LK 810 users

My LK 810 came with a plug only, not the backflow preventer! I emailed the seller and notified them that it was incorrect, and the plug would hinder my thermosiphon. Or thats what I understand. Received an email back saying that the unit will thermosiphon with either the plug in or the backflow preventer. How?
Also, how am I going to get my end of burn heat to the tank with the plug in there? The brochure states for the function of the backflow preventer, "In case of power failure the hot water will self-circulate to the tank" isn't that the thermosiphon? It also states "The function of the backflow preventer can, if needed, be blocked. The backflow preventer is then replaced by a plug LKA art.no: 187022." Am I missing something?

I know of one of the Termovar loading units (about the same thing, it looks) that came with both so you could insert whichever you wanted according to your plumbing scheme. Be nice for all the makers to do that.