The Hearth in the House as a System

The cold hearth syndrome

A common complaint of fireplace and stove owners is that cold air and odors leak from it when it is not in use, a problem referred to here as the cold hearth syndrome. The causes of this problem and its solutions reveal some of the basic physics of chimneys.

A chimney is a vertical shaft enclosing at least one flue for conducting air and/or flue gases to the outdoors. When it is operating, the chimney flue contains air or gas that is warmer than the outdoor air. Because of its buoyancy, the warm air or gas rises, creating the desired upward flow in the chimney. This describes the proper function of a chimney.

There are cases in which the house performs better as a chimney than the chimney. The main evidence of this condition occurs when there is no fire in the appliance (standby mode) and cold air flows down the chimney, into the appliance and onto the hearth. There are two distinct causes of this problem.

Cause #1: Outside chimney and appliance low in the house

The first and most common cause of the cold hearth syndrome occurs when the appliance is located below the neutral pressure plane of the house AND is served by a chimney that runs up outside the house envelope.
On a cold day when the homeowner goes downstairs to light a fire, he or she might notice that the door and body of the fireplace or stove are cold to the touch. When the door is opened to light a fire, cold air flows out. If they light a fire, the smoke flows into the room. The syndrome also reveals itself by the presence of stale hearth odors in the room.

Here's what is happening: When an outside chimney is at standby, the air inside can easily cool to below room temperature. Since draft is dependent on a temperature difference, less draft is produced.

The house, on the other hand, is at a stable temperature from top to bottom. Because the appliance is located below the neutral pressure plane of the house, it is exposed to slight negative pressure, enough to pull the cold air down the chimney. This is a classic example of the house being a better chimney than the chimney.

 Cause #1: outside chimney and appliance low in the house

Note that the back of the fireplace and its chimney project out beyond the building envelope. When it is cold outside and there is no fire in the fireplace, negative pressure low in the house can draw air down the chimney and onto the hearth

The cold hearth syndrome occurs with masonry fireplaces and chimneys that share an outside wall or project out beyond an outside wall.  It also happens to a factory-built fireplace and its chimney that are installed in an external chase (right). In both cases, the air in the chimney is inadequately isolated from the outside cold and its average temperature falls below that of the house. Outside chimneys serving wood stoves and furnaces suffer the same failure.

External chases are usually insulated only at the back and sides of the fireplace, extending to ceiling level of the room in which the fireplace is located. It is also common for these chases to be fairly leaky. This combination of low insulation over much of its height, combined with the leakage of cold outside air into the chase, allows the average temperature of the air in the chimney to fall below that of the air in the house. If the appliance is also located below the neutral pressure plane, the cold hearth syndrome will occur.

The cross-section above shows that only the lower part of the chase is insulated and sealed. The upper part of the chase is just a framed and sheathed shell. During cold weather, the average temperature in the chimney at standby can easily be at a lower temperature than the house.

Falling Air?
It is often said that cold air leaks from a wood burning appliance simply because cold air is heavy and falls down the chimney. But this is a misinterpretation of the physics at work in chimneys. While is true that cold air is heavier than warm air, it is does not fall down the chimney. Instead, it is drawn down the chimney towards the zone of lower pressure low in the house. Air flows down the chimney because air always flows to zones of lower pressure, not because cold air is heavy.

Just to get our terminology straight, the term backdraft means the upward flow in a chimney fully reverses and 100% of the combustion gases from the appliance (if it is firing) and air in the chimney flow into the building.  If the fireplace or stove is firing, it is a hot backdraft; if there is no fire on when the reversal occurs it is called a cold backdraft.  Combustion spillage, on the other hand, is when only some of the products of combustion are released into the building, such as when a loading door is opened quickly when a fire is smoldering.

Vertically vented furnaces and water heaters located in basement utility rooms and served by outside chimneys regularly suffer cold backdrafting under standby conditions. This backdraft produces cold air spillage from gas appliance draft hoods or oil appliance barometric draft controls. The resulting pooling of cold air on the basement floor is often mistakenly attributed to a leaky building envelope.

