Many residential mechanical rooms have rigid metal ducts to pull combustion air into the space for safe operation of the gas-fired equipment. Often the water piping for domestic water lies near the air inlets, posing a potential for freezing in our cold Colorado winters. I frequently see air inlets stuffed with insulation or sealed shut. So in the winter, when the appliance needs to run the hardest, it is starved for air, which makes for an unsafe condition.
I have solved the problem of both freezing pipes and air-starved burners by making thermal traps for the combustion air inlets with two 90° elbows and a small length of straight duct on the end. Skeptical homeowners are amazed at how well the simple trap works; the material costs are low enough to persuade them to try it. Our local inspectors like it, too, but still require that the traps terminate within 1-ft. of the ceiling and floor, per ICC code. The warmer room air stops the cold from falling in but allows makeup air to enter as needed.