Hi,I have a gas heater that was added at some point into our 100+ year old farm house. I’ve noticed that there is a spot where water is leaking down through the drywall where the two PCV flues travel. My guess is this indicates a leak in the pipe? It’s only a little moister but is leaving a black spot on the drywall.
I’d like to help, but would need a little more information. If that stain is under or next to the PVC flues, it might have nothing to do with a leaking water pipe. I’m not sure if those PVC flues are from the furnace or the water heater you’re talking about, but PVC flues sections are jointed together with PVC cement, which has a tendency to deteriorate when exposed to acidic liquid condensate / acidic exhaust gases that are produced during high efficiency furnace or water heater operation.
The acid just leaks through the joint and might be dripping onto your drywall sheet. If the stain looks dark, there’s a chance that you already have some mold growing on the other side of the drywall. Other possibility is an exterior wall or roof flashing around those PVC flues … rain water might be just running along the pipe exterior if there are some gaps around penetration.
So first check exterior (those PVC flues should be sloped towards the appliance at at least 1/4″ per foot) for any gaps around penetrations, and than I would open the drywall, to ensure that you don’t already have some mold colony growing, and if it is leaking joint on the PVC pipe, it should be re-glued, or more likely replaced with a new one (silicone or applying PVC cement on surface will not work) … or it might be a leaking water pipe.