High-efficiency furnaces save on energy use and are getting more common every year.
The fire in a gas furnace is a reaction between natural gas or propane and oxygen. The furnace burns methane or propane and oxygen to produce water and carbon dioxide. But the flame also needs air and therefore brings in atmospheric nitrogen, which makes up 70% of the atmosphere, and the fire is hot enough to force the nitrogen to react with oxygen. The resulting nitrogen dioxide and nitrous oxide dissolve into the water vapor in the exhaust stream to form nitric acid.One of the results of the
