The most common type of hot tub heating options is electric heat, in which a heater is connected to an electrical outlet to warm up the water. It is usually quick and consistent, though it can cost a significant amount of money to run such a heater. Wood-fired hot tub heating units heat the tub with no electricity, and using such a unit will allow a tub owner to place the tub far from electrical outlets. The heating may not be as consistent as other methods, however, and the water will go cold as soon as the fire is out.
Wood burning hot tub heating elements come in two general varieties: stoves that sit directly in the tub water, and stoves that sit outside the tub unit. The stoves that sit directly in the water will heat the water quickly, but the stove itself will take up valuable sitting room within the tub. Units that sit outside the tub will heat the water less quickly but also take up less room. The fires in either type of stove will need to be continuously stoked and fed with firewood to ensure the water stays warm during use. Once the tub is not in use, the fire can be put out, but the water must be covered or drained to avoid freezing in cold weather.
Propane hot tub heating units use tanks of propane gas to fire a heating element that in turn warms the water. This is a more consistent option than wood fired hot tub heating, but again, once the propane runs out or is turned off, the water will go cold. Propane can be a less expensive option than electric hot tub heating elements, and it is more environmentally friendly, but the consistency of the heating will not always be the most reliable.
Solar hot tub heating systems are less common than other methods. They are not the most consistent heating options and they rely on direct or nearly direct sunlight to heat the water, which can be problematic on cloudy days or at night. The solar cells will store energy, but it may take up to an entire day to heat the water. While not the most reliable or consistent heating method, solar heating systems are perhaps the most environmentally friendly option, as no fuels are burned or otherwise used during the heating process, aside from natural sunlight.