I took a photo of this device attached to my friend's pool heater as I hadn't seen one before. This is a new build and a very high end one at that. After some research I have found that this is some sort of check valve which I presume is designed to stop high levels of concentrated chlorine from the salt cell from going back into the heater when the pump basket lid is removed. There seems to be a bit of debate whether this is needed or not as I thought titanium core heat pumps were resistant to corrosion.
My question is, why is it plumbed in this way across the inlet and outlet of the heater and not inline with the outlet?