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 the 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 fills 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).
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.
Figure 2.
Chimneys built on an outside wall, whether exposed or boxed with a chase, are prone to backdrafting (top).
One solution is to move the chase inside and to vent it to the interior so warm air can circulate (middle).
The best solution is to locate the system 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.
Successful Fireplaces in Tight Houses In addition, many modern fireplaces are used strictly as a design element, and many designers have no training in what makes one work.
On top of that, many of the masons and other heating contractors who build fireplaces carry over outdated design traditions that are at the root of performance problems.
It doesn’t have to be that way.
Building scientists now understand why traditional fireplace designs perform poorly, and masons, manufacturers, and hearth installers have responded with new products and techniques that eliminate past problems.
In this article, I’ll discuss the common causes of fireplace problems, and propose solutions for masonry fireplaces and heaters, as well as less expensive, efficient factory-built wood-burning fireplaces.
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" when it is cold outside 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.
In all cases, the cold hearth syndrome has two necessary ingredients without which it will not occur: a misplaced chimney (outside or too short) 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.
Unfortunately, moving a problem fireplace is not usually practical after it’s been built, but you may still be able to minimize the 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 using the same materials and techniques as sealed house wall construction.
The chase can then be vented to the inside so that warm house air circulating in it will keep it at about house temperature.
But 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 that doesn't fall below room temperature, the two ingredients that form the basis of reliable and stable draft.
Straight venting systems also work better, so elbows and offsets in the chimney should be avoided.
Makeup Air While improper design and location is a major cause of poor fireplace performance, tighter house construction and powerful exhaust fans must share some of the blame.
By installing vapor barriers and using doors and windows that have sealing gaskets, builders commonly reduce air leakage by more than 75% compared with the standard construction of 20 years ago.
And homes are now commonly equipped with high-volume exhaust fans, such as those in downdraft kitchen ranges, which can move air out of the house at a rate of 600 cubic feet per minute (cfm) or more.
Because tightly sealed house walls will not allow this much air back into the house through leakage, these powerful fans create negative pressure that can cause a chimney to backdraft and fill a house with smoke (Figure 4).
The reason is simple: Air flows to zones of lower pressure.
If a room is depressurized to the point where its low pressure overwhelms the chimney draft, smoke will flow into the room.
Obviously, ducting makeup air to the fireplace doesn’t work.
In fact, building code authorities are currently removing mandatory outdoor air requirements for fireplaces that were added only a few years ago, just before research debunked the idea.
Where a notorious air-guzzling downdraft kitchen range causes excessive depressurization, many homeowners will simply not use their range exhaust when the fireplace is burning.
But another solution is to install a makeup air system that is interlocked to the range exhaust switch.
This kind of makeup air system would force air into the house to compensate for the kitchen range exhaust flow.
This would prevent depressurization, and solve the smoky fireplace problem.
Controlled Combustion The design of the fireplace itself plays a big role in the level of satisfaction it provides.
The internal features that produce efficient, smokeless combustion tend to be the same as those that produce reliable chimney venting and trouble-free operation.
To help guide fireplace design, here is a simple rule of thumb that neatly summarizes a lot of expensive research: The more air a fireplace demands for normal operation, the more fussy and spillage-susceptible it will be.
Open fireplaces are the worst because they consume huge amounts of air — much more than is needed for combustion — which cools the system, thereby reducing draft.
If you insist on a traditional fireplace, make sure to equip it with doors.
Blocking most of the dilution air to the firebox causes the average exhaust temperature to go way up.
Higher average temperature means stronger, more stable draft.
If you don't want glass doors or much heat, consider a gas hearth.
Do the same if the architect’s plans call for a hearth in an outside chase situated at the low eaves of a cathedral roof.
If you can't relocate the fireplace more centrally, you will probably be happier with a direct-vent gas fireplace.