Passive Solar Basement Efficiency

We use a computer to monitor the temperature of the house and basement and ingest data from a weather station. The computer also controls the solar-basement-to-house fan and the backup propane furnace. This detailed monitoring allows us to measure the backup furnace use and estimate the solar performance of the system. The propane furnace is the only source of backup (nonsolar) heating. The table below summarizes the performance for each year.

Season Degree Days
(F) 		Num Days
Tavg<10 		Weighted Solar
Transmission 		Furnace Use
(Hours)		 Furnace Use
(106 BTU) 		Propane
(gallons)		 Heating Intensity
(BTU/(F-day-ft2))
2004-2005 8171 6 0.591 106 7.44 82 0.60
2005-2006 7670 4 0.626 34 2.40 26 0.21
2006-2007 8547 10 0.592 83 5.84 64 0.45
2007-2008 8865 26 0.611 93 6.53 72 0.49
2008-2009 8224 4 0.587 56 3.92 43 0.32
2009-2010 8788 8 0.587 41 2.85 31 0.22
2010-2011 7813 10 0.599 10.4 0.73 8.0 0.062
2011-2012 7876 8 0.579 4.6 0.32 3.6 0.027
2012-2013 8390 19 0.576 16.9 1.18 13.0 0.094
2013-2014 8512 14 0.585 0.5 0.04 0.4 0.003
2014-2015 7757 8 0.540 13.6 0.95 10.4 0.081
2015-2016 8090 14 0.542 14.1 0.99 10.8 0.081
2016-2017 7630 9 0.536 28.7 2.01 22.1 0.174
2017-2018 7155 0 0.550 0.0 0.00 0.0 0.000
2018-2019 8547 10 0.535 17.3 1.21 13.3 0.094
2019-2020 7918 8 0.558 7.3 0.51 5.6 0.043
2020-2021 7846 1 0.553 2.7 0.19 2.1 0.016
2021-2022# 7506 5 0.632
2022-2023# 8387 3 0.629
2023-2024 7527 1 0.629 5.9 0.41 4.6 0.036

Notes: Heating degree days are the sum of the difference between 65o and the daily average outside temperature, including only positive degree days. For comparison, Minneapolis, MN had 7981 heating degree days on average from 1961 to 1990. The weighted solar transmission is the measured daily average solar flux on the horizontal divided by the top of atmosphere solar flux and weighted by the heating degree days. The weighted solar transmission is a measure of how sunny the heating season was with more weight given to the colder days. (Note: There is a slow decline in the solar flux calibration over the years until mid-November 2021 when a new weather station and solar sensor were installed.) The furnace BTUs are calculated from the furnace running time assuming the furnace rating of 70,000 BTU/hour. The gallons of propane are calculated assuming 91,000 BTU/gal.
# The backup furnace was intentionally never run, so furnance use variables are not relevant.

The heating intensity index is the ratio of BTUs to heating degree days divided by the floor area of the house (1508 square feet). There is some discussion of the Home Heating Intensity on the HHI calculator web page. For more details about each year including graphs of daily average measurements, please follow the annual links above. The last line has the link to the current heating season.

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