What are Burn Times?
A burn time is the total amount of time a single load of wood will burn from ignition through to smoldering. This includes from the time you light a stove, to when the coals are hot enough that you can add another log and the fire ignites again (without having to use another match). Burn time does not refer to the amount of time a piece of wood will display visible flames and be producing large amounts of heat (BTUs).
There is a lot of confusion around the concept of burn times for wood
burning fireplaces, stoves, and inserts. While the idea seems
straightforward, in reality, it is anything but. A burn time refers to
the maximum amount of time a wood unit will burn from a single load of
wood. This seems simple right? Put a log in and measure how long it
burns, therefore that is the burn time. Unfortunately, it is not that
straightforward and there are a number of factors that affect burn
times, the most influential being the fireplace settings – burning on
high vs burning on low.
When looking at a fireplace you will likely see two key elements – BTUs (heat output) and burn time (total duration). Typically, consumers think the stated BTUs of a unit are the number of BTUs that the unit will always produce, when in fact it is the maximum output. The same goes for burn times, it is the maximum a fireplace can burn for under ideal conditions, not the average duration.
These numbers are highly related and depending on how a unit is burned a consumer can maximize the BTUs, maximize burn time, or end up somewhere in the middle, but they can never have both. Typically, BTUs are listed at their maximum or High Heat Value (HHV) and burn times are listed at their maximum or Low Heating Value (LHV).
- A fireplace burning at on high (HHV) will produce 55,000 BTUs and burn for a duration of 2.5 hours.
- The same fireplace burning on low (LHV) will produce 18,000 BTUs and burn for a duration of 8 hours.
Therefore, the units listed max BTU will be 55,000 and the listed burn time will be 8 hours. The fireplace is capable of doing one or the other, however will not be able to do them at same time. HHV fires result in quick and hot burns that rapidly use fuel and LHV fires result in slow, stable, less intense burns that lengthen the fuel consumption.
Factors Affecting Burn Times
Many different factors affect the burn time of a wood burning stove, fireplace, or insert. Aside from the design and model of the fireplace, the following are the most influential factors for burn times:
Type of Wood
Different wood types will yield longer, hotter burns, while some will yield cooler and shorter burns. Typically, hardwoods like oak, walnut, and cherry burn longer than softwoods such as pine and fir.
An air damper setting that is wide open (no restriction) will cause the fireplace, stove, or insert to burn very hot and short as the fire is able to consume as much oxygen as it wants, therefore using all the available fuel quicker. For longer burns, restrict the airflow to the fire to draw out the length of the fuel burning. Low airflow fires will burn at a lower BTU output for long periods whereas high airflow fires will spike with a quick rush of BTUs and quickly burn out, requiring additional wood.
The altitude and humidity also affect the burn times of fireplaces. Higher altitudes decrease the burn times of most fireplaces as there is less available oxygen and the efficiency is decreased. High humidity also decreases burn times. The moisture in the air decreases the fire’s efficiency and lowers the expected burn times.
How to get your fireplace to burn as long as possible
The following is the best practices to ensure a long, warm fire for as long as possible:
- Use only well-seasoned wood
- Burn hardwood
- Fully load the firebox
- Set air damper to ‘low airflow’ setting
- Sit back, relax, and enjoy hours of warmth!
Typically catalytic or hybrid wood stoves produce the longest burn times. Catalytic stoves and inserts produce long-term stable heat outputs due to their design and the nature of a catalytic combustor.