Natural gas heating system
Scenario: Initial heat up time and cost for a 75,708-L (20,000-gal) pool that requires a 20 degree temperature rise. (Note: 3.78 L [1 gal] of water weighs 3.7 kg [8.33 lbs]).
20,000 gal x 8.33 lbs = 166,600 lbs of water in the pool that will need to be heated
166,600 lbs x 20-degree rise = 3,332,000 Btus required
Btu is a unit of power measured per hour. Therefore, 138,833 Btus per hour is needed to achieve the 20-degree temperature increase:
3,332,000 Btus / 24 hours = 138,833 output Btu per hour required
Going back to the original example, a new 400,000-Btu heater that is 85 per cent efficient, or in other words, has a 340,000 Btu output, will take 9.8 hours to raise the same pool’s water temperature by 20 degrees.
3,332,000 Btus / 340,000 = 9.8 hours
Gas companies charge by the therm (thm), so to figure out how much it will cost to heat the pool water it is important to know that 1 thm is equivalent to 100,000 Btus. If it takes 9.8 hours to increase the water temperature in a 75,708-L (20,000-gal) pool by 20 degrees using a 400,000-Btu heater rated at 85 per cent efficiency (i.e. 340,000 Btu output), the cost would be as follows:
9.8 (heat up time) x 400,000 (Btus) = 3,920,000 (total Btus)
3,920,000 / 100,000 = 39.2 thms used
If the cost of gas is $1.20 per therm, then:
39.2 thms x $1.20 = $47.04 (total cost to raise temperature)
A facility using a five- to seven-year-old, 400,000-Btu heater that was originally rated 78 per cent efficient, but is now operating at 65 per cent efficiency, is only outputting 260,000 Btus. To achieve the same 20-degreee temperature increase with this heating system, it would cost the facility the following:
3,332,000 Btus required / 260,000 = 12.8 hours
12.8 (heat up time) x 400,000 (Btus) = 5,120,000 (total Btus)
5,120,000 / 100,000 = 51.2 thms used
51.2 thms x $1.20 (cost per thm) = $61.44
This means it costs the facility an additional 30 per cent to heat the same body of water with their older heater.
