Abstract: A water or other fluid containing system is protected by a thermostatically controlled drain valve having input, output, drain and control ports. A slideable barrier within the valve selectively directs the flow within the valve between the ports. A small coupling orifice through the barrier allows equal static pressures to develop on opposite sides of the barrier, and a thermosensitive relief mechanism at the control port relieves the pressure on one side of the barrier when ambient temperature drops below a predetermined level. Unequal pressures upon the barrier shift the barrier, redirecting the flow within the valve.