Sizing a pellet stove: the method that avoids returns

The basic rule: start from the volume, not the floor area

A pellet stove heats a volume of air. The manufacturer gives a "heatable volume" in m³, calculated for an averagely insulated house. Multiply the floor area of the zone to be heated by the ceiling height, then apply a requirement per m³ according to the insulation:

• House built before 1990, poor insulation: 40 to 50 W per m³.
• Normally insulated house (1990s to 2010, double glazing): 30 to 35 W per m³.
• Recent, well-insulated house (EPB A or B): 20 to 25 W per m³.
• Passive or low-energy house with heat recovery ventilation (MVHR): 10 to 15 W per m³, and there the real issue becomes the minimum output.

Example: a 55 m² living room and kitchen under a 2.6 m ceiling comes to 143 m³. In a house built in 2005, count 143 × 32 = 4.6 kW of demand. A 6 to 7 kW nominal stove covers it with some reserve for the coldest days.

Look at the modulation range, not just the nominal output

A stove modulates between its minimum output and its nominal output. If the minimum output is too high for the room, the appliance stops and restarts constantly: the burn pot soots up, the igniter is worked hard, the fan is heard on every cycle. In a well-insulated house, a stove that turns down to 2 kW or less is more comfortable than a more powerful model that only turns down to 3.5 kW.

On every product page you will find the nominal output and, when the manufacturer publishes it, the minimum output. That second value is the one to check for recent houses.

Ducted stoves: add up the rooms, not the kW

A ducted stove sends part of its hot air into one or two neighbouring rooms through ducts. Count the volume of every room served, but bear in mind that the room with the stove always gets the largest share of the heat. A duct longer than 6 to 8 metres with two bends loses a lot; manufacturers give a maximum length per outlet.

Models with independent fans (the Girolami Vert range, for example) let you shut off the duct to an unoccupied bedroom without stopping the heating in the living room. On a basic ducted stove, the split is fixed or set by a damper.

Hydro stoves: think like a boiler

A hydro stove gives 70 to 80% of its output to the water and the rest to the room. Sizing is done on the total heat loss of the house, as for a boiler, with a buffer tank to smooth out the cycles. A Cherie H 14 Evo or a Soft 18 feeds a properly insulated 150 m² house without difficulty, provided the radiators are sized for water at 60 or 65 °C.

The three mistakes that keep coming back

1. Oversizing "to be on the safe side". The customer complains of stifling heat, closes the living room door, and the stove cycles. Take the right output and let the modulation do the rest.
2. Forgetting the mezzanine or the open stairwell. Hot air rises: a living room open to the upstairs landing heats the upper floor too, so that volume has to be counted.
3. Not checking the air supply. A non-airtight stove uses the air in the room; in a house with MVHR or a powerful extractor hood, you need an airtight model with an external air intake, whatever the output.

What we check with you before ordering

Send us the floor area, ceiling height, year of construction and type of insulation, the rooms to be ducted and whether there is mechanical ventilation. We confirm the output and suggest two or three references in stock. Five minutes on the phone saves a return.