PVC combustible?

Metal water heater flue pipe <1" from high efficiency furnace PVC flue pipe. Is the PVC in this instance considered combustible? I don’t know the reason for the silicone caulk between.

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The installation is wrong. The exhaust from the high efficiency condensing furnace does not get discharged into a masonry chimney. Someone took a shortcut or did not read or understand the manufacturers installation requirements.

The PVC is extended all the way through to the top of the flue, so that is correct. Anyone have any comments on the combustibility of the PVC in this situation?

what stops the flue gases from the water heater vent from drafting back down thru the unsealed chimney or does the flue for the waterheater go all the way up, if so that product should have been bvent. Charlotte pipe has taken up a position that they never manufactured chimney vent pipe so the manufacture will never show a preferred clearance. The whole thing is a short cut, just recommend having a contractor review the install.

What does the manufacture 's installation manual say regarding PVC???.

Both vent pipes terminate above the chimney. No manual present.

My plumbing and heating back ground would say the 26 gauge single wall vent will deteriorate at a rapid rate. It should have been a double wall pipe. I would bet money that was done without a permit or inspection. It does look like recent work from the photo. The single wall in time will rot and then you will probably end up with a back draft issue. How was the top of the chimney sealed or was it.

Original double wall flue goes through a sheet metal chimney cap & is sealed.

Not commenting on the installation but to answer your question PVC is combustible; however, has a higher ignition temp than wood. Ignition temp is 750 degrees in comparison to 500 of wood. PVC also contains chloride which makes ignition more difficult. I would not be concerned with the PVC igniting from the metal flue in the picture you provided; however, it is still considered combustible. I am guessing the silicone is present to prevent any vibration of the PVC pipe against the metal flue when the furnace runs.