Richard T. Kreh, a masonry consultant in Frederick, Maryland, and author of Building with Masonry (The Taunton Press, 1998), replies: I doubt that the downdraft problem you describe has anything to do with the fireplace components. More likely, from what you said about having to open windows and doors, the problem stems from a lack of air supply for combustion in the fireplace.
You did not indicate in your question if there was a vent in the hearth or room that provides air when the fireplace is operating. If no vent is present, I strongly recommend installing one. It’s best to have the vent installed directly in the hearth, but if that is not possible, then a vent should be installed through the wall in the room somewhere.
If, as you stated, the house is built “tight,” then negative pressure can be created when a fan from an appliance or the furnace pushes air out of the house. The result is that the house has to suck in outside air, and a chimney is a likely source. If you have a fire burning, the downdraft will force smoke into the room at the same time. Again, a vent that introduces outside air can help to solve this problem.
Another possible cause of downdraft is an improperly sized flue liner. The size of the liner can be checked by measuring the size of the flue at the top of the chimney. The bluestone caps you placed over each chimney could also be the culprit here. The caps might create swirling air currents that can result in a downdraft. If you’ve eliminated all other possibilities, I’d suggest removing one to see if it makes a difference.
Another possible cause of downdrafts could be an air inversion from strong winds blowing over high trees or hills close to the house. In that situation, the usual solution is to add extra height to the top of the chimney. And while you’re at it, check that the chimneys have been built at the proper height above any adjoining roofs or the nearest ridge. The general rule is that 3 ft. above the roofline is sufficient, but I am sure this detail is spelled out in your local building code.
Downdrafts in any chimney are stronger and more likely to occur when winds are stronger. Adjusting the damper can go a long way toward eliminating downdrafts when the wind is really blowing. In my own fireplace, I have to close the damper almost completely to cut down the excess draft during strong, gusty winter winds.