Heat cool until warmed up

01 caravan 3.3 I've read the posts about no or low heat, and most being either t-stat or low coolant. The problem I have is intermittent in that the heat will be cool until it warms up enough to open the t-stat. Then the hot heat will come. I checked the coolant level, blend door and the t-stat, all seem fine. The water pump is 4 month old. And the engine warms up as it normally does. Never overheated and the gauge goes to the line just before the middle. The odd thing is it wont do this all the time just sometimes, and once the t-stat is open it works properly.
any help is most welcome.

also this might be related but what is the small black canister(almost like a 35mm film container)that is inline with the heater hose. no rear heater lines.could that be a check valve?