Quote Originally Posted by Judo View Post
When I purchased my Provent I got with it a 1 way valve that is inline in the drain from Provent to sump. I think it was a separate but relatively cheap item. I'm assuming you don't have one of these?
Yes it is best to always use the non-return valve in the drain.

For anyone buying one new - it is probably worth going to the PV400 if you can squeeze one in.

http://cdn1.bigcommerce.com/server20...0.1280.jpg?c=2

They are quite a bit bigger (but that will mean lower pressure drop and longer life), and they have the inlet and outlet both at the top.

Dougal's case was the first report of oil getting back through the intake, and there are many thousands of these fitted in closed vent applications worldwide. I do agree though it is not how I would have designed it - I am sure their was a reason though.... If you are reversing the flow make sure you remove/disable the valve in what is originally the outlet.