Rob R. wrote:
I asked a long-time code office about it once. He said it came about after the early 70's oil crisis when everyone added wood stoves/boilers to their existing chimneys. When a wood-burning appliance is connected to the same flue as an oil or gas appliance, the flue temps tend to be lower from the additional air pulled through the oil/gas unit's draft control. That increases creosote formation...and when there is a chimney fire, the draft control opens and feeds lots of air to the chimney fire. Many homes were burned down from this, and many more had the chimneys damaged or destroyed.
Apparently, its still allowed in Canada.
The National Standard of Canada, Installation Code for Solid-Fuel-Burning Appliances and Equipment, CSA-B365-01 clause 5.2.9 states that:
Two or more appliances shall not be connected to the same chimney flue unless:
a) The appliances are located on the same storey;
b) A negative pressure is maintained at the flue collar of each appliance;
c) Each appliance is provided with a means to control the rate of flow of air or flue gases through the appliance combustion and heat exchanger chambers. Where limitations on flue pressure (draft) are contained in the manufacturer’s instructions, such means shall be capable of maintaining at the flue collar, the pressure specified by the appliance manufacture;
NOTE: For the purpose of this Clause, draft controls include ash pit air control dampers, dampers or draft slides wherever located, over fire and flue key pipe dampers, and check draft controls located on or in the flue pipe.
d) The flue pipes of the appliances are connected to a manifold as close to the chimney as practicable or directly to the chimney in the following order:
(i) If only solid fuel is used, the flue pipe from the smallest appliance shall be on top (downstream);
(ii) If different fuels are used as permitted in Clause 5.2.7, the oil flue pipe shall be on top (downstream); and the solid fuel flue pipe on the bottom (upstream); and
e) The chimney flue is capable of venting the flue gas by natural draft when all appliances not interlocked to prevent simultaneous firing are firing at their maximum rate at the same time.
Not sure if this answers any of our questions though. I'm still looking for an answer as to "WHY" its not allowed. Seems like the creosote answer you gave makes the most sense when applied to wood. But it'd be nice to get an answer from an official source...like a town or code book...maybe even a few examples. Although, I feel we may be grabbing at straws ultimately.