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Burning Hunk
The damper inside of the plenum parallel connection is also not a permitted add on furnace connection in Canada.
You can only add on a wood burning furnace using a downstream connection or the 'divider plate method'.
As per the certification standard CSA B366.1 and by the installation code B365 these are the only two add on connections permitted. Also note, some add ons themselves aren't certified for divider plate installations, in which case it would leave only the the downstream option. Further, cert requirements for add ons to oil were developed in '81, for electric in '84, and gas in '85.
But the basic requirement of any add on installation is that the original furnace must continue to operate under the same conditions as it did before the add on was installed. Tests should be done before and after for supply air temp and pressure before any alteration. Then tested again after the add on connection and then adjusted to produce the same results as before.
Things are somewhat strict and specific here the moment it becomes a furnace or a interconnected add on. That is what was throwing me earlier I think when people were talking about using a furnace as a space heater. As soon as ducting goes up, the requirements for clearances and air balancing either as a standalone, or the requirements of a add on aren't a simple affair.
Squisher, I think what you are referencing is what my friend has done with his. Its not an electric bypass damper or anything of that sort. Its a slide plate that goes through the pipes added on after the furnace.
Similar to this. See where the pipes are goign in from the DAKA to that add on box? Imagine a large slide plate that interrupts those at the plenum. It has to be manually moved into place.