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Wood Pellet Boilers

The wood pellet burner

A pellet burner
A pellet burner

Wood pellet boilers work by burning pellets using a burner attached to the front of the boiler. This works in a similar method to an oil burner, in that the flames burn into the combustion chamber of the boiler. Central Heating and Domestic water is then circulated through the boiler and heated.

The Viking Bio burner is used on all Varmbaronen pellet boilers. It is a 20kW fully automatic wood pellet burner. This means that the pellets are fed automatically in to it using an auger. It is designed to feed small amounts of pellets often rather than large amounts less frequently. This gives an even flame and high efficiency.

Other important considerations for a pellet burner include:

  1. The burner should have automatic electric ignition of the pellets.
  2. The burner should have a soft start with a gradual flame build up.
  3. Before beginning to burn pellets, the burner should turn on the fan to disperse any remaining fumes inside the boiler.
  4. If the flue is blocked, the burner should not start and give a visible error.

The Wood Pellet Boiler

A wood pellet boiler can be used to heat the radiators and underfloor heating circuits of a house. Some boilers also have the ability to heat the domestic water without using a separate tank (e.g. work as a combi boiler). A sample schematic is shown below.

This schematic is of a sealed system with the boiler heating the Domestic water on-demand. For other schematics (such as open systems or using a water heating tank), please see the Wood Pellet Schematics page or speak with your plumber.

  1. The boiler burns pellets and heats up.
  2. A shunt valve automatically controls the flow of the heating water, which passes through the boiler and out to the radiators etc.
  3. A circulation pump must be used on the heating circuit
  4. The radiators or underfloor distribute the heat out into the house
  5. The boiler in the above schematic is heating the domestic water on demand. When someone opens a tap, cold water is brought in to the boiler, heated and sent out to the tap.
  6. The boiler comes with a safety valve
  7. The boiler also needs an expansion vessel to allow normal operation.

The wood pellet boiler will typically be kept operating at a minimum temperature all year round. This is beacuse they have a very long start warm up time of 20-30 minutes. However, they are very well insulated so keeping them warm requires very little energy.

Flueing

Stainless Steel Flue
Stainless Steel Flue

Above: Acceptable methods of fluing a pellet boiler

To provide proper operation, pellet boilers typically require a draught from the flue of -0.05 mbar. This generally means that the require a fairly high flue or chimney. The pellet boilers supplied by RVR have a built in fan which means the minimum flue height can be much lower. The minimum height 2 metres from the bottom of the boiler. The cross sectional area of the flue should be Ø100- 160 mm. 150mm is recommended. It is important to use an insulated flue so that the flue gases do not drop too low while rising up the flue. If this happens condensation can occur and the corrosive water released can drop back down into the boiler and cause damage.

RVR supply a range of insulated stainless steel flue.

Installing a pellet boiler

When a plumber installs a wood pellet boiler, it is important that the following items are given special attention:

  1. All requirements outlined in the boiler instructions must be fulfilled
  2. All settings and checks outlined in the burner instructions must be fulfilled
  3. All connections are watertight.
  4. Connections to the boiler and to the flue are sound
  5. The filling/drain valve has been closed properly
  6. The safety valves are working properly
  7. Burner should be adjusted for correct combusion
  8. The installer should fire the system and let it come up to temperature
  9. The installer must adjust the shunt valve to ensure the boiler temperature is greater that 70°C
  10. The installer will carry out a combustion test and check CO and CO2 levels are correct.

Wood Pellet Boilers from Varmebaronen

Sweden is the world's largest market for wood pellet and wood gasification boilers. In fact, in 2006 Sweden used as many wood pellets as Germany, Austria and Italy combined.

We have chosen Varmebaronen, one of Sweden's largest wood pellet and wood gasification boiler manufacturers as our partner for biomass boilers.

Star Pellet Boiler
Star Pellet Boiler

There are two models of wood pellet boiler available. The Star Pellet Boiler has a capacity of 20kW and contains an internal buffer volume of 175 litres of water as well as a high capacity plate heat exchanger. This allows the boiler to provide a high level of Domestic hot water instantly, on demand.

The Apollo Pellet Boiler also has a capacity of 20kW and contains a buffer volume of 90 litres of water. It features an in-built 100L water heating cylinder which offers superior performance for larger homes or when high flow rates of domestic hot water are required.

Benefits and Features:

  • Low cost
  • Simple, reliable products
  • Fan assisted boilers keeps flue height to a minimum
  • Highly efficient burner with electronic ignition and simple controls
  • Easy to clean, everything accessed from the front
  • Low maintenance

More information on these two boilers and on the flue available can be found by visiting the Wood Pellet Boilers product group.

Cleaning the boiler

Wood pellet boilers are designed to operate with minimal maintenance other than periodic cleaning of the boiler. The cleaning schedule will depend on the quality of the pellets but generally speaking the boiler will need to be cleaned after every 100 hours of operation. This will be about every two to three weeks in winter. The user can tell when cleaning is needed by observing the flue temperature. A rise of 50°C above the temperature of a clean boiler indicates that cleaning is required.

Cleaning involves opening the burner compartment and removing the ash which has accumulated in the compartment and the three boiler tubes. This is a very simple process which takes about 10 minutes.

Further Reading

Read on about bags vs bulk storage here: Homeowners:Wood_Pellet_Bags_vs_Bulk