Frank and Jacque answer a couple of listener questions. The first question is about properly sizing a pool heater and brand recommendations. Then, they answer some questions about salt systems, corrosion, and CYA issues.
How do you determine the heater size you need for your pool or spa? First, many pros will agree that bigger is better. We will discuss how much heat it takes to achieve the temperature rise in the water, but one of the most important things to consider is how fast you want to achieve the desired temperature rise. Then you have to consider overcoming heat loss while heating and maintaining the heat rise, and of course, the cost.
BTU = the amount of energy it takes to increase the temperature of one pound of water to one degree Fahrenheit.
One gallon of water weighs 8.3 pounds.
It costs around $4.00 to $8.00 Per hour to fire a 400,000 BTU gas pool heater. Prices will vary between areas of the country. One Therm = 100 cubic feet of natural gas = 100,000 BTU. So, to determine your actual cost, you will need to see what you pay for natural gas per therm at your location. In North Texas, we are currently paying around $1.80 per therm.
Calculate if a heater can achieve your desires in heating your swimming pool (or spa, but spas are much smaller). It takes one BTU to increase the temperature of one pound of water to one degree Fahrenheit. So, if you have 10,000 gallons of water, you must first calculate how many pounds of water that equals. 10,000 (gal) x 8.3 (lbs per gal) = 83,000 lbs. So it takes 83,000 BTUs to heat 10,000 gallons by 1 degree.
If you need to increase 20,000 gallons by 15 degrees, then 20,000 x 8.3 = 166,000 lbs of water x 15 degrees = 2,490,000 BTUs are required.
Now, how much heat can your heater produce? You have to consider the efficiency of the heater. Every pool heater will have a rating. Most common residential heaters will be listed at around 80 to 84% efficiency. That means if a 400,000 BTU heater is 80% efficient, then it will actually only result in 320,000 BTUs of energy being transferred into the water. Please note that the efficiency rating on a pool heater is determined under optimal operating conditions. The proper gas volume provided to the heater, proper water flow through the heater, and proper airflow through the heater is critical to achieving the maximum rated efficiency of that heater. A heater that is not operating with optimal conditions will be much less efficient than it is rated at. So make sure your heater is properly installed and maintained.
If you have a 10,000-gallon pool and need to increase the water temperature by 15 degrees with a 400,000 BTU 80% efficient heater, and you are paying $2.00 per therm for natural gas, then this is what you can expect. 10,000 (gal) x 8.3 lbs = 83,000 total lbs. 83,000 lbs x 15 degrees = 1,245,000 btu required. 400,000 btu heater x .80 (efficiency = 320,000 actual btu per hour. 1,245,000 / 320,000 = 3.89 hours to achieve desired heat rise.
Now we have to consider that the water in the pool is losing heat to the atmosphere and surrounding surfaces. This can be very significant. While heating, you must be able to overcome this heat loss faster than it occurs. In a perfect scenario, we would say 3.89 hours in our example. But if the air is cold, and there is wind whipping across the surface of the water, it can take much longer to heat the pool, and in some scenarios, the heat you are putting into the water may not be able to keep up with the heat being lost. This and other factors are why many believe bigger is always better for heaters. Even if you have properly calculated the minimum size heater required, circumstances can make it insufficient and incapable of achieving your needs.
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Do you have a pool question for Frank? You can email him at [email protected]. He just might read and answer your question on air!