Monday 16 March 2009

The efficiency drive - stoves pickup from the closure of the open fire

As the regulation of the air-tightness of our homes evolves, the first item to face the chop was inevitably going to be the open fire-place.
A possible throwback to the bronze-age, the open hearth and chimney acted as a method of preventing your cooking area from engulfing your ráth and sheepskin bed linen.

The chimney, essentially a wind-path into the innermost chambers of a residence, was decorated and centrally located in any room bringing warmth and comfort to the occupiers. It has remained as such to this date in time, even where modern central heating and insulation have removed the basic need of an open fire.
The homely fireplace serves to do much more than to simply provide warmth. You can't close off an open fire and leave a bare chasm and a slightly off-white radiator on the opposite wall. The loss of this family muster-point can't be undervalued from a social perspective, it along with the TV provide the backdrop to the process of the evening wind-down; time to reflect on past and present, catch up on happenings and occurrences.
The drive for efficiency has demanded that we lose the open fire, but we need to fill the physical and social void created by it's loss. It has to be replaced with a substitute which will provide ample heat through efficient combustion of fuel and a flickering flame, all in a small and neat unit and also provide that essential social centre-piece. That's where the solid-fuel stove makes hay and wins out over rivals such as the electrical-effect fireplace.

Typical efficiency figures for open fires, when burning, are put between a lowly 10 and 25 %. Throw three Bord Na Móna briquettes up the chimney for every one you derive heat from! But, neglected in these figures is the effect that the fireplace has when it's not in use. Add to these value possibly another 15% for heat loss up the chimney on a gusty night.

A good draw on a chimney is both a cure and a blessing. Acting like a central vacuum, it whips the warm air from a room with astounding speed. Where in the past we had manually adjustable chimney dampers fitted to the throat, we somehow forgot about their usefulness in the recent years and neglected to consider the open fire as a major source of heat loss.

The stove on the other hand is the almost-ideal solid fuel consumer. By nature, the adjustable vents throttle the rate of airflow through the unit, eliminating heat loss through the updraught otherwise present. More control on the rate of combustion means that the fuel is completely consumed instead of being lost in fuel or gas vapours, or deposited thickly on the chimney pipe.

Taking a standard Bord Na Mona briquette, we can calculate the following outputs for heat released into the room for both an open fire and a closed stove:

For 2kg of BNM Briquettes burned in one hour:

Open fire (10 to 25% efficiency):
Output in MegaJoules: Min: 3.54 Max: 8.85
Output in BTU: Min: 3,355 Max: 8,388

Stove (40 to 80% efficiency):
Output in MegaJoules: Min: 14.16 Max: 28.32
Output in BTU : Min: 13,421 Max: 26,842

Where the Energy Density of BNM Briquettes is 17.7 MJ/kg approx.


The outlook for the solid fuel stove is good. It fits relatively well with the green movement's checklist for suitable sources of primary or secondary residential heating systems.
  • It fills the gap left by the now outlawed and uneconomical open fire.
  • It reduces our dependence on externally sourced fuel such as oil and gas.
  • It supports local business through the provision of fuel production and sales.
  • It allows the householder to warm a central area of a house without having to warm a whole central heating zone.
  • And dogs love curling up beside a stove which doesn't spit char and sparks.

The rise of the humble stove didn't go unnoticed in Waterford-Stanley's boardroom with a substantial article broadcasting their outlook for the devices.


Bootnote: The Wikipedia article about the stove credits Ireland for having some of the most efficient stoves at 80% efficiency when compared to some stoves in China with 15%.
It appears to make out that back boilers are somewhat unique to our shores.

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