The paper describes modulating control for low-pressure hot-water systems. Compensators and mixing valves are described, and their advantages and disadvantages discussed. The use of time switches is described and typical programmes given. Frost-protection devices for overriding time switches are reviewed, and a strong plea is made for greater simplicity. Difficulties which can arise from faulty positioning of thermostats are discussed. Thermostats detecting lower temperatures than they are intended to can cause safety devices to shut the plant down unnecessarily, and examples are given of such occurrences. Heat retained in the boiler refractories can cause the water temperature to go on rising after the burner has shut off. A heat balance is drawn up and suggestions made for preventing this phenomenon from operating high-temperature safety devices. The difficulties of sequence control of two or more boilers in parallel are discussed. It is difficult to find a method of detecting part load which does not make the boilers hunt; this problem is discussed and a solution for one particular case is offered.
