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 |
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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