Open Fires

Many homes have a fireplace purely for aesthetic reasons. A glowing open fire is a pleasing sight and generates considerable heat, but at the expense of much of the oxygen contained in the surrounding air.

This air, when heated, cannot be circulated to warm the room by conduction because it is mixed with the products of combustion. Air, smoke and a lot of heat go up the flue; in fact, about 85% of the heat released is lost, (i.e. about 15% efficiency).

To distribute heat more effectively a more enclosed, freestanding fireplace was developed. Although this provided more radiant heat it still allowed most of the warm air to escape with the smoke up the chimney

The convection open fire was a further development, dividing the air into "air for combustion", which passed through the fire and up the flue, and "room air" which, by means of grilles and dampers, was conducted around a fire box, thus gaining heat to be convected back into the room in an unpolluted state. This improvement was also applied to the freestanding, enclosed fireplace or stove.

Stoves and Heaters

Solid fuel burning stoves are built with a chimney or flue and can be either adjacent to a wall or freestanding. In both cases, the main heat distribution is by radiation from the contained heat and by heat convection in the surrounding air. Generally, some form of air control damper is provided; either at the air inlet under the grate, or in the flue, or both.

Stoves fall into two categories;

1. "Air-tight"
2. "Non Air- tight".

These stoves allow uncontrolled air into the combustion chamber resulting in less complete combustion with efficiencies at around 30%. Convection stoves have additional metal jackets which form airways. As the stove heats up, room air is drawn into low level vents to be heated and expelled at the top, thus providing convective heating.

Some stoves are provided with water jackets or "back boilers" so that water can be heated for domestic purposes at the same time as room heat is provided. These stoves are connected by pipes to a low pressure storage tank. If the tank is elevated sufficiently above the stove, thermosiphon circulation causes the heated water to rise to the tank.

If the tank is not high enough or if the pipe connections are too long or inadequately sized, a small circulating pump may be required.

Note: It is not permitted to connect a stove of this type to a "Mains pressure" domestic hot water vessel.

Many stoves are made from cast iron. This material is resistant to fire corrosion and has sufficient mass to retain and distribute heat. It should be free of cracks, pitting and porous surfaces. Most manufacturers recommend "seasoning" by lighting a number of small fires before allowing the stove to fully heat up.

Steel is also used for some stoves, with advantages and disadvantages. Steel stoves, being of thinner construction, are lighter and easier to move about. However, steel is more prone to rusting or burning out, while warping may affect its airtightness. When burning coal or coke, a higher temperature is achieved.

As it burns, coal releases gases which ignite and can cause localised spot heating. To prevent damage to the stove body, it is desirable to install a fire brick lining to the combustion chamber. A special refractory fire brick, with a higher alumina content giving it the ability to withstand heat, must be used.