Catalytic Converter for Wood Stove Safety Considerations

Catalytic converter for wood stoves and boilers is incorporated to reduce harmful pollutants such as particulates and hydrocarbons, but some people have concerns over its safety. Are these justified?

The main danger when burning wood in a confined space is carbon monoxide, and this is always present in the flue gases. If the chimney is drawing properly then all the flue gases will be extracted from the fire and none will leak into the house, but if the chimney is not drawing properly then some of this poisonous gas make leak out from the stove leading to a dangerous concentration indoors.

What are the conditions which may prevent a chimney from drawing properly? The main three are as follows:

1. A cold plug. Hot air is less dense than cold air, which is why it rises up the chimney. However, on a very cold day, especially if the chimney is poorly insulated, the temperature of the air in the chimney can be too low to rise, leading to what is known as a ‘cold plug’.
   
2. Restricted air supply to the stove. Flue gas can only move up the chimney if there is fresh air entering the stove to replace it. If the air valve on the stove is completely closed, or the room in which the stove is installed does not have an air brick, or there is an extractor fan which is creating a partial vacuum inside the room, then the air supply may be inadequate to allow fumes to flow up the chimney.
   
3. A blockage in the chimney or stove. Chimneys and stoves can get blocked up. This can happen in one of two ways – from the top or from the bottom. Blockages from the top are caused by soot, masonry or birds’ nests falling down the chimney. Blockages from the bottom are due to a build-up of ash, soot or creosote.

This last category is where concerns about catalytic converters have arisen, because the presence of a catalytic converter for wood stove makes it easier for the flue gas passages to get blocked. It is undeniable that it is possible for a catalytic converter for wood stove or boiler to get blocked, leading to carbon monoxide escaping from the stove into the owner’s home.

However, stoves without catalytic converters can also get blocked up, and as we have seen, there are other reasons why chimneys might not draw properly apart from blockages. Therefore, no woodstove is completely safe unless the room in which it is installed has a working carbon monoxide monitor. These are available at very low cost and are actually compulsory for any new woodstove installation in the UK.

Although a carbon monoxide monitor is the only way to make a wood stove (whether incorporating a catalytic converter or not) completely safe, nevertheless there are design factors which can reduce the risk of blockage. The most important in this regard is to make the channels through the catalytic converter (known in the industry as ‘cells’) big enough so that ash can pass straight through them. They should also be straight – if the flue gas has to change direction, as in the case with a foam substrate, the chance of blockage is much greater.

A manually controlled bypass is also a good idea. The main benefit of this is that it allows the catalytic converter to be bypassed when lighting the stove, which is the time when the draft in the chimney is at its lowest. A secondary benefit is that, if the catalytic converter for wood stove does become blocked, it can be bypassed until it is a convenient time to remove it for cleaning.

Regulations in Europe require a permanent bypass around catalytic converters as a back up plan, however, as with any part of the flue ways and chimney, a bypass can get blocked up too, and also, as discussed above, there are other reasons why a chimney might not be drawing apart from a physical blockage. Therefore, for complete safety with any woodstove it is still essential to have a carbon monoxide alarm.