Back in 2014 HETAS ran an article on slumbering in Technical Bulletin #4. Slumbering can cause issues if an appliance isn’t designed to do so. With air quality a headline issue, HETAS is issuing a reminder to registrants.
Slumber mode is when an appliance is purposely set at a low or minimum output normally for overnight burning to be revived in the morning without the need for relighting. Some mineral fuel burning appliances are designed to operate in a slumbering mode and would be CE type tested to show that they are safe to operate in this manner. A small number of wood burners have been CE type tested for continuous operation and have been shown to be safe and able to slumber for the required 10 hours minimum and be revived again at the end.
Users should however be aware that there are particular problems associated with using an appliance for extended periods at low output especially if the appliance is not designed to operate in this way. Similarly, these problems occur if the appliance has not been sized according to the heating load applied to it so that it must be turned down to low output in order for the living space to be comfortable.
Problems with slumbering
Running appliances at low output can lead to incomplete combustion, especially when burning a high volatile content fuel such as wood logs. This will lead to the formation of sooty and/or try deposits on the flue lining as well as increased levels of carbon monoxide and particulates in the products of combustion. In addition, when appliances are used at low output for extended periods of time the temperature of the chimney reduces and there is a tendency for the products of combustion to also be lower in temperature – which will lead to an increased likelihood of condensation forming in the chimney.
The presence of condensation together with flue deposits that may contain acidic compounds will present a risk of corrosion where metallic components are used for the flue lining. If the intended use of the appliance is for slumbering on a regular basis then it is wise, whenever it is necessary to reline the chimney, to choose a lining system that does not have metallic components. See ‘flexible liners’ below.
Even when an appliance is designed for slumbering it is necessary to adopt certain practices in order to reduce the negative effects of operating at low outputs. Usually this involves running the appliance at high output following slumbering to hopefully burn away any flue deposits and to ensure the chimney warms up sufficiently to provide a good draught and reduce the risk of condensation. Never overload an appliance.
Flexible liners, whilst being easier to install and replace, are not designed to last the life of the building, but may (when using the appliance correctly) last in excess of 10 years. Long periods of slumbering and/or infrequent chimney sweeping can cause corrosion damage which has been known to reduce the expected life of a flexible metal flue liner to less than five years.
The efficiency and life expectancy of any chimney is dependent upon correct use and maintenance. Masonry and pre-cast chimney products – whilst usually offering long life and high resistance to risk of corrosion – tend to involve more installation work when compared with metallic chimney systems.
Metal liners and insulated metal chimneys offer fast and convenient installation. However, they can be less resistant to damage by corrosion, particularly if subjected to abuse or inadequate cleaning. Allowing soot or condensate deposits to accumulate in metal lined chimneys and also prolonged periods of burning in slumbering conditions, particularly on closed appliances, can cause high concentrations of corrosive condensates to build up and attack the metal liner. This situation can not only considerably reduce the life of the flue lining, but can also invalidate the manufacturer’s warranty.
Carbon Monoxide, particulates and slumbering
It is important not to allow the appliance to repeatedly remain in slumber mode for long periods; this practice ultimately starves the appliance of air giving rise to an increase of carbon monoxide and particulates. Leigh Greenham of CoGDEM advises that many of the calls he gets on the COGDEM helpline surround consumers slumbering an appliance and leaving the door open to get extra heat into the room, increasing further the risk of CO.
Users aren’t always aware of the dangers of doing this. If they don’t want to put another log on the stove at the end of the evening, they often open the doors in an effort to extract the last bit of heat from the stove. However, the heat-recovery design of the stove normally causes the products of combustion to take a tortuous route around the stove to the flue, but when the stove cools there is not enough energy to drive the products in this way.
So the embers, which are still producing quantities of CO, have insufficient heat energy to drive the products to the flue in the normal way, so the products take the easier route and spill into the room through the open door. Fortunately, the cases we get to hear about are those where a nearby CO alarm has reacted appropriately.
It is imperative that the output of the appliance is considered when planning any stove installation; should the appliance be oversized then this may give rise to the end user operating the appliance with the controls turned down, effectively slumbering the stove. In contrast an appliance with an output too small for the room it is being used in will lead to inadequate heat output. The HETAS Guide gives appropriate guidance on appliance sizing.