Hvac chimney question

I have read not to tap into your boiler or hot water heater ducting to get heat out of the house. Can I put a 6" vortex fan on the rigid 6" ducting that connects my boiler to my chimney? I would like to spill over extra heat the boiler room (portable dual hose ac or chiller heat). The boiler wont be in use 98% of the time. I have a radiant heat problem. Two 1000w ballast and 1 400watt and a 400watt veg tent are in the boiler room. I cool the 2 1000 watt lamps by sucking outside air and pushing it through both hoods then expelling it back outdoors. The 7x7x7 room has radiant heat problems. Thinking 14000 btu dual hose ac mounted high so it fucks with my Co2 less. Use the boiler room as a lung room . The Boiler room has a 12x12" passive intake so any air I blow through the chimney can easily be replaced with new cooler air.

What the guy in the video is talking about is teeing into a main stack, which yes, will definitely create a back draft of CO. If I'm understanding the OP correctly he wants to turn a passive boiler exhaust into an active exhaust.

If you put the 6" vortex fan between the boiler and the chimney (run it 24/7 max speed) the area around the boiler will constantly have negative pressure. Do a smoke test to be absolutely sure you have neg pressure. I've never heard the term "lung room" before but it describes pretty well what you are doing, exhausting your grow room into the boiler room, then separately exhausting the air in the boiler room. I wouldn't try to pump more CFM into the boiler room than pumping out. So if you are exhausting the boiler room with ~440 cfm with a 6", I wouldnt try to use an 8" in the grow room, forcing 800CFM into the boiler room. hopefully a 6" (on a speed controller and/or thermostat) for your grow room will suit your needs. Of course it helps that your lights are independently cooled and you are just trying to get rid of radiant heat. Always do a smoke test to be absolutely certain and for good measure get a second CO alarm (you already have one near the bedrooms, right?) for the boiler room.

Here is my setup. The damper only opens when the boiler is on. Any problems?

this is a bad idea. If you were never gonna use the boiler I'd say sure do it. But since it MAY be used at some point emphatically no.

Hvac by trade.

Thanks so much bro! My HVAC knowledge is quite limited. Anyway to get it to work safely? Few questions will the fumes/gases effect the inside of an inline fan? I know there are special fans to move vapors. I wouldn't the natural updraft of the hot water heater and boiler keep gasses going up the chimney. I understand your afraid of the 4" fan pushing all it's CFM through the 6" boiler ducting when the damper is open (boiler on) but why won't it just go up the chimney (heat rises).