If you have an Ecobee thermostat calling for heat, connected to a heat pump and the flame icon is white instead of orange , check the heat pump settings on the thermostat. The factory default is to prevent the compressor from running when the outdoor temperature is 35 degrees Fahrenheit or below. Most newer heat pumps, especially inverter driven units, will run safely at ambient temperatures lower than 35°. Check the equipment specifications and adjust accordingly. You can also disable the minimum temperature if the heat pump handles that feature.