This is a question that is often asked when looking at a wood fired pool heater or hot tub heater. The answer to this is based on the size of the pool and the temperature of the water. The type of wood used will also affect this as a hotter fire will produce more heat energy than a weak fire. The universal measurement of heat energy is defined as a BTU or British Thermal Unit. This definition of a BTU is the amount of heat energy required to raise 1 lb. of water 1 degree Fahrenheit. It is not a measure of time rather a measure of energy input.
In a perfect world all the BTU energy going into the pool is used to raise the pool temperature, however in reality while energy is going in, energy is also being lost through the walls of the pool and the top of the pool. The amount of heat loss is a variable that cannot easily be calculated as it is affected by temperature differences, so the colder it is outside the greater the amount of heat loss that will occur. When dealing with heating times of a pool we normally deal with the theoretical values and disregard heat loss. Using a blanket or good insulated cover will greatly increase the heating time for a pool or a hot tub and should be an important investment along with a pool heater. Wind also plays a very big role in heat loss from a pool. Wind cause evaporation. Evaporation is actually a physical reaction whereby water turns to gas in this process heat energy in the water is removed. This is the same concept used in evaporative air conditioners to cool a house. However using a cover will also minimize heat loss caused by wind.
Weight of water – 1 US gallon of water weighs 8.34 lbs. So, the first step to calculating the rise in water temperature is to calculate the volume of your pool or hot tub. This can be done by contacting the manufacture or using cubic volume calculators found on the website. Once you know how much water is in your hot tub you can then convert this to the weight. So, a 15,000 gallon pool has 125,100 lbs. With this number you can now determine how much energy is needed to raise the water temperature. For each 1 degree Fahrenheit rise in temperature you will need 125,100 BTU. So to raise the pool 5 degrees you will need 625,500 BTU of heat energy.
Now each wood burning pool heater puts out a rated amount of energy based on the size of the unit. This rating assumes maximum burning temperature. So our Inferno pool heater is rated at 105,000 BTU per hour. Based on this rating you would need to operate the heater for 5.9 hours to raise a 15,000 gallon pool 5 degrees (625,000/105,000). Remember this does not take into consideration heat loss that is occurring while you are heating up your pool. In some case such as the fall or early spring, the pool many be more heat than a heater can produce. Always use a cover to minimize the heat loss when heating a pool!
When sizing a heater is better to go with a larger size to minimize the heat up time and the amount of work required tending to the fire. If you are planning a pool party in the summer you can now calculate the approx. hours you will need to heat the pool so your guests can enjoy a warm pool during their visit.