October 8, 2019 Jeff Rosenblum
All fuel-burning appliances require oxygen to burn safe, cleanly and efficiently. In this case, safe means without excess CO (Carbon Monoxide). A safe level of CO is lower than 100ppm (Parts per Million) and stable while tested undiluted in the exhaust.
Many fuel-burning appliances are 70-80% efficient with open combustion chambers that draw air from the surrounding area for combustion.
This can cause a negative pressure in the area. If this air is not replaced, it can cause un-safe combustion and the release of CO and negative draft in all the appliances drawing combustion from the space.
Always make sure to follow the National Fuel Gas Code for proper combustion air guidelines. Following this will prevent issues that can arise with not having enough combustion, ventilation and dilution air.
Not only are direct vent (90%) appliances more efficient on the fuel utilization side, they can also prevent the home from going into a negative pressure because many will draw their combustion air from the outside of the structure.
When inspecting or designing a system, make sure you have an adequate combustion air to supply all fuel burning appliances in the space with the air for proper combustion. Again, units that draw their combustion air from outside the space are not a candidate for concern.
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