There are many brands of external heaters available in the marketplace today. Among these, the "Chofu" heater is particularly popular. Chofu and other brands of external heaters most often utilize a process known as "thermal siphoning." Heat transfer in thermal siphoning is relatively slow but can be increased by adding an electric pump. On its own, an external heater will take upwards of two-thirds of the time to heat a similar amount of water compared to a submersible stove. External heaters must also be completely drained in the winter (for those who live in cold-winter climates). The significant advantage of external heaters is that they do not use up your valuable hot tub space!
The biggest advantage of a submersible heater over an external version is the speed at which water is heated. Simply put, submersible heaters heat water far more effectively. Whereas an external heater loses some of its output into the air around it, the submersible heater's output goes directly into the water since it is submerged. Two heat exchanger pipes trap the heat usually lost through the chimney and return it to the water.
Submersible heaters are made of the highest-grade marine aluminum. This material is highly resistant to corrosion and contributes to the stove's high-efficiency output. There are two sections for each submersible stove. The bottom section is kept cool because the heat is continuously and quickly transferred away into the tub water. The upper section is very hot but protected by a cedar barrier, ensuring hot tubs are kept safe.
Our submersible stove will heat an 85 cubic foot hot tub (six feet diameter and three feet high) in approximately 2.5 hours. This heater takes up approximately one-quarter of the space in the hot tub, but there is still ample room for four adults.