How the machine works is of little interest. What we are concerned about is comfort. Remodeling is a great time to consider upgrading these systems to gas or solar energy. Sealed combustion furnaces and hot water heaters bring in fresh air for combustion rather than competing with house air for oxygen. While natural gas is more efficient than electricity, gas appliances cause combustion to occur in the home.
Combustion requires oxygen to burn.
That air either comes from the house air or it is vented in from the outside. The result is like keeping a window open all winter long. To save energy and provide safety at the same time sealed combustion heating units duct air into a jacket surrounding the furnace or hot water heater. This keeps the combustion air where it belongs and eliminates cold air flowing through the house. Sealed combustion also doesn't allow deadly carbon monoxide to escape from the combustion chamber into the house when you turn on an exhaust fan or clothes dryer.