Cob Chimney?

[Don Stephens]

Chuck Learned <clearned at bminet.com> writes:..."Mike I am currious why you
have two flue's. Also I am wondering how high above the roof the chimney 
needs to be?"  

Might I assume that "two flues" implies that each has its own path within
a single chimney?  Is each flue lined and if so, with what?  It would seem
that a liner of fired clay with rounded corners might be the best bet, but
I wonder if, when expanding as heated, the liner might apply undo stress
on the cob around it, if they were in direct contact.  A thought on a
possible fix would be to wrap each liner with a "crushable" like rock wool
or fiberglass blanket (unfaced) and then wrap the two together with
"poultry" wire to provide a bond to the cob applied around both.  This
might also help address Frank's concern about low flue temps and moisture,
see below...

Frank Duffy (frank at chimney.demon.co.uk) wrote:..."The main problem could
be moisture from the inside rather than from the outside;...when the
chimney serves a stove, cooker or other closed appliance...(they) have
high flue gas temperatures though a smaller flue gas volume.  So you
have less actual heat put into the chimney.  This means that condensation
occurs in the flue and can lead to liquid running down the inside or
soaking in and destroying the cob.  The other problem could be that the
(modern stove) flue (may be) taking higher  temperatures...550C  (1000F)"

Frank-what part of UK?  As to Chuck's question about how high - some codes
call for 3 to 5 feet above highest point of roof within 10 feet.  If
one were using a combustable roof surface like wood shake or thatch or
wanted more draft lift, more might be worth considering (along with a
spark screen), but the higher she goes, the more vulnerable to tumbles and
the more need for vertical reinforcing!  How about some input from folks
that have lived with cob chimneys for a number of years, to help inform us
mudpuppies? - Don Stephens, Spokane Wa USA