Keep in mind that heat pumps have a limit how much they can pump (it also depends on temp., there is less heat in 35F air). If your house is not well insulated,

Keep in mind that heat pumps have a limit how much they can pump (it also depends on temp., there is less heat in 35F air). If your house is not well insulated, at a lower temperature it would be loosing more energy and eventually it would reach the threshold where it's performance is not enough to keep up.