperature needs to be heated by about 60 F.  I estimate that between showers, the dish washer, and laundry, we use about 30 gallons of hot water per day.  Crunching the numbers, that requires 0.12 therms/day, which nicely matches the actual use in the plot above!  Nearly all of the warm weather gas use if for water heating, not compensation for heat loss.  Good to know.  

Armed with this information I can actually determine what fraction of our 160 therms used per year goes to water heating and what fraction to air heating.  Below is the result.  On the left is the breakdown of air heating vs. water heating.  On the right, the water heating component is broken up into original heating and replacing losses.  Obviously, attacking the water heat loss is not the way to go.  Furthermore, I'm already pretty efficient with the amount of hot water I use so the water heating in general is tough to significantly decrease.

All of this leads me to think I need to tackle the green wedge, or how much I need to heat the air in my house.  Since I already keep the temperature set pretty low in my home, the best way to decrease this will be better insulation.  That's probably my next step.