Successful Fireplaces in Tight Houses

Cold Hearth Syndrome But the biggest source of trouble is the location of the fireplace. Over the past 50 years of residential design, fireplaces have migrated from the center of the house to a position against the exterior walls, or even into chases that are completely outside the house. This causes cold hearth syndrome, which is the source of most fireplace failures. The most dramatic effect of a cold hearth is a predictable blast of cold air when the fireplace doors are opened to build a fire. Smoke tends to fill the room when someone tries to light a kindling fire. This is a common, even chronic, characteristic of North American fireplaces. The syndrome usually has its origin in the decision to place a fireplace outside an exterior wall in a frame or brick chase (Figure 2). Figure 2. Chimneys built on an outside wall, whether exposed or boxed with an uninsulated chase, are prone to downdrafting (top). One solution is to insulate the walls of the chase and to vent the chase to the interior so warm air can circulate (middle). The best solution is to locate the chimney properly in the first place. The ideal location is in the center of the house (bottom), because the surrounding air will keep the chimney warm and the chimney will penetrate the roof at its highest point. The cold outside air sucks warmth from the fireplace and chimney structure, causing the temperature of the air in the flue to drop. When the flue temperature is lower than the house temperature, air begins to flow down the chimney and onto the hearth. This is called a “cold backdraft” and contrary to common belief, it does not happen because cold air is heavy and falls down the chimney. The air is not falling — it is being sucked down by the house. Just as hot exhaust in a chimney produces a pressure difference called a “draft,” so the relatively warm air in a house produces a pressure difference called “stack effect.” The buoyant warm air rises, producing a slight low pressure zone downstairs and higher pressure upstairs. Since most fireplaces are installed on lower floors, they experience negative pressure due to stack effect when it is cold outside. As soon as the air in the chimney falls below room temperature, the house becomes a better chimney than the chimney itself, and a cold backdraft gets started. The backdraft tends to stabilize because as the chimney becomes full of cold air, it cannot produce any draft to resist the suction of the house.

Tall stack effect.

A similar problem is caused by chimneys that fail to extend higher than all of the living space in a house. A chimney that is not tall enough competes with the living space above it to establish the dominant draft (Figure 3). Figure 3. Chimneys built on outside walls are often too short to counter the house’s stack effect and are prone to downdrafts. Again, moving the chimney closer to the center of the house ensures a tall chimney with a strong draft. If the upper part of a house envelope leaks enough air through windows, attic access hatches, and wall penetrations, the house will again act as a better chimney than the chimney itself. In these cases, the house is said to have a “taller effective stack” than the chimney. Air will tend to flow down through the chimney, then loop through the house to exit through the attic or upper-story wall leaks. To avoid these problems, chimney tops should always penetrate the highest section of the conditioned living space. In all cases, the cold hearth syndrome has two necessary ingredients without which it will not occur: a misplaced chimney and a fireplace located low in the house. If we could move the fireplace and its chimney towards the center of a house, the syndrome would vanish. Or we could move the fireplace to the highest floor of the house where the higher pressure caused by rising warmer air would ensure a good draft. Unfortunately, moving a problem fireplace is not practical after it’s been built, but you can still prevent cold hearth syndrome by keeping the chimney from falling below room temperature. One way to do this is to trick the fireplace into thinking it is inside. This requires building a sealed, insulated chase that thermally matches the house wall construction. The chase should be vented to the inside so that warm house air circulating in it will keep it at about house temperature. A better solution is to design out cold hearth syndrome at the planning stage by bringing the fireplace and chimney in from the cold. Ideally the fireplace should be located centrally, in the heart of the home, so that the chimney will penetrate the roof closer to its highest point. This makes for a tall chimney with a large temperature differential between combustion exhaust and outside air — the two ingredients that make for the most reliable and stable draft. Straight venting systems also work better, so elbows and offsets in the chimney should be avoided.