1.   For example, there may be inadequate combustion air.

2.   If you do this, you must provide the furnace with its own combustion air.

3.   It all boils down to maintaining heating appliances properly so they work at top efficiency, providing combustion air, making sure chimneys and vents work properly.

4.   To operate safely, gas- and oil-fired furnaces, boilers and water heaters need adequate amounts of combustion air.