Water heater ties in with furnace, furnace is a 80% with a combustion gas blower fan. There are other issues with what your looking at but want to know if vent pipes sharing like this are acceptable?

A shared chimney or flue is allowed by both the IRC and UBC.

Thanks jeffrey I kinda knew that, what I meant was does the water heater vent need to be located higher than the furnace vent, specially in this case due to the furnace having a combustion blower on it?

An 80% furnace and water heater should not be vented into a masonary chimney. The exhaust is not hot enough to properly draft and will condense in the chimney and cause the chimney to deteriorate. The chimney should have a metal liner installed.

Recommend a Level 2 Inspection of the chimney/flue.

That chimney looks hideous! Was this picture shot in a dungeon?

What does the appliance Mfg. name plate show? The Mfg tells you what and how that unit should be installed including proper venting requirements.

The key is Mfg’s instruction first and next your local AHJ.

If these systems are natural draft, negative pressure then:

In general terms, natural draft , Negative pressure, Cat 1 , N.G. burning appliances can share a common flue/ chimney BUT only if they share a common factor. Both should be Negative pressure, Cat 1 systems.
No mixing of negative pressure with positive pressure systems.

As for which flue should be first at the chimney breach? My local gas company taught me the smaller of the two should be higher. ( In a perfect installation), but as we all know that is not always the case or possible during the installation.:wink:

The other factor is called the 7 times rule . This applies to the size of the flue of the largest with respect to the smallest flue. The larger flue should not be 7 times larger in diameter then the smallest flue since the “draw” of the chimney can not effectively “draw” the products of combustion from the smaller flue if that were the only appliance working.

Brick chimneys ,as pointed out earlier, for natural gas burning appliances require a liner. The temperature of products of combustion from N.G. is much cooler then wood burning appliances. The corrosive action from moisture in the products of combustion will “eat” mortar and eventually the chimney will fail.

Check out : for a great explanation of venting basics under “venting done right”.