Once the backdraft starts, it becomes stable and is difficult to reverse because the chimney is further cooled by the outdoor air flowing down through it. The stability of the cold backdraft is the reason it is so difficult to light a fire in a system that is cold backdrafting without smoke spillage into the room. Vertically vented gas-fired appliances that have draft hoods are often unable to overcome the backdraft when they fire and may spill their exhaust into the house for the entire combustion cycle.

Solution #1: inside chimney

Note that the fireplace and chimney are enclosed within the house and that the chimney has a higher NPP. A chimney that is inside the building envelope will always vent more reliably than an outside chimney because, even at standby, there is always enough temperature difference to create upward flow.

The first cause of the cold hearth syndrome can be prevented by installing chimneys only within the building envelope. The importance of this principle cannot be overemphasized. The chilling of the air and/or flue gases in a chimney exposed over much of its length to the outside will almost certainly inhibit its performance. Every fireplace or stove will benefit from having its chimney within the house envelope. A chimney installed inside the house envelope is not affected by the negative pressure low in the house due to stack effect because an inside chimney is always at or above the house temperature. Also, since the neutral pressure plane follows the leaks, the large opening at the chimney top means that the chimney has a neutral pressure plane that is higher than the one in the house. The result is a consistent upward flow of air through an inside chimney at standby. An inside chimney is always a better chimney than the house, subject to cause #2.
If a factory-built fireplace must be installed in an external chase, the chase must be sealed and insulated at least to the top of the building envelope and there must be no insulation between the chase area and the house. In order for chimney draft to compete with stack effect, the average temperature in the chimney must be the same as the temperature in the house.

If a chimney is kept warm by being inside the house, it will always make more draft than the house makes stack effect. The resulting low pressure zone at the fireplace opening will suck air up and out of the house.

Note that the chase is sealed and insulated right to the top of the building envelope and there is no insulation between the chase and the room. These characteristics will help to keep the air in the chimney at the same temperature as the house.

In a retrofit situation, the chase may need to be vented in to the room so that air circulation can keep the chimney warm.

Where a factory-built fireplace in an external chase is being retrofitted to an existing house, or to correct a cold backdraft in an existing system of this type, provision must be made to keep the fireplace and chimney at the same temperature as the house. One possible strategy is to vent the chase into the room so that warm room air can circulate within the chase (left). This strategy is only practical and viable if the chase is well sealed and insulated so that cold drafts are minimized. Note that if the system passes through two levels of the house, each level would need to be vented separately so that building code firestopping requirements would not be violated.

Cause #2: heated space higher than chimney top

In the illustration, notice that the top of the chimney is lower than the upper part of the house envelope. In standby mode (no fire) the house is a taller effective stack than the chimney. If the ceiling of the second floor of this house has leaks such as poorly sealed attic hatch and electrical penetrations, the house can be a better chimney than the chimney when no fire burns in the appliance. This is another example of the house being a better chimney than the chimney; the system would almost certainly suffer from the cold hearth syndrome, depending on the location and extent of leaks in the envelope.

 

The appliance and chimney should be located on the wall next to the two storey section of the house. In this way, the chimney could be installed to run inside the envelope and penetrate the highest part of the envelope.

The illustration shows an inside chimney installation, but if the chimney ran up outside the house envelope, the problem would be far more severe.  Effective stack (height) refers to the relative performance of the stack (house or chimney), in standby mode, rather than specifically to its linear height, but is influenced by actual height as well as temperature difference.  In the illustration above the house is a higher effective stack than the chimney.

Solution #2: chimney top must always be higher than the highest part of the house envelope

The chimney should penetrate the building envelope at or near its highest point, as shown above.

To avoid the cold hearth syndrome, it is essential that the top of the chimney be higher than the highest section of the house envelope.

The problem is created early in the design phase of the original house or of the addition to an existing house. The hearth retailer, installer or builder, faces obstacles to improving the design because the client, whether it be the home builder or homeowner, has already made a commitment to the design before selecting and arranging for the installation of the hearth appliance. Adjusting the design to prevent the cold hearth syndrome may not be as simple as adding height to the chimney.

