Background: New home and see-thru fireplace (Lennox Magna-Fire). It has an outside air kit feeding the inside of the firebox (think it is a 4" feed). The chimney is 13" round and extends well above the roof ... it has also had an additional 3' added to try and fix this problem.
With no fire, if I seal the fireplace with plastic (with the flu and fresh air kit both open), the plastic is sucked to the front of the fireplace. The only way to get it to drop off is to turn the kitchen vent fan on high ... no other vent fans and/or dryer, or open doors, in any combination cause a negative pressure.
The home is all electric so I do not have a furnace or water heater that is taking air out of the home.
When it starts to vent smoke into the home it is after the fire has been burning 30 min to several hours. It is usually a reasonable flame at the time. Opening a door or window seems to have no effect and can even make it worse by causing air to flow thru one side of the fireplace pushing smoke out the other.
At the time of the problem, no vent fans, dryer, ceiling fans or furnace blower, are running.
My thinking: Is the fire burning and taking air out of the home causing a negative pressure after X amount of time? I have no idea how to test this without some expensive equipment.
Solutions? Add an exhaust fan to the top of the chimney (http://www.northlineexpress.com/itemdesc.asp?ic=5CO-96214) ... expensive, noisy and not sure if I have a negative pressure problem if it will totally fix it. Or, add a blower to the fresh air kit to push more air into the firebox so that it does not draw air from the rest of the home. Not sure how much cfm to push into the firebox and this might take some trial and error, if it works.
Both of these solutions assume that I am creating a negative air pressure in the home.
Any other ideas that I have missed? I am getting tired of living in a smoky environment or doing without the fireplace.
Thanks for listening,