Carol and I looked at a couple of houses today. Both houses we looked at were older houses with low efficiency natural gas furnaces and gas water heaters. And in both cases the water heater shared a flue with the furnace.
I’m under the impression this is a bad idea. But this is definitely something I am not an expert on, so I’m open to learning something.
Here are a couple of pics:
In this one the furnace is off to the right of the picture:
So what’s the deal?
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Replies
In this neck of the woods it's not at all uncommon to have a shared flue. It must be sized right, of course, and there are several other restrictions, but done right it should be perfectly safe. The biggest potential problem, IIRC, is that the flue may be too large for the water heater alone (and hence have a poor draft), but I think it works out OK for most residential setups.
However, the first shot above appears to be feeding a masonry chimney (vs metal flue) and that can (in theory) have draft problems with the small water heater, due to its large bore and cold surface. (However, my MIL had this setup in their old ca 1930 house and never had a problem.)
Both of those installations would meet today's codes.
Even if the first one doesn't have a metal liner inside the masonry chimney, it would still be OK. The main concern about using a masonry chimney for venting gas appliances is the possibilty of condensation forming, which is acidic and could deteriorate the masonry. However, appliances with draft hoods, like the WH for sure, and maybe the furnace from what I could tell--such appliances allow a continuous flow of dry air into the chimney, which shortens the "wet time" and dries any condensation that has formed, as well as lowering the dew point of the flue gases to where condenstation is greatly reduced, or eliminated.
And, if in doubt, you could always do your own simple depressurization test to ensure that the draft is strong even under worst-case conditions.
on the first water heater, there are plastic collars around the water lines, in this case red and blue. Check to see if they are melted. If they are, you have a draft issue.
On the second heater, make sure the insulation blanket is above the air intakes on the bottom of the heater. Better yet, if you buy that house, get rid of the blanket altogether.