Wood burning water heaters and wood-fired furnaces are heating appliances that use wood as the energy source for home heating and domestic hot water (DHW) preparation. They are not as popular as they used to be, mainly because of today's modern life, urban living, and the availability of gas and electricity in almost any home.
The wood-fired appliances for DHW preparation will be reviewed here because they are pioneers in the water heating industry. We will also provide some useful tips if you are interested in buying or making one.
Also, due to the rising cost of other energy sources, including gas and electricity, you might decide that this is the most affordable if not the only way for your home or cottage. They are ideal for heating hot tubs, above-ground pools, and storage tanks.
We will cover different types, from the simple wood stove hot water preparation in the large pot to the outdoor wood-fired furnaces.
Owning a wood stove and heating DHW with wood is rare today. The wood stove is usually used for cooking and home space heating but rarely for hot water preparation.
Note: Wood burning efficiency for the average quality wood is around 70%.
The main problems with wood-fired hot water heaters or boilers are the inconvenience when using, the time it takes to get the fire started, maintain the unit operational, difficulty controlling the temperature, accidental burning, pollution...
If looking for a wood stove, buy one equipped with a water storage tank that can hold water so that you can use it for water and space heating and cooking, such as the type from the picture. Modern stoves tend to be decorative and can use logs or pellets.
You can also rebuild the old stove. The most efficient layout is to install the storage tank as close as possible to the stove, in the vertical position. Old wood stoves use the draft and wood quantity to control the temperature, while new units come with the controls.
The simplest way to get hot water is to put a large pot full of water on top of the wood stove so you can use it for dishes or even fill up the bath or a hot tub.
However, it is hard to heat a large amount of water at once, plus the process is labor-intensive.
The jacketed wood stove is equipped with coils of metal pipes running through or around the firebox. The tank is usually located above the wood stove, so the heating system or a loop can function on the thermosyphon principle. The hot water is moving up while the cold using gravity is going out to be heated.
Keep in mind that the collector installed in the firebox affects combustion by cooling the fire. Taking a lot of heat out of the firebox can lead to smoky fires and too much of creosote.
The thermosyphon effect is not that strong, but it still provides a pumping action due to the rising hot water, and that is why the tank or collector has to be installed above the stove.
When the hot water collector is installed above the stove, one of the main problems is that it takes the heat from the combustion; burning will become dirty, and it will cause the stove to fail. EPA emission certification almost prevents this type of heating.
Also, if the plumbing system is open, the temperature is limited by its boiling point, while in closed systems, it is dangerous if the temperature and pressure rise above the boiling point.
If hot water has to be controlled, external controllers and pumps can be used. Special attention should be taken as this wood-burning water heater works in a higher temperature range, and if the system is closed, a pressure and temperature relief valve must be installed, also the expansion tank.
The hot water temperature is high, so the recommendation is to use white CPVC pipes with a special glue that won't dissolve in hot water, plain iron pipes if exposed to the flame, and lead-free soldered copper pipes. Try to avoid cheap PVC pipes and galvanized pipes in the area of the fire.
The outdoor wood-burning water heater is actually a wood-burning furnace situated outside, at a safe distance, and it provides hot water through the pipes buried in the ground.
If looking for a wood-fired hot tub water heater, check out Chofu, a Japanese hot bath equipment company. The Chofu heater is made of high-quality stainless steel, and it can rise 247 gallons of 47 F hot water in about 3 hr and run it without using the electricity, but thermo-siphon principle. You can even retrofit an existing hot tub.
It is inexpensive to operate this type of a wood-burning water heater, as the cost is only wood. It is ideal for use in the cottage, even homes for heating hot tubs and pools.
Wood burning is not a practical way to heat the water, especially during the summer months. You just don't want to fire the unit up every time you want some hot water. But if you cannot avoid it, use it in combination with solar heating. Be sure that the tank size is large enough to absorb the heat between the usage cycles and prevent overheating. Also, this type of water heating is not for the older and disabled as it requires much physical input. There is also a lot of planning involved, and it requires ordering and sawing logs and storing them.