Reasonable expectations

From this type of heater you can expect the following results.The water going in to the heater will come out of the heater 3-5 degrees warmer on the medium unit. The water will come out 4-7 degrees warmer on the large stainless unit.The variance will depend on the temperature of the fire you build.It takes practice to learn how to keep a fire between 450 and 500 degrees. If the fire drops to 200 your heating will obviously slow to the lower end of the range.

The air temperature has an effect on your heating also.If the air is colder than the water, the air will be cooling the water as you heat it, thus minimizing your gains.An insulated pool cover is quite helpful in keeping the heat in. Realistically speaking the same applies for the gas or electric or solar heater. The best use of the heater is when the air temperature is warm enough to swim but the water is too cold. The heater will warm your water and the air will not be fighting you to cool it. As your water temperature rises it will keep the heat since the air temperature isn't trying to cool it.

Upon installation of a medium unit on a 14000 gallon inground pool, I decided to compare with the neighbors non heated pool. Both pools were at 60 degrees at the start of the test. The temperature at start was at 65. Fire was started at 3PM and the temperature started to rise. The air temperature began to fall as the sun set. A cold front came in and the temperature dropped to 35 degrees. The fire was continued the next day and the weather stayed cold reaching a high in the low 50's. The pool temperature got as high as 69 degrees and fell back to 66 degrees that evening when the temperature dropped to the 30's again. No insulated pool cover was used in this test. The temperatures of the pools were once again compared. Wood heated pool was at 66 degrees and the non heated pool had fallen to 52 degrees. The heater made a considerable difference and when the cold spell was over it continued to heat the pool up to swimmable temperatures.As the unheated pool slowly worked it's way back up to 60 degrees the heated pool was heated to 80 degrees with another two days of burning.