whats the cause? just noticed my neighbor's roof. water is running
from the base of the chimney. outside temp is 30, its a hot air gas
fired furnace, in Maryland. i didnt notice this on other roofs.
Rain or snow there recently? Perhaps there's a hollow spot someplace
trapping moisture that's now melting.
Sounds like something worth mentioning and investigating as could be
indicative of a chimney problem that might affect draft and hence
perhaps lead to high CO inside at worse end or roof or other structural
damage at the lesser....then again, it just might be from designed-in
weeps doing their job for external drainage if it's a masonry chimney.
When gas burns, it does give of water vapor. At the right temperature, it
can condense in the chimney and drain down.
Combustion of NG is: CHI[g] + 2 O2[g] -> CO2[g] + 2 H2O[l] + 891 kJ
That is, one molecule of methane (the [g] referred to above means it is
gaseous form) combined with two oxygen atoms, react to form a carbon dioxide
molecule, two water molecules (the [l] above means that the water molecules
are in liquid form, although it is usually evaporated during the reaction to
give off steam) and 891 kilajoules (kJ) of energy.
most of the time you see the vapor cloud above the metal chimneys but
this one had a stream of water. nothing like it on the other similar
houses. maybe he's got a real humid envirnoment inside?
where others aren't and so is condensing more external moisture than
Also, relative efficiency of the furnaces could have an effect -- a
low-efficiency furnace would use more fuel in a comparable house...is
there a cap, perhaps that is a condensation point? If it's
flue-moisture, it should just be vented as steam--the water (if from
combustion or internal humidity) has to have some surface on which it
either condensed or collected to run from.
Did he happen to upgrade his gas furnace recently to a high energy
efficiency model? If so, google "orphaned chimney". (here's a link:
In this setting, typically what happens is that a home owner used to
have both his old gas furnace and water heater both vent out of the
chimney. But one day they upgrade the furnace to a newer high
efficiency model, that vents out of the the side of the house with PVC
pipes since the flue gases coming out of a high efficiency furnace are
too cool to vent out of a chimney properly. So now the only thing left
venting out of the chimney is just your water heater, which depending
on the chimney flue size, may not be able to vent all of its combustion
gases properly (CO2 and H20) in cold weather, causing the water vapors
to condense and freeze up inside the chimney, only to melt when it gets
The solution for this is to get your chimney lined with a smaller metal
liner (often 3" or 4" diameter) so that your water tank can properly
vent out of your chimney. Or, change your hot water tank from a
chimney venting model, to a model that vents out of the side of the
house (power vented or direct vented), and cap your chimney since
you'll no longer be using it (unless you have a fireplace).
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