Q. I’ve got both a fireplace and a free-standing wood heater spaced 15 feet apart apart in my open kitchen and living area. The fireplace has a fresh-air vent in the hearth. I’ve only used the fireplace twice in 15 years because it pulls a sooty smell from the wood heater. Surprisingly, I do not get the smell when I use the wood heater. I’d like to install gas logs in the fireplace. Will gas logs pull air from the wood heater and cause the same smell?
A. I would be willing to bet money on my answer: Yes. Based upon your description, you are experiencing back-drafting.
When the fireplace is burning, it is consuming vast amounts (perhaps 250 to 300 cubic feet) of air each minute. The fresh-air vent in the hearth may be blocked, or it may not be able to supply enough air to satisfy the appetite of the fire. The additional air to feed the fire must come from somewhere. Simple physics will tell you that air enters a room through the path of least resistance. In your case, it appears the air has decided to enter the room through the flue of the wood heater.
Back-drafting is an enormous problem. It is a byproduct of the constant push to make homes more energy-efficient and airtight. The trouble is, homes are equipped with devices, such as bath fans and clothes dryers, that blast air, and lots of it, out of the living space. They also create a partial vacuum in your home.
If you do not have an air inlet to replace the exhausted air, the replacement air will leak in through windows, doors, chimneys, water heater and furnace exhaust piping, cracks in the house exterior, etc. If several appliances are operating, and the demand for air is great enough, exhaust gases from fuel-burning appliances can be sucked backward into your home. This can cause dangerous carbon monoxide to be pulled back into your home.
Many modern furnaces and water heaters have built-in pathways for the air needed to burn the fuel. In fact, virtually every residential building code mandates that combustion air be supplied to fuel-burning appliances. But more air than combustion air is needed in the average home. For years, commercial and institutional building codes have required this extra ventilation, or “make-up,” air. This air creates a healthier interior environment, ensures that plenty of fresh air is entering the building and that dangerous back-drafts are avoided. It is in your best interest to provide this ventilation, or make-up, air.
This thinking is not new by any means. There are companies that actually sell sophisticated air-exchange devices that provide some or all of this needed ventilation, or make-up, air. In my opinion, there may be more practical and far less expensive solutions.
One device I have seen is ingenious. It looks similar to a dryer exhaust hood, only larger. Instead of having one pipe feed into this hood, there are two 4-inch-diameter pipes that connect side-by-side to this fresh air vent. One of the pipes connects directly to the cold-air return duct of your forced air heating and cooling system. When your furnace or air conditioner operates, fresh air is pulled into your home. The other 4-inch pipe serves as a primary or auxiliary combustion air supply. This pipe terminates near your fuel-burning appliances. In your particular case, you could possibly terminate this pipe very close to your fireplace to provide the necessary added combustion air.
You may wonder if the introduction of cold or hot air into your home will increase your energy bills. In actuality, they may stay the same or drop. The air that is consumed by the fans, vacuums, fireplaces, etc. is coming in anyway to offset the vacuum produced when these things operate. By installing a simple device to cause this air to enter in just one place instead of many different places, the thermostat that controls your furnace and air conditioner may actually cycle on fewer times each day.
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