backdraft through furnace with fireplace lit

hot water tank and furnace vented through same pipe into 6x9 flue when i lite a fire in fireplace smoke comes out of furnace and hot water tank it is fine unless i have fire going can i use a draft inducer fan to correct prob have lennox model g2oq3e-75-4 fireplace has 2 12x12 flues have tried raising flues chimney caps on and off please help

Back drafting the furnace and or hot water heater is a very dangerous situation as it can pull carbon monoxide into your house and potentially kill you. Seriously, I would get a carbon monoxide detector or two and install them in the bedroom and in your central living space. Then I'd consult a chimney expert to see why the chimney flues are interacting this way.

Not an expert but what i know about fireplaces is that your problem is your fireplace. It is not vented or the vents are clogged not your flues. When your fire is burning it is sucking oxygen from the house and not from outside thru the vents.

Pete:

Why not try opening a window when you're using your fire place. (I'm presuming you're not using your fireplace for heat since you have a furnace.) I'm thinking that air, just like any other fluid, is going to take the path of least resistance, and a big open window prolly offers less flow resistance than your furnace's flue pipe.

Test the effect of the window with a candle. Get the fireplace going, open the window and place a burning candle under the boiler or on the furnace heat exchanger. If the flame is stable, watch what happens when you close the window. If the flame starts to dance around, then you've got downdraft through the furnace flue pipe. If opening the window causes the flame to become stable again, then that's at least a means of controlling the situation.

Question:
My understanding is that natural gas only burns to form carbon monoxide when there's insufficient air for complete combustion. Under normal circumstances natural gas burns very cleanly to form CO2 and H2O. If Pete has both his chimney and furnace going, he's noticed that there will be flue gas spillage from his furnace. Doesn't that mean that there's a downdraft through the furnace flue pipe and an updraft through the chimney, and doesn't that mean that there's an air draft through the house that will ensure there's plenty of oxygen for complete combustion of the natural gas?
I'm fully aware of the dangers of carbon monoxide. I just can't really see why carbon monoxide would form if there is a downdraft through the furnace flue pipe providing the furnace with plenty of air. The flue gas from the furnace would be CO2 and H2O, and it would migrate toward the chimney, where the stronger updraft is and be removed from the house.

If you have a backdrafting issue, get a professional to check the drafts.
The problem comes when you start changing the dynamics of the home, opening windows will help to change the flows, but as soon as you close them to go to bed for the night, you are back to square one. This is when the CO will sneak up on you.
It is not always the furnace that will give off the CO, the fireplace has it's own demons...wood.
Next town over, 3 kids died in a sleepover , one fireplace backdrafted into the basement they slept in. The fire got low, the furnace kept kickin on because the flue was open in the upstairs fireplace....the kkids closed the window at night because it was cold...and the downstairs fireplace became makeup air for the furnace. Dragging the CO across the rooms they slept in.

Please, Just get it fixed so the next person does not need to possibly loose a family member.

thanks for the help have 3 co2 detectors in house what about the draft inducer fan? anyone install one of these?