My friend has a 2010 Seneca, first trip and he's getting warm water and can not find the third valve. I think hes mixing his hot and cold because third valve must be open. Where is it located, it is not by the heater. Did they put it in a compartment somewhere?
I think you're asking about the heater by-pass valve, correct?
I don't know on the Senica, I'm just hoping to clarify the question so that someone else can help... Or try over at irv2.com - more activity there.
Generally the by-pass is near the heater on every RV that I've owned. Can he find the heater and inlet line?