Since no home I personally know of has a make up air system installed, (granted, none in my social circle have a custom home, all tract builder here) there must be some assumption/saftey margin that some amount of make up air is drawn from doorways or "somewhere safe" besides a dedicated make up air system. What "cfm" is typically assumed on a recent (<5yrs) home without risk of back drafting from furnaces and water heaters/chimneys?
After all, we all run clothes dryers, kitchen vents and bathroom exhaust. So take my home for example, ~2600sf, 3 baths, 1 interior utility room. The cheapo builder bath & laundry fans are 50cfm, there was the standard 150-200cfm kitchen vent fan, and the clothes dryer.
Is it safe to assume then that in my home I don't require make up air for 4x50cfm + 1x100cfm + 1x150cfm (bathroom/laundry, dryer, kitchen) 450cfm?
When does an HVAC professional decide a home needs make up air when designing an HVAC system for a given home?