Understanding Wood Pellet Boilers – A simple guide

April 30, 2008

What are wood pellets?

Wood pellets are small pellets of compressed saw dust. Simply put saw dust is a by product of other more lucrative wood processing. This saw dust is dried to a specified moisture content, and compressed by a very high pressure machine. Generally no glue or binding agent is required other than the sap from the timber itself.

The benefits of wood pellets are

  • Its easy to transport
  • It is clean
  • It complies to standards. Things which have standard sizes, shaped and moisture content can easily be designed for

So these pellets, how do they get to the boiler?

No, a shovel is not required. The pellets are blown from a delivery truck up to 20m to a storage unit. In the past there has been a lot of hassle with dust, but this was largely down to the use of agricultural feed trucks being used to deliver the pellets. As dust is not really an issue for agriculture, they did not have dust minimisation technology, and it was a mess. If you use one of the large suppliers these days, there is little problem.

So the pellets are in the storage unit… what now? Well the next stage is getting it to the boiler. The simplest solution here is to use a screw system. These systems turn very slowly and draw the pellets from the storage unit to the boiler.

Understanding Wood Pellet Boilers

And.. Combustion

Combustion… Explain?

Wood pellet boilers are fed into the “burner” where they are burned. The burner in all boilers does much the same job. It’s a place where the fuel is mixed with air to provide the optimal conditions for combustion, and lit. Before you know it the combustion gases are travelling through the heat exchanger where it heats up the water.

Understanding Wood Pellet Boilers

Understanding Wood Pellet Boilers

Heat exchanger

The flame from the burner, and the resultant combustion gases escape into the boiler, transferring their heat to the water circulating through the heat exchanger within the boiler. The heat is then distributed the house using a wet system. In most cases water flows through the heat exchanger in the boiler, absorbing heat from the flam and the combustion gases. This hot water is then circulated around the house to radiators, underfloor heaters and hot water cylinders.

For more infor See  my boilers post

Thanks to





for the pix!

Understanding – Immersion heaters – hotwater cylinders

February 29, 2008

Immersion heaters, otherwise known as electric water heaters or hot water cylinders, double as water heaters and hot water storage systems. Most combine a number of heat sources to maximise the flexibility of the system.

Understanding - Immersion heaters - hotwater cylinders

The typical hot water storage cylinder has two electric elements and one or more hot water heat exchangers. These electric elements and heat exchangers heat water in the cylinder to be used for hot water in the house, operating in an order which best provides the hot water as efficiently as possible.

Firstly, most immersion heaters are heated by the main boiler system. A heat exchanger inside the cylinder is piped from the boiler, though is often on a different circuit than the heating system (so the heating can be switched off without switching off the hot water from the immersion). As electricity is more expensive than other fuels in most countries it is cheaper to run than the electric elements, and so is the default option.

Next, a small electrical element located towards the top of the cylinder is used to heat small quantities of hot water. In many switching arrangements this the control for this element is marked “sink”, though it is generally sufficient to provide water for a shower.

A higher rated electrical element is located lower down in the cylinder and can be used to heat the full cylinder (often marked “bath”). Electrical heating is generally quite easy to use and for this reason it is quite popular, but it is by far the most inefficient from an environmental point of view, and also the most expensive.

In many cases renewable heating systems such as solar panels or geothermal systems are also piped into the immersion heater. Heat exchangers reject heat from these systems into the immersion heater, heating in a similar fashion to the central heating system. In some cases these renewable systems can not heat the water to a sufficient level, and so require conventional methods to heat the water the final amount. While this is not an ideal situation, the energy required from the conventional methods to heat the water the last bit is significantly reduced.

Understanding – how condensing boilers works

February 27, 2008

Ok… So post assumes you’ve just checked out this post about how a boiler works… then what about these condensing boilers then…

Condensing boilers have all the same elements as standard boilers, but have increased efficiency. The efficiency of a boiler is a measure of its ability to convert energy from the fuel that goes in, to heat in the water that circulates around your house. Condensing boilers do this by having an extra heat exchanger in the flue. As the flue gases (fumes, smoke… and the like) heat up the water (or some cases the air) a bit as it is feeding into the boiler. This means that the boiler has less work to do to heat the water in the main body of the boiler. It also means the flue gases are cooled down a bit more than usual, and so some of the particles in the gases will “condense”. The condensed liquid can be a bit nasty, and so higher quality materials are necessary in the boiler. This causes the boilers to be more expensive but the reduction in fuel costs can be significant.

Understanding - how condensing boilers works

Another key point to note is that the flue gases are at a far lower temperate than the gas straight after the burner. This lower temperature gas is really only good for heating cooler water, and so works best on a lower temperature system. So with condensing boilers, the lower temperature you run the system, the more efficient they run.

Lower temperature systems means the water circulating around your house will be at cooler, meaning that you will either

need larger radiators (or have underfloor heating)

Accept that it will take longer for them to heat up the room (not a lot, just a bit)