Furnaces are expensive and so are roofs and windows. I recently talked to a neighbor who says her furnace has not worked properly for three years and she can not afford a new one.
There are some low interest and no interest loan programs available for people who wish to buy and energy efficient furnace. In most cases the income limits are pretty liberal.
There are also some tax incentives for purchasing energy efficient heating or cooling systems. Here are some local programs for low or no interest loans that can be paid off over periods of up to 20 years. Some of these programs also work for replacing windows, doors and even roofs.
City living program - City of St. Paul
Keep in mind that especially here in Minnesota a home has to have a furnace and when you when you put a new one in your homes does not increase in value by the amount of money that the furnace cost.