The 10 foot rule is part of the National Building code and is something that is demanded of all ventilation installations, regardless of type or where it is locate (ie residental, commercial or industial). Ther absolutely **HAS** to be a minimum ot a 10 foot separation between a fresh air intake and *ANY* exhaust duct, be it from the lampworker ventilation system or the furnace/hot water heater.
The reason for this is very simple: with less than 10 feet of separation there is a strong possibility that fumes from the exhaust system will be sucked into the fresh air ducting. If you have a gas-fired furnace or hot water heater, stand outside next to the exhaust duct while the appliance is running. You should be able to smell the exhaust fumes (mostly NOx). Now, slowly step away in a straight line until you can’t smell the fumes. Generally speaking, this is around 10 feet or so, if the appliance has been running for several minutes.
Quite often what happens is that there is a localized “dead spot” on a house where fumes will hang around, and the 10 foot separation rule eliminates that entirely.
The rule is applied as follows: for straight wall measurements, measure between the inside of the exhaust duct and the fresh air duct. The measurement must be a minimum of 10 feet.
For “around the corner” measurements, where the ducts are located on different outside walls, measure around the corner inside to inside of the two ducts, and again, the measurement must be a minimum of 10 feet.
For roof exhausts, measure from the nearest part of the opening of the duct to the nearest opening of the fresh air duct, again, it must be a minimum of 10 feet.
Don’t measure inside the building from point to point, that is meaningless, especially if the ducts are on different walls, it is quite conceivable tha the measurement inside could be 8 feet but the outside measurement as you go around the corner will exceed 12 feet.