Give this idea a try:
You can buy smoke bomb (emitter, generator). Get one going near your fireplace with the fireplace damper open. Now turn on your exhaust.hood.
You can tell if you should have the fireplace going when the hood is operating. CO back draft is a problem with a powerful exhaust fan. That's why many areas have specific code requirements for makeup air.
If you have gas or oil heat with no proper makeup outside air intake, you should really make sure that your boiler room door is well sealed.
dcarch
