Venting into return duct?

I am going to pull air from the return duct through the room and down into the crawlspace. The room is going to be sealed.

this return duct you plan on tapping into is What size?
11.5*7.5

how mnay other retuns in your home?
4, however it looks like to me that 3 of them are connected to the line that I tapped into. The other one is closer to the unit on the middle floor of my tri-level house.

what is the size of your furnance ( BTUs )


A/C size if you have A/C coil and condenser unit.
Not sure where to find ac size, it is outside and it looks like most units however maybe a little smaller.

Do not pump hot air into your ducts! Pulling air from it is fine,as u are just pulling air from the home. If your system is running and your pumping hot air into your R/A ducts not only is it not going to cool your home and run incredibly inefficient but your also going to increase the pressures in your system and throw off the refrigerant cycle by changing the superheat/subcooling due to unnecessary heat being pulled through the evaporator coil.

Bear I don't use my hvac system for heating only a/c just using cold air held within ducting. Bowl of ____ ? Might want to educate yourself then teach. My unit is a 4 ton making its "certain amount of air" 1600 cfm, visualize one thousand six hundred 1' cubes of air traveling in and out of ducts every minute. A human's load on a/c is about 600 btu, to create your estimated 10x load demand 800 of my closest friends would have to figure out a way to fit into my home. You would have to take the total load of my home divided into area, then subtract closet and add closets actual load to find the increased demand. In my case its about 1200 btu or a much more comfortable couple of friends. My room is sealed all incoming air is hepa filtered then treated with uv lighting before entering room. I used to pull from outside before sealing and controlled it with a thermostat. I use a swamp cooler most of the summer and this setup keeps me cool for pennies on the second floor. I'm guessing when its +95* outside a/c will be necessary so I'll turn closets intake into filtered exhaust.

No then I misunder stood you you can not pull air from the return , def wont happen , you can however disconnect that duct from the return and use it ,and just leave the whole in the return main duct( dont patch over) ,no harm there ,except you may find the room you stole that return duct from with a differantal temp drop or rise from the other parts of the house, do you know the sqare footage of your home please tell me , that return duct size you said could handle 500 cfm ,that is a pretty big return duct , I really dont advise using that duct that is a main that returns a good portion of theat area where its originates from