Rianne Error Code 10: what's left!?!

Pooped_Muhself

*sorry if this should be in another forum. Mods feel free to move this accordingly*

Hey Team,

Read a lot of code 10 error fixes and I've tried almost everything I can think of but to no avail.

I have a RC80HPi (5 years old) and recently have been getting an error code 10.
So I've done a complete tear down:
-removed and cleaned fan
-removed and cleaned burner plate
-replaced primary hx with a new unit under warranty
-flushed the entire unit with vinegar
-cleaned out water filter
-replaced all thermal gaskets
-replaced all venting with new (original installer used incorrect galvanized steel core venting when it should have been polypropylene).

Code 10 usually exhaust/intake error but the venting is all 100% brand new (literally 2 days old), same with the hx and now the flame is perfect and super quiet... (venting is only 2-90's, one 28" horizontal run, and the 36" roof termination piece).

Prior to the swap, the venting was severely corroded and the hx had such bad build up on it I couldnt even file off the corrosion between the fins... this was obviously causing terrible exhaust/burning, and what I though would be the cause of an error code 10.

Alas, I clean and replace everything, run the shower and if the radiant floor heating kicks on (uses a separate heat exchanger) the unit kicks off and displays error code 10.

So is this a flow issue somehow? If I turn off the radiant floor heating I can shower without an issue, or vis versa. The unit is super quiet again after the new hx and venting installed.

I never had this error code 10 for the first 4 years of ownership running both radiant floor heating and shower simultaneously...

Any ideas?

I'm at whits end with what else I can open up and replace after all the reading and tinkering I've done.

Really appreciate any help.
Merry Christmas!