Many wood burning stove retailers believe that you should oversize your purchase. They claim bigger wood burning stoves will take away the risk of you being cold. This is incorrect. Wood burning stoves that are oversized are less efficient, resulting in a flue that becomes glazed. Oversized stoves will use more fuel and end up costing you more money.
The most common calculation used by installers is a formula that even the most mathematically challenged can understand. Measure the length, width, and height of your room in meters and multiply. Then, divide that number by 14. The result will be the approximate kW output you need. If you have a room that is 5m by 4m by 2.4m, then the calculation is 48 cubic meters divided by 14, which gives you a running kW output of 3.4. This means that you will need a 4 kW stove.
The formula is based on good insulation. Divide by a different number if your case is unusual:
1- Poorly insulated, single glazed room, older, solid-wall house, divide by 10.
2- New, modern, well-insulated house, divide by 25.
3- Open staircase or an open floor plan, add 10-20% for the heat escaping upward.
When researching a wood-burning stove, you must understand that manufacturers publish two separate heat output numbers. “Nominal” output is a rough estimate and is the larger of the two numbers that manufacturers usually print on their boxes. “Maximum” output is a number that is closer to the actual output that the stove can produce when fully ramped up. If a listing only cites nominal output, request for maximum output and only then decide if the stove is too small.
Another important input that most calculators gloss over is firebox size. Two stoves that are both rated at the same output in kW can have very different sized fireboxes, and a smaller firebox will require feeding more often to sustain that output, which will change how much a stove requires your attention on a daily basis.
If you already have a radiator that stays on in the room, you can size down slightly. A 2 kW radiator running alongside the stove means that a 7 kW calculation can safely be reduced to a 5 kW wood burning stove, since the radiator is taking a portion of the load rather than the stove doing it all on its own.
Rooms larger than 5 kW nominal output require dedicated outside air ventilation by law. This includes a vent rather than just a slightly open window. It is better to check this prior to purchase and avoid a surprise visit for building control.
Our Honest View: Under sizing is better than oversizing. A stove that is too small heats the room, even if it does take longer and burn a hotter than ideal. On the other hand, an oversized stove will burn everything in the room to the ground to prevent starting a flash fire since that is the shortest route to wearing a stove out quickly and making it dirty.
