Now ya see why PVC on mod con flue is not a good idea

yeah, that looks like the glue was allowed to dry too long before assembling or they went over it a couple times because it dried

MA has basically banned it I think.

If the existing PVC flue is in a wall (difficult to replace) you can reattach a new boiler to it if the new boiler is listed for a PVC flue and the pVC shows no damage or overheating but then they want you to go to the new boiler with "approved pvc" like the Ipex so now you have a mix of PVC so its an issue.

That PVC pipe and coupling was not pushed in all the way. There is no way that pipe bottomed out in the coupling. My guess is the installer didn't notice it was pushing apart before it dried. The PVC is still white like its relatively brand new.

very first picture looks like it could have gone in a little bit further, maybe 1/4 to 1/2 inch but kind of hard to tell. my unprofessional view is operator error but again, what do i know. thanks matt for the topic. i learn something new all the time.

I have used PVC venting on scorched air systems for over forty years. That is clearly a poor installation job. A proper connection is actually a solvent WELD, impossible to disassemble after curing.

You can armchair quarterback your opinions and guesses as to the failure but without lab analysis you are still just grasping at straws. Was it installed per the mfrs. listed instructions as well as those of the PVC pipe mfr, the primer and the solvent cement? Was the pipe used within the approved applications by the pipe mfr? Was the pipe properly supported? Was it leak tested prior to use? Was the appliance inspected and tested including gas pressures, orifices, firing rate, MUA, intake air obstructions/ sizing/ pressures? Flue gas outlet temperatures? Any signs of overfiring in the unit or historical evidence such as tripped safeties? Was the heat exchanger ever replaced? Lot's of questions in a forensic investigation.

I don't like PVC for a number of reasons: The pipes out there were never listed for this use but specifically forbidden per most pipe mfrs. While the pipe itself might withstand short term temps upwards 170F, the solvent cement routinely fails at 140F or lower. I've never seen an HVAC tech properly cut, chamfer, deburr or join PVC for venting. Never. I see pipe cut with a Sawzall held with one hand and the pipe with the other. The end of the pipe resembles a hole saw with all its jagged edges. The end is never chamfered nor deburred and there is no way to tell externally once the joint is made. Meanwhile, inside, the rough joint plows the cement pushing a dam of it ahead while scraping cement from the outer walls. The result is a very weak, often leaky joint.

The pipe mfrs. expressly FORBID pressure testing with gas, such as done with gas piping. You can do a low pressure water test but how are you going to test the seal on a flush wall termination like a Bayvent? You would have to test with the pipe extended, tested, cut to length and attach AFTER the test. You might be able to rig a test ball in some cases. Also, on long runs through crawlspaces, you may get a few drips that harbinger a future failure yet not manifest during the test duration. You have to rely on one end of the pipe or another just as with gas pipe testing. That brings in the gauge resolution, test duration and allowable leakage. It has to be reasonable yet reflective of the conditions.

The UL 1738 test seems fairly reasonable and reliable. It is also repeatable.

Everything made by man can and will eventually fail. It's just a matter of when and at what cost.

high temperature systems with PP and long horizontal run can be a problem if not supported properly.

I have seen the piping sag and leak condensate at the joints.

HTTP failed , standard PVC and CPVC isn’t the best solution, some stainless suffered gaskets or silicone adhesive failures,

Hopefully PP holds up better. It is a harsh environment for the tube and seals

Dirty or plugged HXers can run the flue temperatures up pretty hot. Hot enough to trip temperature safeties.
Mod cons in DHW mode are generally exceeding the PVC, maybe even CPVC temperature limits

Not to mention the lack of listing for flue gas conveyance on typical PVC pipe

Yet manufactures are still sending equipment out with PVC or CPVC “starter pieces” PVC fittings and rubber sewer pipe connectors😲 Some brands sent ABS start pieces, which would often get connected to PVC. Crazy!

Some of the early HTP products included a short piece of ABS, the Voyager maybe?

What I find is

PVC 140°F max

IPEX 636 CPVC (flue gas certified) 194°F, with their solvent and glue

CPVC 200°F max

Centrotherm PP 230°F continuous

DuraSeal single wall stainless 480° operating, 550°F max

Flue gas sensor on my Lochinvar Knight is 225°F to shut down burner. So essentially only the Centrotherm and SS vent pass the potential flue gas temperature question

acids are one of the things PVC does seem to handle well

https://ipexna.com/wp-content/uploads/2022/08/chemical-guide-caen-ipex-pvc.pdf