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Boiler stove fuel consumption

Initially the fuel consumption in the early stages of firing will appear high as the damper will be fully open, allowing a fast burn rate and rapidly raising the heating circuit water to the desired temperature. During the initial stages of firing the return temperature to the boiler will be low having the effect of maintaining a rapid burn rate.

When the whole system is up to working temperature, the return flow water will be high giving only a low differential temperature across the boiler jacket. This will mean the water is up to the desired temperature and the damper will reduce the air to the fire box and the fuel consumption will drop rapidly.

As mentioned in the ‘Boiler over-run’ section, setting the thermostat is important to prevent the boiler from cycling high to low and burning fuel very inefficiently. If a boiler is constantly rising to maximum output and falling back again under damper control, large amounts of fuel are wasted.

During the period when maximum firing is reached and the damper is throttling back the air, a considerable amount of the heat produced will not be absorbed by the water but rather passed directly into the room or up the flue. This is a result of the maximum heat absorption rate of the water being reached before the fire box temperature can fall back under the influence of the damper closing ('control lag').

Best results are obtained with a high quality anthracite fuel, such as Ancit, with around a 20% output to water reduction when burning seasoned hardwood.

Refuelling a boiler stove

The most efficient way to re-fuel the boiler stove is not to continuously top-up with fresh fuel, but rather allow the stove to burn back to a bright ember bed and re-fuel with sufficient fuel for the burn period required. Constantly topping up causes the fire box and flue temperature to fall rapidly as the door is opened and the cold fresh fuel load will use a percentage of the original fuel to heat it to combustion temperature. This will reduce the water outflow temp and cause the damper to open and rapidly increase the burn rate wasting fuel.