This graph represents one of
several household thermodynamic tracking projects that are being
conducted at The Ravina Project. We are
interested in tracking the heating efficiency of our house as we
make various changes both to the house operation and to the
insulation. Are we getting better? That's always the question.
Heating Degree days are
calculated by subtracting the average temperature for the day
from a standard temperature. The difference is the number of
heating degree days generated by that particular day. On that
day the house uses some number of cubic meters of natural gas.
Once these values are known then one can calculate the thermal
efficiency of the house.
We can define the thermal
efficiency of the house by answering the following question. How
well does my home retain heat? Since heat flows only in one
direction, from hotter to colder, then the question really
becomes, how well does my house's physical structure and
its operation impede the flow of heat through it. The perfectly
insulated house would have no heat flowing through it ... the
heat would be stopped at some point, never travel farther and
certainly not reach the outside.
We do not live in perfectly
insulated houses. Heat will escape the house when the outside
temperature is lower than that inside. Since the house is heated
by natural gas, the amount of natural gas used should be
proportional, in the winter time, to the amount of heat escaping
the house. The relationship should be close to this: the greater
the heat leakage, the greater the amount of natural gas used to
keep the house within a certain temperature range.
Heating Degree Days allow us
to make a standardized comparison among houses' structural
insulation efficiencies. It does this by fixing the temperature
inside the house at 18 degrees centigrade. If the outside
temperature is 18 degrees centigrade then, using this model, no heat flow occurs
from inside the house to the outside world. As the difference in
temperature increases, the greater the flow of heat because
there is a greater heat flow pressure between the hotter
areas and the colder areas. Simply put, as the difference
between inside temperature and outside temperature increases the
greater the pressure driving heat through the walls of the
house. As heat is lost, natural gas is
burned to keep the house within a desired range of internal
temperatures. The greater the number of heating degree days; the
greater the number of cubic meters of natural gas used. We can
calculate a ratio as follows: 'the number of cubic meters of gas
used' divided by 'the number of degree days'. This will give us
the number of cubic meters of gas used for every heating degree
day.
All good stuff so far
however, how does one separate out the gas burned to heat the
house from the gas used for preparing food, drying clothes and
heating water? Good question! We summed the total gas
used during the warm months from June to September each year. For each of these totals we
divide by the total number of days in those four months.
We came out with a number of cubic meters of natural gas used
each day. That daily use, net any use for heating, became a
baseline use number for the next heating season. On the efficiency analysis spreadsheet
below the entry Net NG Used (CM) signifies the gas used
less the baseline usage. This is as close as we can get to a
number that represents the use devoted to heating the house.
Note we use the previous summer's baseline calculation. See how
our baseline has changed over the years. One adult was added to
the household in the Fall of 2005 and you can see the change in
the baseline calculation for the next Summer. This presented a
problem which we have recently discovered. There is no previous
Summertime baseline calculation for this new person. The
previous Summer's baseline reflects Natural Gas usage for only
one person. What we have done to correct this problem is to use
the baseline calculation (from the Summer of 2006) of 1.89
CM/Day for BOTH these sets of Winter months. The net effect is
that the Winter of '05'06 shows less net cubic meters of gas
used and hence a slight increase in the efficiency of the
household. We believe that this new number more accurately
reflects the household efficiency for the Winter of '05'06.
Notice that the baseline
usage increased to 2.28 CM/day for this past Summer ('10).
Another adult was added to the household. More gas was used for
personal hygiene, cooking, clothes drying when it was raining
for long periods and the like.
Here's an example of a
calculation. House A
uses net 300 cubic meters of gas in a month. The total number of
heating degree days for the month is also 300. House A uses (300
cubic meters / 300 heating degree days) = 1 cubic meters of gas
per heating degree day.
Notice that if the net number of
cubic meters of gas used increases while the number of heating
degree days remains the same value, the resulting ratio changes
from 1 (one) to a value greater than one. For instance, consider
House B. Its numbers are: '600 net cubic meters of gas used' / '300
heating degree days' = 2. We see that House B used two (2) cubic
meters of gas whereas house A above used only one (1) per
heating degree day. The House B must be less efficient than
House A.
Heating degree days may be
available from a weather office near you. We use the stats for
"Toronto Center" from Environment Canada. See our new paper on
Household Thermodynamics October 2011 on the Project
Papers page detailing all the changes we made to the house
over several winters.
Here's our Graph and Data for
the last several years of natural gas usage and heating degree
days.
Note that the number of days
varies because the dates are defined by billing periods and not
by a simple calendar.
Efficiency Analysis Using Previous Summer's Baseline 
























Winter 0405 
Winter 0506 
Winter 0607 
Winter 0708 
Winter 0809 
Winter 0910 

Oct/04  May/05 
Oct/05  May/06 
Oct/06  May/07 
Oct/07  May/08 
Oct/08  May/09 
Oct/09  May/10 







Total NG Used (CM) 
3024 
2724 
2710 
2154.6 
2330 
2032 
Total Baseline Used (CM) 
369.0 
462.2 
455.4 
436.4 
463.7 
470.1 
Net NG Used (CM) 
2655.0 
2261.8 
2254.6 
1718.2 
1866.3 
1561.9 
Total HDD 
3478.5 
3277.7 
3507.3 
3400.3 
3736.4 
3242.4 
Efficiency CM/HDD 
0.763 
0.690 
0.643 
0.505 
0.499 
0.482 







Year over year % increase 

9.59 
6.84 
21.40 
1.15 
3.56 
% increase over baseline 

9.59 
15.78 
33.80 
34.56 
36.89 















Winter 1011 






Oct/10  May/11 












Total NG Used (CM) 
2328 





Total Baseline Used (CM) 
559.2 





Net NG Used (CM) 
1768.8 





Total HDD 
3804.2 





Efficiency CM/HDD 
0.465 












Year over year % increase 
3.47 





% increase over baseline 
39.08 





Baseline Calculation 









Baseline calc for June through September 2004 









days= 
121 




CM NG used 
183 




Baseline 
1.51 
CM/day 








Baseline calc for June through September 2005 









days= 
121 




CM NG used 
187 




Baseline 
1.55 
CM/day 








Baseline calc for June through September 2006 









days= 
123 




CM NG used 
233 




Baseline 
1.89 
CM/day 








Baseline calc for June through September 2007 









days= 
123 




CM NG used 
220 




Baseline 
1.79 
CM/day 








Baseline calc for June through September 2008 









days= 
121 




CM NG used 
229 




Baseline 
1.89 
CM/day 








Baseline calc for June through September 2009 








days= 
121 




CM NG used 
239 




Baseline 
1.98 
CM/day 








Baseline calc for June through September 2010 








days= 
124 




CM NG used 
283 




Baseline 
2.28 
CM/day 

