I have a house with an asbestos flue. The water heater and gas furnace
have regular sheet metal flue duct, then up in the ceiling the metal
flue slips into this ceramic/asbestos flue - 6" diameter.
There was no heat for about 1 year, just this past week got everything
One problem, there appears to be a brown liquid dripping out of the
asbestos part of the flue, it then trickles down the metal pipe. Not
alot, just a small drip from time to time - but messy and concerning
Any idea what this is? Should I be concerned?
Yes, you should be concerned. The brown liquid is most likely water with
various contaminants mixed in. In your case it is probably caused by the
flue not staying hot enough all the way out. When you burn gas one of the
resulting products is water vapor. The way 80% and lower effeciency heaters
deal with that is to keep the flue temp high enough so the water vapor does
not condense out. In the original installation there was probably enough
heat going out the flue to do that because the original furnance was not
very efficient. As furnaces have gotten more efficient there is less heat
in the exhaust. It may be possible to install a liner in your old flue to
prevent this. Or you may need to have a newer flue installed. You do not
want to just ignore this because the other combustion byproducts make that
water corrosive and it will damage the flue and the furnace.
Well, the liquid is definitely condensation.
The flue from attic up to roof is 6" diameter asbestos/concrete
product - difficult to remove.
So I want to explore my other options. I tried lowering the fan speed
on my furnace - thought was slower speed = hotter exhaust through
flue. No luck.
The other problem I have, which may also be a cause is my thermostat.
Its a new digital programable Honeywell, and it attempts to keep the
house at exactly the set temp, without deviating a single degree. As a
result the furnace short cycles. Heat goes on for maybe 5 minutes,
There are no settings on the thermostate to help out, so how to remedy
Ideally, if I set the temp to 68, I want the furnace to kick in at 66,
then shutoff at 69. Having that 3 degree gap, will allow the unit to
run long enough so there is no condensation. But how do you force a
thermostat to do that?
There may be an adjustment in the thermostat. Check the paper work that
came with it. Otherwise try the original one and see if that helps.
You still may have flue problems. They make liners than go inside older
concrete/chimney flues. They are flexible. Your local hvac folks will have
plenty of experience putting liners in and it should not be too expensive.
Your current flue may be too big even for the combination of the water
heater and newer furnace. Condensation is acidic and will deteriorate the
concrete and cause it to develop leaks. That can result in carbon monoxide
problems or cause a fire. I'd get it fixed right rather than attempt to
work around the problem by playing with the furnace run time.
Maybe wrap it in insulation inside (the section of it inside the
That means more heat up the flue, which means a less efficient furnace
which means larger heating bills. No, you don't want that.
There might be a setting on the back of the stat, like a jumper or
small switch. It might be referred to in the instructions as setback,
span or hysteresis. It might be settable through the menu system on
it's front panel. If you can, set it to 2 or 3. It is probably set
to 1 right now.
Also, put a cover or shield over the stat (like the clear plastic box
you see in commercial situations like offices). This might help keep
it running longer in between calls for heat.
??? Are you for real? Wrap a oil furnace flue pipe with insulation up
in the attic where you dont usually see it?
You are scary man, real scary. Have you ever wondered why you need an
inch or two of clearance completly around the flue?
Here's a hint. Its NOT to keep the flue pipe from making babies.
Holey chit! You got something right!
More genius material at work.
Dumb, dumber and dumbest.
Longer run cycles mean longer off cycles. Hmmm, that ought to be a
really comfortable house.
Give it up "havclueless". You havent a chance
If his flu pipe is condensing then it's too cold to cause conventional
fiberglass insulation to "combust". Conventional fiberglass
insulation can be used up to 250 f (according to Owens Corning) and
probably higher (fiberglass insulation is non-flammable last time I
checked). There is pipe insulation that is rated for up to 800 f.
Because wood burns, and most framing, flooring and ceiling joists are
made of wood.
An attic flu that is not insulated is radiating heat inside the attic
and that is bad news in the winter for all sorts of reasons.
Insulating it will raise the inner flu temperatures and hopefully
prevent water from condensing inside it.
Did it ever occur to you that setting the span from 1 degree to 2, or
from 1 to 3, does the same thing (longer on time and longer off time)
That is exactly what the OP is looking for.
He could also turn down his burners so that the furnace runs longer
(but cooler) so that the heat output of the furnace matches the heat
loss of the house.
.....and like I said......If you wrap fiberglass insulation around a
oil burning flue pipe in an attic you are a clueless retard that needs
to borrow Stormy's rock.
Currently, my 90+% gas furnace puts out less than 10 PPM at the flue
outlet (thats, outside incase you didnt figure that out).
This is well within the norm.
You really should get a clue on what you are doing and the physics of
air and fuel combusting.
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