First, raising the height of the chimney enough to exceed the height of the building envelope can make it look unacceptably tall and unsightly, or even create stability problems. Second, the extended chimney will be outside the building envelope for much of its length. With so much of its length exposed to outside, the additional height may be offset by excessive cooling and the problem may not be resolved.

It would be preferable to locate the fireplace or stove on the wall next to the two-storey section. In this location the chimney would have to be taller than the house envelope and could be installed up through the warm space.

Houses with cathedral ceilings present particular problems because the heated space extends to the peak of the roof. Ideally, the chimney in such houses should penetrate the cathedral ceiling close to the peak.

Ideally, the chimney should penetrate the envelope near the peak of a cathedral roof.

 Houses with cathedral or vaulted ceilings sometimes pose severe problems for chimney venting because the house envelope extends to the roof peak. Ensuring that the chimney is always a higher effective stack than the house limits the range of locations within the house that the combustion appliance and chimney can be placed.

In the illustration to the left, it is apparent that for the chimney to be a higher effective stack than the house and to be visually acceptable, it would have to exit the roof far closer to the peak than it does now. Unfortunately, from a design perspective, it is often desirable for the hearth to be located in the house at the lowest part of the eaves. Nevertheless, it is essential to deal with the issue because this configuration often produces chronic and severe cold backdrafting and hearth odors, particularly if the fireplace is installed in an external chase.
The cold hearth syndrome has become the most widespread problem with wood burners in modern homes. Complaints of hearth odors and cold drafts from the hearth have been heard by builders and hearth specialists all over North America. It appears that several housing trends that have emerged since the 1950s have combined to create this epidemic of cold hearths. First, house designers began to favor external chimneys and fireplaces in external chases whereas centrally located chimneys and fireplaces were previously the norm. Second, cathedral or vaulted ceilings have become far more common during the same period. And third, inexpensive, lightweight manufactured fireplaces with air-cooled chimneys have become more popular than masonry fireplaces in the past twenty years. If these air-cooled chimneys are not kept warm, a reverse flow will be induced by the slightest negative pressure in the house. The problem of the cold hearth is often mistakenly blamed on the fire's inability to "get enough air" through the tightly sealed envelope of modern houses. Certainly, negative pressure in the house resulting from the operation of exhaust fans can induce flow reversal in the chimney (and we will cover that issue later), but the majority of cold hearths can be traced to outside chimneys or chimneys that do not have adequate effective stack height.

Why the cold hearth syndrome is more than just a nuisance

Since air is not a pollutant, a fair question to ask is: What's the problem if cold air comes down the chimney, aside from affecting comfort? The problem is that the cold hearth is merely a symptom of an underlying system design flaw.

The chimney's function is to flow air and/or exhaust gases up and out of the house. When a backdraft occurs, the event must always be considered a failure of the chimney to do its job. Considering the chimney's role, a backdraft is a very serious and unacceptable event, rather like the wings falling off an airplane. It should not be permitted to occur.

Looked at another way, cold air comes down a chimney that is backdrafting, whether or not the connected appliance is firing. A chimney that is capable of backdrafting because it runs outside the house envelope or because its top is lower than the house envelope may either continue to backdraft when the appliance fires, or may go into backdraft while the appliance is still producing pollutants.

A chimney that is capable of backdrafting because of its relationship with the house envelope is an inherently unstable venting system that should not be trusted.

Never minimize the significance of backdrafting in a chimney. A backdraft in standby mode is always a problem, one that should be corrected if possible. Air pollutants, dust, odors and cold accompany the air that flows down a chimney, so both health and comfort can be effected.

Retrofit solutions for a cold hearth

If you are involved in the design of a new house or a renovation, always be aware of the potential for creating the conditions that produce backdrafting at standby: outside chimneys and chimneys that do not exceed the height of the building envelope.

Resolving a cold hearth problem in an existing building is extremely difficult since the root cause is the relationship between the building envelope and the venting system. The only sure solution is to change the relationship so that the chimney is enclosed within the envelope and penetrates the envelope at its highest point. This is often impossible, and even where it is possible, the cost may be prohibitive. Unfortunately, therefore, the solutions offered here are either complicated, expensive, or only partly effective.
Enclose in a sealed, insulated chase

An outside factory-built chimney could be enclosed within a sealed, insulated chase. However, unless special care is taken in the design and construction of the chase, this strategy may not be successful. To be effective, the chase must be well sealed and insulated so that it resists air leakage and heat conduction. Also, to ensure that the air in the chimney during standby conditions is at or near the indoor temperature, the chase cavity would probably need to be vented into the house.

