We started preheating the house. We had unseasonably late warm temperatures, so I hadn’t been thinking about. Then all the sudden on Sunday morning, the 27th, there was frost on everything. So, Monday morning, we switched the valves to start heating the sand from the solar panels. We are still heating the hot water tank too. The above image shows temperatures before and after turning on the sand.
Since we have been in the house for a few years, I analyzed previous years to see if it mattered when we started heating the sand from the solar panels. Results are highly inconclusive. These plots show the average sand temperature the first week of November and December as a function of the day we turned on the sand. Last year we turned the sand on first on September 22, but not long after we turned it off again because the house was too hot. We turned it on for good October 8th, 2014. I plotted the data both for the first day we turned the sand on, and the second time.
And for my own reference, the dates we turned the sand on were
- 2011: October 3
- 2012: October 8
- 2013: October 21
- 2014: September 22, October 8
- 2015: September 28