A hot-water heating control consists of an outdoor liquid expansion type bulb connected by a capillary system to a double-ported three-way automatic mixing valve (see Figure 10-59). It is designed to blend the hot water from the boiler with the cooler return water in inverse proportion to the outside water and deliver the blended water to the circulating system.
In operation, the outdoor bulb reacts to changes in temperature and creates pressure. This pressure is transferred through a capillary system to the indoor bulb and then to the mixing valve, which is positioned to increase or decrease the amount of hot water from the boiler. Temperature-range adjustments can be made by turning an adjustment on top of the valve. A typical installation in which a hot-water heating control is used is shown in Figure 10-60.
