What do you mean 'through the furnace'. Many homes have the water heater
vent on the same chimney as the furnace. New codes in some states
require a separate flue for each device. They are trying to drum up more
business and make you pay more.
thats the trouble both will run at the same time occasionally.
plus have you ever noticed a hot water tanks chimey is smaller than a
the device BTU should be sized in regards to the chimney.
to get good draft the chimney must heat up. that takes a long time
with a too large chimney
but a furnace on a too small chimney may not exhaust well.
I asked a lot about this after nearly dying after carbon monoxide
the chimney cap cracked allowing rain water to get between ceramic
liner and bricks, freezing weather expanded the trapped water the flue
liner fell in and we all got ill. could of died if a buddy, a
volunteer fireman hadnt visited, stayed a while and said lets check
the flues. he got it too.
a very close thing
I'm told it is not just flue gasses, but the potential for some unburned gas
too, that could be ignited by the other appliance. Seems it would be rare
though. I do know that many homes wee built for many years with both heater
and water heater in the same flue.
Changing codes and the shift to forced draft instead of natural -- when
everything was 50% or thereabouts efficiency w/ natural draft there
wasn't much problem. 80% and above require forced draft.
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