The are four basic styles of sauna heater: gas, electric, infrared, and wood fired. Each has advantages and disadvantages that should be considered when installing a sauna heater. In all cases, you should have a sauna heater professionally installed if you are uncertain about any part of the process, and you should make sure that the sauna is adequately ventilated. Once you have narrowed your choice down to a type of sauna heater, check to see how much clearance it requires, as this may have an impact on whether or not it will work in your sauna, and also make sure that the heater is large enough to heat the entire sauna. In most cases, the packaging for a sauna heater will include information about the square feet of coverage that it provides.
A woodstove is the traditional sauna heater. A fire can be laid with aromatic wood, and quickly brought up to a high temperature that will heat the sauna and sauna rocks, which will help distribute the heat. Woodstoves can typically be used wet, meaning that they can be sprinkled with later to create löyly, sauna steam. However, they do have disadvantages, including the large clearance needed to reduce the risk of fire. It is much more difficult to control the temperature, and the woodstove will need to be fed by the sauna bathers with additional fuel so that it will continue to heat the sauna. There are also issues associated with fire risks, pollution caused by burning unclean fuel, and acquiring fuel for the woodstove.
Many saunas come with electric sauna heaters, which are turned on using a timer. The heat level can be more precisely controlled, and the sauna can get formidably hot if a sauna heater of the right size is installed. Not all electric heaters can be used wet, so if you want löyly, check with the manufacturer about whether or not the sauna heater can be wet. Electric heaters carry a low fire risk when installed properly, and do not require ventilation, as they should not release gas. They can also be used in small saunas, as they have a minimal clearance. However, they are not terribly attractive, although the top can be covered with sauna rocks to make their appearance more pleasant.
A gas sauna heater can be fired with natural gas or propane, and is a good choice for a large, frequently used sauna. Gas can be less costly than electricity, and the sauna heater's temperature can be controlled, just like with an electric sauna heater. Once again, not all gas heaters are safe to use wet, and they also require professional installation to reduce the risk of fire. The sauna will need to be ventilated, and a gas heater requires more clearance than an electrical heater.
Far Infrared (FIR) sauna heaters, also known as infrared heaters, are another option. These heaters heat the body, rather than the sauna, so they take less time to warm up. However, a FIR heater cannot be used to create löyly, and some sauna bathing enthusiasts believe that the effects of a sauna are not as beneficial with an infrared sauna heater. The overall temperature will also be lower, allowing sauna bathers to stay in longer, but the experience will not be the same as a traditional sauna.