Cheater vent placement

[Kyle9075]

Hey everyone! I moved into this house two years ago and have been having constant problems with the kitchen drain. First of all, it runs about 25 feet under a crawl space to the new addition (see photo). There isn't much slope and there's a 'belly', so I plan to drill and lower the pipe. I also think having air flow will help keep things moving. Where do you think I should place the valve?

Thanks,
Kyle

 

[MicEd69]

You have an illegal S-trap. Assuming you cannot get an atmospheric vent for that trap, you need to install an AAV (Air Admittance Valve).