Gas Heating
Measurements
Our gas consumption using ducted gas heating only, March 2009:
Therm | Hours | Outside | Inside | Curtains | Gas m^{3} | m^{3}/hour·°C | |
---|---|---|---|---|---|---|---|
Night | 21 °C | 1.15 | 13.5 °C / 12.5 °C | 22.4 °C / 20.7 °C | 20% | 4662.6 / 4663.5 | 0.09 |
Night | 21 °C | 1.00 | 16.4 °C / 16.2 °C | 20.4 °C / 20.4 °C | 0% | 4676.5 / 4676.8 | 0.07 |
Day | 20 °C | 2.32 | 10.6 °C / 12.4 °C | 19.4 °C / 19.6 °C | 10% | 4689.2 / 4690.7 | 0.08 |
Estimated heat loss = 0.08 x 38.6 = 3.1 MJ/hour °C (3.7 cents/hours·°C) = 860 Watts/°C
So to heat the house 20°C warmer than outside requires 17kW. Sounds about right as our heater is rated as 25kW and it does handle 20°C difference 'okay'.
Gas supplier's data
March 2009.
Cost: 1.20 cents/ MJ = 1.20 cents per 277Watts per hour = 0.004 cents per Watt per hour.
Heating value: 38.6MJ/M^{3}
Wood Heating
To measure the effectiveness of our fireplace I measured gas heater gas consumption when the fire is lit.
Measurements with fire lit (March 2009):
Therm | Hours | Outside | Inside | Curtains | Gas m^{3} | m^{3}/hour·°C | |
---|---|---|---|---|---|---|---|
Day | 20 °C | 1.17 | 10.4 °C / 10.7 °C | 20.2 °C / 20.7 °C | 5% | 4724.7 / 4725.1 | 0.04 |
Day | 20 °C | 0.90 | 10.7 °C / 10.9 °C | 20.7 °C / 20.7 °C | 5% | 4725.1 / 4725.5 | 0.04 |
So, the wood fire saves us 0.04 m^{3}/hour·°C gas, which means that the wood fire's output (measured at 10°C difference), was equivalent to 10 x 0.04 m^{3}/hour which is 15 MJ/hour or 4.2KW. This is a cost saving of 17 cents per hour.
So, If we use the fire, on average, for 4 hours a day for 5 months then it saves us about $100 per year. The fireplace insert we installed did cost us about $2,000 so it will need to last 20 years to break even. This seems unlikely, so the wood fire is nice but not a financial success unless we only heat the lounge room (turn the gas heater off). Funny that, being green seems to always come to consuming less.
Double plane windows
(Using SI R-values)
Single pane glass window - R-0.18
Double pane glass window - R-0.35
Considering cost savings of changing to double pane glass, the difference is R-0.17.
W = K·m²/R = K·m²/0.17 = 6·K·m²
So for:
K = 15°C
m² = 1
W = 90
That is, to maintian a 15°C difference accross a window will require 90Watts less power per m² if double pane.
This is a cost saving of 0.36 centers per hour per m².
So if we heat 8 hours a day a 2 x 1.5m window, the cost saving is 2x1.5x8x0.36 = 8.6 cents per day = $2.60 per month. Not worth the expense.
Average Temperatures
For Mount Dandenong (°C):
Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | |
---|---|---|---|---|---|---|---|---|---|---|---|---|
Mean Max | 22.1 | 22.9 | 19.7 | 15.4 | 11.7 | 8.8 | 8.2 | 9.6 | 11.6 | 14.8 | 17.3 | 19.9 |
Mean Min | 11.5 | 12.6 | 11.3 | 9.0 | 6.9 | 4.4 | 3.6 | 4.2 | 5.0 | 6.8 | 8.3 | 9.8 |
Calc mean^{1} | - | - | - | 12.2 | 9.3 | 6.6 | 5.9 | 6.9 | 8.3 | 10.8 | 12.8 | - |
So, from the above table I take the average outside temperature for April to November (8 months) as 9.1 °C.
R-value
From wikipedia:
The world-wide definition of R-value is kelvin square meters per watt (K·m²/W), using the SI system.
American customary units, used in the United States, measure R-value in degrees Fahrenheit, square feet hours per Btu, (ft²·°F·h/Btu). This is commonly written in the form R–## (eg. R–19). The conversion is 1 ft²·°F·h/Btu ≈ 0.1761 K·m²/W, or 1 K·m²/W ≈ 5.678 ft²·°F·h/Btu.
I'm using SI system.
R = K·m²/W
so ...
W = K·m²/R
Links
- Measuring Home Heat Loss (Physics forum)
- Calculating your Home's Heat Gain and Loss
- R-Value Table (for selected materials)
- Home Heat Loss Calculator
- Home Energy Analysis
- Heat Loss From Buildings
- R-Value
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