Passive Solar Basement Efficiency

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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 above 0.6, and cloudy days have lower values.

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 to (usually) 75oF at 4:00 pm MST, 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 55oF from 9:00 pm to 6:00 am (or after). The computer operates the backup furnace within a ±1.0oF 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. 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.
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