So-called universal boilers with common furnaces for solid fuel and for oil-firing are generally known. These boilers are primarily intended for solid fuel, such as coke, but can also be provided with an oil burner at the top. A universal boiler of this type is shown for example in the DE-AS No. 1 102 368. In this known boiler, a damper closes an upper passage between the common furnace (combustion chamber) and the flues of the boiler, when the boiler is fired with oil. In this case, the flue gases enter the flues instead via a passage in the lower portion of the boiler. On the other hand, when the boiler is fired with solid fuel, the upper flue is opened as a result of the fact that the damper is moved up and shields the oil burner. This known boiler has the disadvantage common to all universal boilers with a common furnace that the furnace cannot be made in a manner which is optimum for both firing with solid fuel and for oil firing.
Another limitation is that it is not possible to fire simultaneously with solid fuel and with oil, something which may be desirable when the maximum amount of heat is required. This possibility is offered, on the other hand, with the boiler as shown in FIG. 3 in the Swedish Pat. Spec. No. 94 956. In this boiler, the furnace for solid fuel is formed primarily only with a view to burning rubbish and serious disturbing operation must be expected in the oil burner working in the same combustion chamber, particularly as the flue gases from the rubbish combustion sweep directly past in front of the nozzle of the oil burner. Furthermore, the Swedish Pat. Spec. No. 360 458 shows a heating boiler with separate furnaces for firing with gas and/or oil and for solid fuel. The flame from the gas or oil burner is directed towards a perforated wall in the opposite end of the furnace, behind which there may be solid fuel. The hot flue gases from the gas or oil burner do not, however, pass through the perforated wall into the furnace for the solid fuel whether when firing is taking place in this furnace or when only the gas or oil firing is being used. Instead, the flue gases from the furnace (combustion chamber) of the gas/oil burner are always taken off directly upon into a heat-exchange chamber through a pair of flues in the roof of the combustion chamber. The aim of the arrangement is that with a high-speed supply of fuel the flue gases should pass up through both flues but with a low speed supply of fuel they should only pass through the one flue. The possibility of improving the heat economy which this self-regulation can afford is, however, very moderate and the possibilities for a satisfactory heat economy which a suitably formed furnace for solid fuel can give have not been utilized at all. In consequence all possibilities of leading the flue gases from the gas/oil burner into the furnace for the solid fuel have also been completely ignored, something which often happens in boiler constructions with separate furnaces for oil or gas and for solid fuel. Among such known constructions, a heating boiler manufactured by the Swedish Company Exoverken AB under the name "Exo oljepanna typ G och H" may be mentioned for example. On the other hand, this lacks a possibility which the boiler according to the Swedish Pat. Spec. No. 360 458 affords, namely of leading the flue gases from the combustion chamber of the oil burner directly up into a heat-exchange chamber, that is to say up into a system of flues in contact with water, without passing through the furnace for solid fuel, which may be desirable with a low heat requirement.