As the current energy crisis intensifies, more and more people are in search of alternative heating solution with wood fuel, since the gas price is too high and supply for the upcoming winter is not sure, and the same thing is valid for pellets also.
Wood is the natural fuel that nature provides us for thousands of years. It is a neutral fuel, with no emissions, causes no pollution and is environmentally friendly. The new technology can assure big efficiency in wood combustion, making it an economic form of heating with high autonomy and automatization.
So, if you are thinking of having a wood boiler as a heating solution, here are 10 things you need to know.
1. Types of wood boilers
There are three main types of wood boilers classified by function and regulation mode.
Chain regulator boiler: This type of boiler is the most simple solution and it is purely mechanical. There are no digital or electric components, and the combustion is regulated by an air door which is controlled through a chain according to the boiler temperature.
Advantages: low acquisition and maintenance cost, easy to install and use.
Disadvantages: poor control of the boiler and installation, high fuel consumption, low autonomy.
Fan assisted boiler: This type of boiler is equipped with a fan, and a controller which regulates the combustion air through the function of the fan. The controller has at least a boiler temperature sensor, and usually also has additional sensors, functions and controls over the boiler functionality and heating installation.
Advantages: precise combustion, higher autonomy, integrated control of the heating installation, lower fuel consumption.
Disadvantages: a little more complicated installation and maintenance, depends on electrical components for function.
Gasification boiler: This type of boiler burns wood in two stages: The wood is first burnt with little air and hot gases are produced, and then these gases are mixed with oxygen to have a full combustion. These boilers are always equipped with a fan for control of the combustion, which can be installed on the inlet (push fan) or the outlet (exhaust fan).
Advantages: high efficiency, high autonomy.
Disadvantages: high acquisition cost, can only work efficiently with dry wood.
2. Fuel
Wood is a renewable fuel, just like the sun. The most important factor of wood as a fuel is humidity. The less water it contains, the higher its calorific value. It is recommended to use wood with no more than 20% humidity content. This way the boiler lifespan is significantly longer, and almost 30-40% fuel savings can be achieved.
The best method is to store wood aſter it is cut in a well ventilated and sheltered place for a period of at least 18 to 24 months. Hardwood is better for longer lasting combustion, while soſtwood can be used better for creating a layer when starting a fire.
| Wood species | Calorific value kwh/kg |
| Oak | 4,2 |
| Beech | 4,2 |
| Maple | 4,2 |
| Birch | 4,3 |
| Willow | 4,1 |
| Fir, pine | 4,4 |
| Wood briquettes | 4,0-4,9 |
3. Dimensioning
The correct dimensioning of the boiler is extremely important to ensure comfort and fuel savings. There are many factors which influence the correct calculation, and in any case the best practice is to have a professional calculation by an engineer.
Here is some main advice what you should consider.
Surface – There is no single answer of boiler power according to the house surface. However, the bigger the house, the more it needs for heating. A practical rule is to calculate boiler power 150-180 W/m2, according to the insulation quality.
House insulation – The main factor influencing the heat losses of your house is the insulation. The same surface might need 20-30% less power if well insulated.
Installation type and automatization – Depending if you have a conventional radiators installation or floor heating, temperature control for each room, thermostatic control of the heating installation devices and overall automatization, you can achieve 10-15% less heat loss, meaning you need a smaller boiler.
Domestic hot water – A hot water boiler is usually used to produce domestic hot water, using energy from the boiler. If you have a big capacity boiler and use it often during the day, you might need to increase the necessary power by 20-25%.
4. Buffer tank
A buffer tank is a hot water storage which has the role of a “battery”. A buffer tank is not needed with gas or pellet boilers, where combustion is stable, and start/stop are controllable. But in a wood boiler, the combustion is not stable (it is more intense when it is filled with wood and while it burns it loses intensity), and the start/stop of the combustion cannot be controlled.
A traditional wood boiler has autonomy of 3-4 hours, but with a buffer tank it can reach 8-12 hours. Also, excess heat which is produced by the boiler but not needed in the house is stored in the buffer tank and used only when needed. This way, the buffer tank also helps save fuel.
A practical tip of how to calculate the correct buffer for your boiler is at least 50 lit/kW. So, if you have a 30kW boiler, you need 1.500lit buffer tank.
5. Bypass and return temperature protection
It is extremely important to install a bypass system to ensure return temperature protection for the wood boiler. It ensures good function, long life of the boiler, and avoids formation of condensate and corrosion. Most wood boilers producers mention that it is a necessary measure for validity of the warranty.
The bypass is made by making a loop between the boiler outlet and return, and a temperature mixing device to ensure steady temperature at the boiler return, as shown in the following picture.
The mixing device can have a pump integrated or can only be a thermostatic 3-way valve and a boiler pump needs to be installed. In this case, the boiler pump must always be installed before the 3-way valve!
6. Open or closed expansion vessel
The expansion vessel is a crucial safety element of any heating installation. It compensates the water expansion in the heating installation due to temperature increase. There are two type of expansion vessels:
Closed expansion vessel: a flexible membrane with air pressure is moving to compensate the water expansion.
Advantages: Easy to install, low cost, reliable, pressurized installation
Disadvantages: If not dimensioned correctly or has a defect it can lead to dangerous situation
Open expansion vessel: a simple steel water tank installed at the higher point of the heating installation.
Advantages: Works in all situations, protects in case of extreme overheating
Disadvantages: Complicated installation, can freeze during winter, system works with hydrostatic pressure
According to European Standards, only open expansion vessel is allowed with wood boiler installation, unless the boiler has an overheating protection heat exchanger.
7. Safety
Apart from the expansion vessel, there are many other safety elements which are necessary for the heating installation. The minimum required is an air-relief valve and a pressure safety valve. Usually with wood boiler, it is recommended to install two safety valves, in case of failure.
Another important safety device is a UPS, to ensure electrical supply (at least) to the boiler pump in case of power loss.
8. Chimney
All wood boilers, even if they have a fan, need a correct chimney installation, and rely on natural draft for safe and efficient function. Make sure that your chimney is correctly dimensioned, installed and connected to the boiler. Check that it is air-tight and avoid too many and direction changes that introduce pressure drops.
It is strictly forbidden to install the boiler inside rooms that are inhabited by people, as well as chimney elements to pass through these spaces.
Always keep the chimney clean and make an annual inspection and maintenance.
9. Cleaning
All wood combustion produces ash residues; therefore, the boiler needs to be regularly cleaned. Ash will block the circulation inside the boiler passes, will deposit dust on electrical components, and being very strong insulator, will reduce the heat transfer efficiency of the combustion.
This is why it is essential to keep you boiler well cleaned and as often as possible. At least once a week is recommended.
10. Maintenance
It is necessary to make all service and maintenance procedures according to manufacturer instructions and local regulations. Always call to official service technicians, approved by the manufacturer, and install only original spare parts.
A good practice is to have a general heating installation maintenance and revision before the start of the cold season, to make sure no defects will appear during its function.
