Wood burning water heater and wood furnaces are the heating appliance that are using wood as the energy source, for home heating and DHW (domestic hot water) preparation. Due to today's modern life, urban areas and the availability of the gas and electricity in almost any home, wood fired water heaters are less and less used.
Even being less popular, wood fired appliances for DHW preparation will be described here as they are pioneers in this industry. On the other side, due to the rising cost of other sources of energy, including gas and electricity, you might decide that this is the most affordable if not the only way for you.
We will cover different types; from simple woodstove hot water
preparation in the large pot, to the outdoor wood fired furnaces.
Owning a woodstove and heating DHW with wood is rare today. It is usually used for cooking and home space heating but rare for hot water preparation. If used for cooking there will be so much energy waste, but the energy saving increases if using it for heating.
Note: Wood burning efficiency for the average quality wood is around 70%.
The main problem with the wood fired hot water heater or boiler is the inconvenience of using it, the time it takes to get fire started, to maintain the unit operational, difficult to control the temperature, accidental burning, pollution...
If looking for a woodstove, buy one equipped with a chamber that can hold water, so you can use it for heating. You can also rebuild the old stove. Old wood stoves are using 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 the top of the woodstove so you can use it to do dishes or even for a bath using a bucket. However, it is hard to heat up a lot of water at once and the process is labor intensive.
The jacketed woodstove is equipped with coils of metal pipes run through or around the firebox. The tank is usually located above the woodstove so the heating system or loop can function on the thermosyphon principle. The hot water is moving up while the cold using the 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 creosote.
The thermosyphon effect is not that strong, but it still provides a pumping action due to the rising hot water, and that is the reason why the tank or collector has to be installed above the stove.
One of the main problems when the hot water collector is installed above the stove is that it takes the heat from the combustion; the burn will become dirty which 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 higher temperature range, and if the system is closed, pressure and temperature relief valve must be installed.
Temperature of the hot water is high, so recommendation is to use white CPVC pipes with 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 fire.
Outside wood burning water heater is actually a wood burning furnace situated outside, on a safe distance and provides hot water through the pipes buried in the ground. Follow the link to explore one of the wood fired heater manufacturers or to get an idea how to DIY.
If looking for a wood fired hot tub water heater, check out Chofu, a Japanese hot bath equipment company. Chofu heater is made of a high quality stainless steel and it can rise 247 gallon 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.
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 the solar heating. Be sure that the tank size is large enough to absorb the heat between to usage cycles and avoid overheating.