Water heater exhaust vent
Our gas water heater exhaust is not drafting sufficiently. The 3" vent pipe runs horizontally about 4 ft, then expands to 7" and enters the wall, where it travels another 7 ft, until it meets up with the (sealed) fireplace flu. I've been told there may be a blockage (there isn't, at least until the vertical flu), or that we need to install a vent liner pipe. Are there any other options? Would installing a 3" pipe through the horizontal section suffice? I don't know how it would be possible to install a liner pipe in the horizontal section, then make a 90 deg bend, because that bend is 7 ft within the wall. Any suggestions would be greatly appreciated.
BTW, I tried to upload a diagram, but the forum will not allow uploading a pdf.