In another post, I calculated how much it would cost to heat our pool based on various assumptions and estimates.
Turns out I was off by a factor of about two. I looked at the gas meter while having the heater on from beginning to start, for one weekend, with the set point being 80°F. Here is the data for early September 2011 (average outdoor temperature of 70 – 75 °F):
| Day | Time |
Water T (°F) |
Meter Reading (cu. m.) |
| Fri | 6pm | 72 | 7281 |
| 11pm |
80 |
7313 |
|
| Sat | 9am |
80 |
7328 |
| 6pm |
82 |
7328 |
|
| 10pm |
80 |
7329 |
|
| Sun | 11pm |
80 |
7340 |
This means that heating the pool by the first 8°F required 32 cu. m. Assuming my price calculation was correct, namely that it costs us $0.25 per cu. m. of natural gas, this means it costs us $1 per °F. to heat our size pool, approx 10,000 gallons.
Also, it required 5 hours to increase the temperature by 8°F, whereas my calculation based on BTU rating of the heater was 2.5 hours for 8°F, so it took double the time that I calculated.
To leave the heater “on” required about a dozen cu.m. of gas during the night, when the outdoor temperature went down to the low 60′s. Note that our heater only turns on when the water temperator goes below the set point (80°F, in this experiment). So it cost, at this time of year (September, in Eastern Ontario) about $3 per night. This indicates the heat loss from the water to the air was about 3°F, which makes sense: this is the drop observed when heater not on the previosu weekend, but solar blanket on. Without the solar blanket, the heat loss can be 6 to 8°F, at least double.
According to TheWeatherNetwork.com temperature statistics for this region, the average daily temperature in September has been 60°F, October 45°F, and November 35°F. This means approximately 15°F of extra heating required, plus more heat loss once the set point is reached, such that one can expect an extra $30 per weekend in October, or triple the cost of September.
Conclusions:
- Time required to heat our pool by 1°F: about 40 minutes
- Cost to heat our pool by 1°F (10,000 gallons): $1
- Total cost for the weekend to maintain our pool at 80°F, when average outdoor T is around 70°F: about $15.
- Given the cost of the pool and that of the pool heater, it is well worth the heating cost to be heating it to 80′ish °F whenever we want even till the end of September.
- Cost for October may be triple that of Semptember, but still bearable. It is possible that we can even justify the cost to end of October, if we can put up with the rather cool outdoor air (which just makes the water feel like a hot bath!)
