Passive Solar Basement Efficiency

The graphs below summarize the results for the 2011-2012 heating season.

Click on the graph for a larger version.

The graph above shows the observed daily average temperatures for the 2011-2012 heating season. The grey squares (along with the y axis on the right edge of the plot) show how many hours per day the backup propane furnace ran. The lower graph shows the ratio of daily average surface solar flux to the top of atmosphere solar flux. The surface solar flux on a horizontal plane is measured by the weather station, and the top of atmosphere solar flux is calculated for the date and location. Sunny days have a solar transmission near 0.7, and cloudy days have lower values.

It was a relatively warm season, except for the cold snap in early December. December was the only time that the basement temperatures were below 80o for a sustained period, and even then most of the heat for the house came from the basement. Like last year we had some problems with propane at the coldest temperatures, though this year the problem seemed to be the regulator. With some warming of the regulator we were able to run the the furnace most days we needed to. The house overheated at the end of September when the outside temperature was warm and we were not home to open the windows. We tilted the reflectors back to level on May 15 before we left on a trip, and the basement temperatures fell rapidly. Keeping the window insulation down during the day starting on May 28 further reduced the basement temperatures. We tilted the reflectors earlier this year, about a week before the end of the month from January through April, which seemed to work well.

Click on the graph for a larger version.

The above graph shows the total heating degree days (HDD) accumulated through the season and total BTUs provided by the propane furnace in our house. These HDD are defined relative to the standard 65oF. The furnace BTUs are calculated using the 70,000 BTU/hour furnace rating and the time the furnace is on as measured by the computer.

This season the furnace was used for 4.6 hours, consuming about 3.6 gallons of propane. The ratio of BTUs to heating degree days divided by the floor area of the house (1508 square feet) is plotted in the lower panel. This ratio is called the heating intensity index, and is a measure of how efficient the house is.