fireplace fumes

Furnaces need combustion air and it will draw it in from outside. Most times it brings it in through al the cracks around windows and doors, but the fireplace chimney is a good spot also.

To add to what Edwin wrote, I suggest that you may want to provide fresh outside air to the utility room with the furnace. It sounds like it does not have enough air now and even without the fireplace it could cause problems, maybe serious CO problems.

Fred

Stop using the fireplace. If it is bringing a smokey smell into the house, it is likely there is carbon monoxide coming in too.

Does it help if you turn off the furnace and water heater when you use the fireplace?

Get someone with a digital manometer to test your house pressures wiuth your furnace and fireplace running. Get THREE carbon monoxide testers in your house. It sounds dangerous to me. I would not try testing this your self, you could wake up dead some morning. If your house has a negative pressure with respect to the outside, it could be backdrafting both the fireplace flue and the flue that serves the furnace and water heater.

Good luck

Stretch

You have to check all the basics, e.g.

1. Does your fireplace chimney draw well? How much higher than the rest of the roof does the chimney cap reach?
2. Does your furnace have an independent source of outside air (now mandatory in Ontario, Canada.)

Frankly I would be concerned about the lack of draft even without the fireplace in use.