Passive Solar Basement Efficiency

The graphs below summarize the results for the 2005-2006 heating season.

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The graph above shows the observed daily average temperatures for this 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.

In September and October there was a fairly steady rise in basement temperatures as the sun moved lower in the sky and the solar energy was absorbed in the basement. There was a period of three cold and cloudy days in mid-October when the average house temperature dropped below 65oF because we were not at home and closed vents prevented heat transfer from the basement. Most of November was sunny and the basement temperature remained above 80oF. However, while we were away over Thanksgiving (late November) the automatic basement window insulation system malfunctioned, causing 1.5 days of sun to be blocked from heating the basement. This malfunction, along with the following cold weather, caused the basement temperatures to drop into the 70s and required the first use of the propane backup furnace. December, January, and February were rather sunny, and it was quite warm at the end of February. Thus we used the backup furnace very little in the winter. As the sun rose higher in the sky in spring, much less solar energy was captured in the basement (due to the vertical windows), and there was significant furnace use during cold or cloudy periods.

To improve the solar heating performance, the computer control implements a ramping thermostat, with the desired temperature increasing smoothly from 65oF at 8:00 am (usually) to 75oF at 4:00 pm MDT, with a decrease during the night. This makes the solar heating much more effective because it transfers heat to the house when the basement is warmest. The backup furnace thermostat is set to 65oF during the day and 60oF from 9:00 pm to 6:00 am. The computer operates the backup furnace within a ±1.0K temperature range around the desired minimum temperature.

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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 only 34 hours, consuming 26 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.