So far, we have discussed systems in which a fuel is burned and heat is produced and delivered into the home. In these systems, we buy all the energy, and (depending on the system) we reject or lose some heat to the surroundings, reducing the efficiency.
One of the ways in which we can improve the heating efficiency is to make use of the heat that is available outside, even on a cold winter day. On a cold winter day with outside temperature at 30ºF, the air still has more energy compared to air at 10 ºF or 5ºF. Air at any temperature above absolute zero (0ºK or -273 ºC) will have energy.
The higher the temperature of air, the higher its energy content. This energy can be transferred to the interior.