What's the difference between "heat output" and "efficiency"?
Heat output from the combustion process is related to the type and quantity of fuel that is being used. Generally the more fuel being burned, the more heat will be produced. Unfortunately this does not always mean that more heat will come into the room as in some cases up to 80% of the heat generated is lost up the chimney. The "efficiency" of a fire is a measure of the ratio of heat produced that is actually thrown into the room. Hence it may be that a small but very efficient fire might be a better supplier of heat than a huge open roaring basket fire. In these days of increased awareness of energy efficiency, it is worth being aware of the different types of appliance that are available and their relative efficiencies.
A traditional basket fire is the least efficient form of fire. Up to 80% of the heat produced may be lost up the chimney and what many people who fit these fires for decorative reasons discover later is - that not only do they not benefit from much heat from the fire when it is lit, they also lose heat from their central heating up the chimney when it is not in use.
However technology has advanced in the fire industry and many modern appliances address these issues very effectively. The secret is to control the air flow across the fire, allowing enough to safely remove the waste products while minimising the heat loss.A modern "box-fire" is designed to improve the efficiency of the appliance. The box is installed in the chimney opening and the fire placed inside. The box is insulated at the back and sides to prevent heat loss and the opening into the chimney itself is reduced thus enabling it to radiate more heat into the room The air flow over the fire is reduced which results in less heat loss up the chimney both when the fire is in use and also when it is not.
Ultimately the answer to efficiency is a glass front on the fire. This has the effect of reducing the amount of air flow across the fire and gives huge gains in efficiency. These fires are either built into the fireplace or the chimney breast or in the case of a wood-burning stove can be freestanding in the chamber. These types of appliance also require much smaller diameter flues ( usually achieved by a liner down the chimney), require less additional ventilation in the room and cause little heat loss when the fire is not in use.
