The changeover valve is a device which reverses the flow of refrigerant in the heat pump. This will allow the freon to reverse and heat the home or cool the home. The Zen as many other thermostat manufacturers uses an O terminal to energize the valve in cooling and a B terminal to energize the valve in heating. Please note however, some manufacturers of heat pumps may use the B terminal to energize in heating, while a vast majority energize the O terminal in cooling.
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