draft is what you have when the exhaust fan on the furnace turns of. it's the air movement through the furnace and out the flue pipe. draft is also air movement up a chimney in a fireplace. draft is air movement out a ventilation fan duct.
the movement of air from the house to the outside creates a negative pressure inside the house. in an older leaky house this negative pressure is negated by airflow back inside from leaky windows or doors. in a tight, usually newer house, this negative pressure is harder to overcome because of better sealant or insulation properties.
something else i thought of (sorry i missed it before)... it's *not* a common occurrence BUT if the gas line is undersized and 2 or more appliances call for heat (stove, furnace, dryer, fireplace, etc.) the gas line may not be able to supply enough gas for the furnace to operate correctly. it would typically have a flame failure or ignition lockout code. it *may* have a limit lockout code if there is an user installed gas low pressure lockout control installed. a technician with a gas manometer can find this out by hooking the manometer to the gas line, then starting each appliance. the pressure drop can then be measured. each appliance will have a minimum required gas pressure listed.
Bupkis mit Kuduchas!
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