[tw.stanley]

If your chimney is in the center of your house, do you need a chimney liner to vent the water heater?

[Milo Dolezal]

Hello T.W. Stanley, I work under UPC...

In my area of operation, you cannot vent water heater using masonry chimney - whether there is a liner or not. If you do, the hot fumes will cool down too fast and deadly gases will fall down towards the heater creating dangerous situation for occupants of the house. It is basic law of physics...

You could utilize your existing masonry chimney to vent your water heater only if you insert double wall vent into it and run it all the way to the roof terminating with vent cap. In this case, chimney is irrelevant, including lining, and acts only as a space to accommodate double wall gas vent.

Hot water heater needs to be vented using type "B" gas vent.

Remember Vitus Gerulaitis, the American professional tennis player from John McEnroy and Jimmy Connors era ? Well, Gerulaitis died when a "malfunction" in his gas heater venting system caused odorless, poisonous carbon monoxide gas to seep into the guesthouse where Gerulaitis was sleeping, causing his death. That's the same gas you are dealing with.

We all are learning from mistakes of others. Let's not repeat them...

[KISS]

I forget the magic number of efficiency. Inefficient furnaces and water heaters could be vented together. High efficiency furnaces cannot.

Water heaters cannot alone be vented without a liner. A liner is a fancy name for small diameter flex tube. A smaller diameter tube has a higher velocity.

Part of the issue is freezing. Without the furnace warming up the chimney, it's possible for the chimney to freeze. The high efficiency furnaces just can't get the chimney warm enough anymore.