In the Mpls/St Paul area we install furnaces designed to heat your home to 70 degrees when the outdoor temperature reached 16 below zero. In other words , the furnace should run almost constantly when the temperature reaches 16 below. This long run time uses a lot of gas, but on the upside, the long run time makes your home much more comfortable than when the temperature is more mild outside and your furnace satisfies the thermostat setting more quickly.