Modern wood burning

Wood burning has always played a central role in our lives and even more so now when the energy prices are increasingly rising. Step by step the wood burning technology has improved and more and more of the heat value are being used within the fuel. Today, the energy content of the wood has been able to be utilized even more with today's modern combustion technology, combined with modern accumulator technology. Today, we even see environmentally certified wood boilers on the market which typically now provide added oxygen with the help of a fan. This then makes the combustion even more stable compared to the old natural draught principle. With todays’ technology wood burning is a very comfortable and profitable heating form for our homes. Many people think that modern wood boilers with a fan require electricity to work. However, if there is a power cut you can continue to fire these boilers anyway. You can simply then open the door to provide the boiler with combustion air and fire these boilers exactly like you would with natural draught boilers. This is worth thinking about.
A good wood burning installation should include a modern boiler that has a small water volume and is provided with an integrated fan. It should also have an after combustion chamber of ceramics or similar and be installed by a professional certified installer.
Finally the wood boiler should be connected to an accumulator tank that contains a large enough volume of water to meet the function requirements to keep the heat for 24h at the coldest temperature.
The fact that the water volume in the boiler should be as little as possible makes the boiler increase to the right temperature faster than in old boilers with larger water storage. As a result, the efficiency then becomes optimal and even radiation and standstill losses are eliminated. Calculate so that the accumulator tank is at least 15 times bigger than the volume of the hearth of the boiler and professionally installed.
The accumulator tank has not only one function for the wood burning, but several.
• First of all the water volume of the accumulator tank should be calculated so that it is able to store the required heat energy the house uses during 24h and at the coldest outside temperature.
• The accumulator tank should also be able to receive the energy from a full load of wood inserted into the boiler without actually boiling. You might want to be able to fill the boiler with wood two times a day, maybe even three while it is already burning and heating up the volume of the correctly sized and installed accumulator tank. If you need to ignite again then it is very likely that you have too small or an incorrectly sized accumulator tank.
• The accumulator tank also makes a good impact on the environment, whereby you might just need to fire only once a week during the summer and once a day in the winter. The amount of ignitions of the wood log boiler decreases radically if you fire wood against an accumulator tank.
In a normal sized house you should need approximately a 1500 to 2000 litre accumulator volume to handle the houses demand of hot water. With the right dimensions on the water volume of the accumulator tank it is then enough to fire once per day during the winter, maybe with the exception during the coldest days during the winter period, when you might have to fire two times, although with the correctly sized accumulator tank this should be very rare.
If you have your own wood you will always then produce the cheapest heat. However, even if you buy e.g. pulpwood to fire, in Sweden for example you would arrive at an energy price of 15- 25 öre per kWh, which corresponds to approximately half the pellet price, or a quarter of the electricity price.
Charging equipment:
To charge an accumulator tank with the "right choice" of wood boiler you will need to install some charging equipment. The charging equipment controls the adjustments needed between the boiler and the accumulator tank. What charging equipment you then choose depends on your chosen boiler and chosen connections. Note, different charging equipment have different functions. For example, the Laddomat 21 charging unit performs so that when you ignite the boiler, the water is just being pumped around internally inside the boiler and within the charging equipment until a temperature of approximately 80°C is reached. Then the thermostat opens and the equipment starts to charge the accumulator tank or tanks (if using more than one), with 80 degrees of charging temperature from the boiler. The return temperature from the bottom of the accumulator tank is then approximately 25 degrees if the tank is regularly discharged. The bottom temperature within the accumulator tank is then blended with the charging temperature from the boiler (approximately 80 degrees). Then the cooling, or we can say mixture temperature, from the charging equipment to the boiler is approximately 70 degrees. When the water temperature of the accumulator tank is close to 80 degrees on the bottom, then the charging of the tank is done. You should not aim to fire to more than 80 degrees. If for example the bottom temperature of the accumulator tank exceeds 90 degrees, then the charging equipment will be completely open until the temperature goes down to 80 degrees again before it closes the flow between the boiler and the accumulator tank. Then a lot of Kwh will be pumped around through the boiler and the charging equipment is of no use until it closes the internal flow. The boiler then operates like a reversed heat exchanger. With a flue-gas thermostat you can easily solve this circulation problem which if left unadjusted, only creates a lot of energy losses.
Flue-gas thermostat
A flue-gas thermostat can be briefly described as a device that controls the charging pump between the boiler and the accumulator tank by measuring the flue-gas temperature. This thermostat operates by measuring the temperature from the boiler itself instead of controlling the combustion from the water temperature of the boiler. The difference between these two is that on a flue-gas thermostat the heat sensor for the thermostat is placed inside the outlet pipe from the boiler. With a normal thermostat the heat sensor is placed inside the boiler with a so called diving pipe that is controlled by the water temperature of the boiler. The flue-gas thermostat is a better solution. It is much faster than a normal thermostat that is controlled by the water temperature of the boiler. When you ignite a wood boiler the flue-gas thermostat measures the flue-gas temperature almost immediately and starts the pump. This does not happen if it is placed in the water of the boiler, as it is a lot slower in feeling temperature changes.
It is the same when the boiler has finished burning, whereby the charging pump stops a lot faster when the flue-gas temperature is decreasing, compared to a thermostat that is placed in the water of the boiler. Even if you have a flue-gas thermostat it is important that you plan the wood loading so that there is glow in the boiler when the bottom temperature from the accumulator tank is 80-85 degrees. Then it is all about making sure that there is glow in the boiler when the charging equipment stands completely open, and set the flue-gas thermostat so that it turns off the charging pump at the right point after the end of firing.
It is very important that the flue-gas thermostat which starts and stops the charging pump is correctly adjusted. A suggestion is to provide the thermostat with an indicating light so that you can see when the pump is operating. This setting is different within different installations.
A short summary of wood burning is that if you choose the right products and right boiler, along with right installation method, it is probably the cheapest heating form when it comes to bio-energy today. If you then have access to your own wood it is even better. You can also buy cheap pulpwood today at very good prices.
Faster ignition of wood boilers with pressing fan.
Most wood boilers with pressing fans have some kind of bypass damper that is being used when igniting the boiler to quickly rise up the temperature in the chimney and by doing so create a good under pressure. However, there are also disadvantages with using these dampers. One example is that when you fire for too long with this damper open a very high flue-gas temperature will then be created. Most modern wood boilers on the market today have hearth volumes that hold between 100 to 150 litres of wood. If you then fire too long with the bypass damper open, there will be very high flue-gas temperatures that can damage brick chimneys and even the insulation within steel pipe chimneys can get damaged. It can also create overheating within the joist floors with resulting fire incidents as a consequence.
Another thing that happens if you fire with the bypass damper open for too long and create over combustion, is that it creates a heavy smoke emission within the chimney. This should not happen when using the fast ignition system typical now with a modern wood boiler installation. This goes the same when starting up wood boiler. It is often here that there becomes trouble with neighbours and people living nearby when the user does not have the correct knowledge on how to light the boiler correctly, despite the fact that the boiler is certified and environmentally approved.

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