I need to purchase a back vent what is it and how to go about buying one.

Hi Donnalynne,

You said, "I need to but in a new back vent under my kitchen sink because the one on the roof "...

The one on the roof, what? What is going on... let us know more... OK?

In terms of a back vent... I have never heard the term. The only vent you can purchase is an automatic air vent... called an AAV or provent (see image). These mechanical vents are not usually allowed to be used in most states, but they can be handy in certain circumstances. They are available at all home improvement stores.

Check out how to install them below. They are male threaded so you need to install them into a female threaded fitting.

Back to you...

Mark