An open masonry fireplace typically converts less than 20% of the wood's energy into room heat. The rest disappears up the chimney, taking warm indoor air with it. Modern closed-combustion units routinely achieve 75–85% efficiency — a fourfold improvement using roughly one-third of the wood.
The efficiency gap in real numbers
For a 60 m² lounge, an open fireplace burns roughly 4 kg of wood per hour to produce a comfortable ambient temperature. A closed-combustion unit rated at 8 kW delivers the same comfort on 1.2–1.5 kg per hour. Over an average Cape winter that is the difference between two bakkie-loads of wood and half a load.
Indoor air quality
Open fireplaces draw combustion air directly from the room, pulling cold air in through every gap in the building envelope. Closed units — especially those with an external air kit — take combustion air from outside, so warm indoor air stays inside. This is particularly relevant in modern airtight builds where an open fire can create dangerous back-draught conditions.
When an open fireplace still makes sense
- Heritage restorations where the original hearth is a listed feature.
- Occasional-use holiday homes where ambience matters more than heat.
- Homes with an existing masonry firebox that a closed-combustion insert can be retrofitted into (the best of both worlds).
Retrofitting a closed insert
If you already have a masonry chimney, a cassette-style closed insert slots into the opening and connects to a new flexible liner. Installation typically takes a day and immediately delivers the efficiency gains above.
Written by
Progress Group
Fireplace, braai and gas specialists since 1990 — showroom in Bellville, installations across Cape Town and the Western Cape.
