If you want to lower the cost of keeping your birds warm during cold weather you have to limit the amount of cold air that is entering your house. It is as simple as that. The more fresh air you bring into your house either unintentionally or intentionally the higher your heating costs will be. The table below shows how many gallons of propane have to be burned to heat various volumes of fresh air based on the difference between inside and outside temperature. For example, if you are operating t...wo 36" fans 30 seconds out of five minutes (2,000 cfm) and it is 90oF inside and 20oF outside (70oF temperature difference) you will have to burn approximately 36 gallons of propane over a 24 hour period to heat the incoming air from 20oF up to 90oF.