Braeburn 505 Mechanical Megaswitch Heat Only Thermostat Single stage conventional or heat pump systems, dependable switch performance, environmentally safe  mercury-free 24 volt AC gas, Oil, electric and millivolt systems System lever cool/off/heat, front mounted wiring terminals, integrated sub-base Fan control lever auto/on, plated terminals reduce corrosion , no leveling required during installation 2 wire hydronic zone systems, heavy duty heat anticipator