In a retrofit situation, vent the insulated and sealed chase into the warm space of the building.

Since this strategy effectively makes the chase part of the envelope, a firestop would have to be installed at each ceiling level, so the chase must be vented into the house separately for each storey of the house. This venting could be accomplished by installing passive grilles between the house and the chase at floor and ceiling level of each storey. During standby conditions, the chase would gain heat from the house, effectively increasing the house's heat load. When the appliance is operating, the house would gain some of the heat lost by the outer shell of the chimney. This type of chase installation means that the chimney components for inside installation would be installed, including firestops and attic shield. The entire installation should be checked for safe clearances to combustible material.

Masonry chimney retrofits

A masonry chimney can be relined in a variety of ways in order to reduce heat loss through the masonry. Relining can be the least expensive retrofit for a chimney suffering from cold backdraft. By providing a degree of isolation for the exhaust gases or air in the flue from outside cold, relining can help to resist the fall in temperature. Of course, at standby it is quite possible that the average temperature in the chimney would eventually fall below that of the house and a cold backdraft would start. A lining system that includes insulation would help to slow the drop in temperature during standby and would likely improve the performance of the chimney over all. Although relining is an effective method for re-sizing and insulating the flue in a masonry chimney, and some performance improvement is almost certain, relining may not be an entirely successful solution to the cold hearth syndrome, particularly in cases of powerful backdrafts.

Theoretically, a masonry chimney could be enclosed in a sealed and insulated chase to make it function like an inside chimney, as was discussed for factory-built chimneys. But, there are practical considerations that could render this strategy unworkable. Primary among these is the fact that inside masonry chimneys require larger clearances to combustible materials than do outside chimneys. If a chase were built around a masonry chimney, the clearances would have to conform to the building code rules for a chimney built inside the envelope. Also, there are no recognized guidelines for terminating a combustible chase at the top of a masonry chimney.
Install a sealing cap on the chimney

The flow of air down an inoperative chimney can be reduced or eliminated by installing a specialized chimney cap that has a sealing damper. The damper in these caps is usually operated by a cable or chain that extends down into the fireplace. When the damper is closed, cold air cannot flow down the chimney.  Be aware, however, that although air cannot flow down from the chimney top, a convection current caused by the cold chimney liner surface can bring odors into the room. A set of tightly-fitting glass doors on the fireplace may be a necessary additional step.

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The use of a chimney cap/damper can be effective in blocking the backdraft in a chimney in standby mode.

Two important cautions apply to the use of top sealing dampers:

1. These damper/caps cannot be used with gas or oil fired appliances that operate on thermostats.

2. These dampers are mainly intended for masonry chimneys and fireplaces and their use on factory-built metal chimneys may be prohibited by the chimney manufacturer—check before installing one.

The fact that reliable remedial measures for the cold hearth syndrome are so complex and costly reinforces the importance of avoiding the problem at the design stage.

To conclude, the available strategies for resolving the cold hearth syndrome in an existing system are of two types: the first, like relining, chimney top dampers and fireplace door assemblies may not eliminate the problem, and the second, like sealed, insulated chases, tend to be expensive because they entail structural changes to the building. The cold hearth syndrome can be avoided at the design stage and this is certainly the time to address the issue so that remedial measures are unnecessary.

Summary

  • chimneys must be located within the building envelope to avoid the cold hearth syndrome

  • the chimney should penetrate the building envelope at or near its highest point

  • insulated chimney relines, tight-fitting fireplace doors and top sealing dampers can help to minimize the symptoms

  • to be effective, external chases must be constructed to be within the building envelope, all the way to the top of the building envelope.

  • backdrafting is a serious failure of the chimney to do its job, whether or not combustion gases are spilled

  • the cold hearth syndrome is extremely difficult to correct so it must be dealt with at the design stage

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