Allocating energy consumption

We use an oil-fired boiler feeding 180°F water to hydronic baseboards to heat the house. It also supplies domestic hot water (very hot water!) for washing dishes and people. I think the boiler has a small, maybe 5 gallon, reservoir, but it might only have a buffer of hot water in the heating coil.

I looked at our fuel bills. Taking the timeline of fuel deliveries and dividing the quantity delivered by the number of days since the last delivery gives a rough idea of the consumption per day preceding that fill up. The suppliers like to fill it up, so at the time of every dot in this graph, we have a full tank.

The black dots are the total consumption per day estimates, calculated as the amount needed to refill the tank (i.e. what was consumed), divided by the number of days since the last full tank. The dots fall into two bands, roughly - below and above 5 liters per day. We can average those below 5 l/d, assuming that's when it's only making hot water, not heating the house. That average is 2.9 liters of oil every day to heat the water used wash dishes and have showers! I don't really believe this, so investigating further.

The blue line connects the low dots, so shows oil consumption to make just domestic hot water over 8 years. Subtracting that off the total gives the orange line, which is the estimated consumption to heat the house. The green swath in behind is the heating degree-day value for each day, which should have a direct bearing on heating. The orange and green curves match up pretty well. There's a consistent time shift because of the fill up is at the end time of the consumption.

Oil is going to be $1.50 per liter this winter!