Failing HEX

waterboy88

Have a Navien NFB200 installed in a large freezer building heating the ground under the slab. Running at 85 degrees supply temp. Boiler is installed in a loadout area kept at 50 degrees. It is plumbed primary /secondary. It was installed 3 years ago and has a fire tube hex. The hex failed (glycol coming out the condensate drain) this spring. After talking to tech support we swapped out the 0015 boiler pump for a 0013 and they sent us a new hex, which also failed 5 weeks later. This time they sent us a new burner with the hex (original burner had some small cracks in the burner gasket). This hex again failed (glycol coming out condensate drain) within 10 days of installation. We now had the attention of every lead tech at Navien and they sent us a new hex and burner head again as well as sending a rep out to check things over. Did some water testing which found ph at 6.8 on one test and 7.5 on another. Propylene glycol content at 36%. Venting was done in accordance with install manual with exhaust extending 1’ out from intake, 1’ above, and 1’ to the side. Our combustion analysis didn’t show any signs of recirculation. Also we extended the intake down another 4’. We have not started the boiler again since, but are circulating the system pump and monitoring the return water temp to make sure we don’t get close to freezing. The pictures are of the second and third heat exchangers after running for 10 days and 5 weeks. What else should we be checking?