Controlling blowdown of a burner heated steam boiler comprising a boiler housing 3 for containing water, a burner 1 and a blowdown control valve 17 to allow water to leave the boiler, the control system 20 controlling the blowdown valve in response to signals indicative of the steam production rate. The control system arranged to open the blowdown valve multiple times over a time period and sdjusts the time the valve is open each time. The control valve is opened for a time which is reduced proportional to the reduction in steam production. Initially the blowdown amount for a given production rate is set and the blowdown amount is then adjusted by comparison of the aggregate production amount with the amount that would be expected at the given production rate, the given rate being the maximum continuous steam production rate for the boiler.
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