Passive Solar Basement Efficiency for the 2004-2005 Season

The graphs below summarize the results for our first heating season.

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The graph above shows the observed daily average temperatures at our house for the first 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.

There are some periods during which we lowered the thermostat and disabled the furnace, because we were not home. During cloudy periods the basement cools and becomes progressively less effective at heating the house. In mid-December we started using insulation at night in the house windows. In early December we switched operation of the basement-to-house fan from a conventional fixed thermostat to a ramping thermostat, with the desired temperature increasing linearly from 65oF at 8:00 am to 75oF at 4:00 pm, with a linear decrease during the night. This made the solar heating much more effective because it transfered 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.

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The above graph shows the total heating degree days (HDD) accumulated through the winter and total BTUs provided by the propane furnace in our house. These HDD are defined relative to the standard 65oF. After the heating season was over the total furnace use was 106 hours or 7.4 million BTUs (using the 70,000 BTU/hour furnace rating), which corresponds to 82 gallons of propane (assuming 91000 BTU/gal). 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 energy intensity index, and is a measure of how efficient the house is. Our heating energy intensity index was 0.66 BTU/(oF-day-ft2), which is very low for a manufactured house.

To compare the furnace fuel consumption with a control house without a solar basement, we used the Home Energy Saver (HES) web application provided by Lawrence Berkeley Laboratory. This application has a fairly sophisticated treatment of energy use in a house, including detailed geometry, orientation, window and window insulation specifications, roof and wall insulation, and the local climate. We used the average house temperature (65.7oF) for the thermostat setting. HES predicted that our house without the solar basement would use 930 gallons of propane for heating in an average year. This estimate seems high based on our experience of furnace use when the basement was not heating the house. We converted the HES estimated fuel use to the energy intensity index using the standard HDD for the weather station used by HES (Alamosa). This heating intensity index is 6.5 BTU/(oF-day-ft2). If this estimate for the control house is accurate then our house this year was about 90% solar heated.