Pellet Stove Fires and What Can Cause Them
Pellet stove fires are rare occurrences, but they're a hazard that can occasionally happen and when it does, the consequences can be devastating.
The most common cause is improper disposal of ashes from the stove.
When a stove is in constant use, it's only natural to want to get rid of ash build up quickly and get the stove on again. That's fine, but the ash from the stove has to be kept safe until it's cool.
You may think that ash from a stove that has cooled down will be safe, but there are often embers in there that only need exposure to air to become active again.
Most pellet stove fires are started by ash which is not entirely cool being spread on or adjacent to a combustible surface or object.
Ash from the stove must be collected into an air-tight metal container which can be put somewhere safe, on a non-combustible surface, until the ash is absolutely cold. 24 hours is a reasonable time to leave it.
I read recently of a fire being caused by someone who cleaned the pellet stove out with a shop vac which had a plastic drum, and then left the vacuum on a wooden floor.
Recipe for disaster. Not only was the ash ignited by the flow of air from the vacuum, it then went on to destroy the vacuum and set the surface it had been left on alight.
Dedicated warm ash vacuums
with metal bodies, flame retardant filters and heat resistant hoses are available for cleaning out pellet stoves.
The reason that the metal container that the ash is collected in must be air-tight is that the ash must be starved of oxygen to stop it re-igniting. Special containers are available for exactly this purpose.
Clearance from Combustibles
Inadequate clearance from combustibles is the second most common cause of pellet stove fires.
The minimum clearance to combustibles will be set out in a pellet stove's installation manual. Clearances from the flue vent also include internal wood framing within the walls of the house which can on occasion ignite if the stove is poorly installed.
These are minimum
clearances, which are adequate if the stove is functioning normally. Common sense would dictate that these clearances should be exceeded if possible. More on stoves functioning abnormally below.
One particular aspect of clearance from combustibles that may be fine when the stove is first installed but gets overlooked later is clearance from the exhaust vent outlet.
Venting into a crawl space is definitely not to be recommended because it is not easy to check if combustible debris has built up near the vent.
Piles of leaves have been known to catch fire near an exhaust vent, as has a pile of sawdust.
Another fire was caused by wooden roof shingles igniting. It's always a good idea to fit a spark guard/catcher at the end of venting, particularly important on short horizontal runs. The spark guard mustn't interfere with the airflow through the venting.
Pellet Stove Fires Inside the Stove
If a pellet stove malfunctions, an abnormal fire can happen inside the body of the stove. At its most serious, this can spread to the hopper that holds the pellets.
Lack of maintenance
is usually the root cause of these types of pellet stove fires. A pellet stove is designed for a small, fierce fire in a particular area of the stove (the burn pot).
Other areas of the stove are designed to be kept relatively cool by the flow of convection air over the heat exchanger. Obviously the hopper and the electronics will not tolerate high temperatures.
A burn pot can become overloaded, which results in a larger than normal pile of pellets accumulating. If this happens, a pellet stove can try to turn itself into a wood stove.
Two things can cause this situation. Firstly, blocked air flow to the burn pot can be the result of a build-up of ash underneath, so remove the burn pot regularly and check the air flow is clear. Sometimes, the combustion air holes in the burn pot can become blocked - they need to be checked and cleared out periodically.
The second situation is when poor quality or damp pellets are slow to ignite, causing a similar build up.
If this happens, the pile of slowly burning pellets can suddenly ignite causing a small explosion within the stove.
Although pellet stoves burn much more cleanly than other solid fuel stoves, and most of the creosote that causes chimney fires gets burnt up, after a while creosote will build up in the exhaust venting.
An exhaust vent fire is very dangerous as pellet stove venting is not designed to handle high temperatures. The joints between the vent pipe sections will be destroyed and the fire can easily spread into the home.
In the case of a chimney fire, normal venting clearances don't apply because the temperatures are so much higher.
Regular inspection and cleaning is the only way to guard against chimney fires. A Brushtech
cleaning kit is extremely useful if you're going to do the cleaning yourself.
Pellet stoves have at least two types of safety sensor that guard against pellet stove fires. The high temperature switch will shut the stove down if this danger is detected. The other safety sensor is the vacuum switch which shuts the stove down if abnormal airflow is found.
All certified pellet stoves are tested to UL 1482 for the USA, ULC S627 for Canada and CEN EN 14785 for Europe. A more stringent test specifically designed for pellet stoves is ASTM E1509.
A safety label is permanently fixed either inside or outside all current pellet stoves giving information about the standard the stove is tested to, appropriate types of fuel, clearance to combustibles, electrical ratings and the type of floor protection required.
Pellet Stove Problems
Pellet Stove Cleaning