Correctly installed, they improve both safety and performance

Here are two documents created for the U.S. hearth industry association on the selection and installation of chimney liners. One is advice for homeowners and the other covers things that dealers and installers should consider.

If you are thinking your chimney might need a retrofit liner, the information in these documents will really help you to understand what the issues are before you make the decision.

Note that the rules for liners are slightly different in Canada, with the result that insulation around liners is not used as often as in the US.

Both documents are in pdf format.
Advice for retailers
Advice for homeowners

Aerodynamic effects are complicated but being aware of them helps in diagnosing venting problems

Air is a fluid that has weight, so when it gets moving it exerts pressure on anything that gets in its way. If you have experienced a fifty mile per hour wind, you know all about it. Just like water, air flows in eddies and currents when it gets turbulent, as it does flowing around obstacles. The fact that air is invisible makes diagnosing wind-induced venting failure mostly guesswork, but there is some science that provides guidance.

The higher the velocity of air flowing over a surface, the lower the pressure it exerts.The higher the velocity of a stream of air, the lower is the pressure that it exerts on the surface it is flowing over. It is this principle that gives an airplane wing its lift. For the same reason, wind flowing over the top of a chimney can increase draft by producing a driving pressure that assists in pulling exhaust gases from the chimney.

Despite the fact that wind flowing over a chimney can produce a driving pressure, it cannot be depended upon for appliance performance because it is variable and unpredictable. The only dependable driving pressure in a chimney operating on natural draft is produced by temperature difference.

Wind can force exhaust back down a chimney without a cap.For example, wind can often flow down towards the top of a chimney after passing over an obstacle like a roof, adjacent building or trees. Wind may also approach the top of a chimney from below after flowing up a roofline to a chimney penetrating the peak. Wind tunnel testing has demonstrated that wind flowing from either above or below the chimney top can be adverse to upward flow by creating positive pressure at the top of the chimney.

Note that the thick black line in this and the other house drawings on this page is the building envelope, which contains the insulation and vapor barrier that encloses the warm spaces of the house.

Wind approaching the open top of a chimney from above can cause positive pressure in the flue.

Less obvious is that wind approaching from below can also cause positive pressure at the top of the chimney.

A chimney with no cap is the most vulnerable to the adverse effects of wind. A cap, particularly one that has baffles to prevent direct line of sight access to the opening (as opposed to a simple flat rain cap) provides significant protection from the adverse effects of wind. In fact, research has shown that caps with baffles (of the sort common on factory-built chimneys) can actually enhance draft regardless of wind direction.

This kind of cap can take adverse winds and convert them to upward flow in the chimney.The cap shown at right is the type of design common on factory-built
chimneys. Note that the baffle, in the form of a band between the cap and the skirt at the base of the cap, prevents direct access of the wind to the open top of the chimney. This simple design consistently produces a driving pressure at the top of the chimney, regardless of wind direction or speed.

Adverse pressure can also occur when the top of the chimney is in a positive pressure zone caused by the velocity pressure of the wind as it flows against a raised part of the building behind the chimney (below). This is one case in which adding to the height of the chimney may help to resolve a wind-related venting problem.

Hearths installed in single story sections of two story houses almost always perform badly.

Adding height to this chimney could get its top above the positive pressure zone and also make it higher than the second floor ceiling.

Some caution is warranted when diagnosing what may appear to be wind-induced venting failure, particularly when the chimney already has a suitable cap. For example, the householder might report the intermittent puffing of smoke from the appliance that occurs only on windy days. The pulsing effect of wind gusts clearly plays a role in this type of smoke puffing, but is it the only cause? Other contributing factors could be low flue gas temperature due to fire smoldering, an outside chimney, or a chimney that is shorter than the building envelope as in the illustration above.

Often, wind gusts simply cause a vulnerable system that borders on failure to spill the distinctive puff of smoke that implies wind-induced downdraft. At one time or other, most chimney sweeps and technicians have recommended the installation of a specialized "anti-downdraft" chimney cap only to find that it did not cure the problem. Adverse pressure caused by wind acting on the chimney top is rarely the only cause of a venting problem. Nevertheless, chimneys in locations such as the one above may be susceptible to wind-induced failure, partly because they were failure-prone to begin with.

The Neutral Pressure Plane

Wind causes pressure changes inside as well as outside a house. This drawing is simplified; the real world is much messier and harder to predict.In cold weather the buoyancy of the warm air in a house causes a slight pressure difference from the highest to the lowest point. The pressure high in the house is positive relative to atmospheric pressure and it is negative low in the house. Between the high and low pressure areas is a zone of neutral pressure called the neutral pressure plane. When the air is calm, the NPP is roughly horizontal. The idea of the NPP and its likely position under various conditions can be useful in describing what happens to pressures inside a house in windy weather.

The force of wind blowing around a house produces a positive pressure zone on the windward side and a negative pressure zone on the downwind side. These pressures act on the leaks in the envelope, causing air flow through them and changing the pressures within the house. These pressure changes are best illustrated by looking at their effect on the position of the neutral pressure plane. The NPP can tilt away from the horizontal, but no illustration can properly convey the ragged, messy shape that the zone of neutral pressure can be distorted into by wind effects. Perhaps the best way to visualize the wind-induced pressure variations in a house is to compare the NPP to the surface of rough water. The plane of neutral pressure will have waves, curves, peaks and valleys responding to the aerodynamic influences around the building envelope. This understanding renders inherently inaccurate any simple attempt to define and illustrate the position of the NPP under windy conditions.

In strong winds, the pressures experienced by the building envelope can be very powerful—several times the normal pressures produced in chimneys through natural draft. In gusting winds, the pressures and position of the NPP are in constant change, further complicating the diagnostic process.

If the majority of leaks in a house are on the downwind side, the whole house can be severely depressurized.The design and setting of a house can influence the pressure environment inside during high winds. Imagine that the house on the left backs onto an attractive ravine and that the architect located most of the windows to take advantage of the view. The majority of leaks in the envelope could be on the exposed two-storey section. When a strong wind blows from the front of the house, the entire interior could be placed under negative pressure. This effect could have disastrous consequences for a hearth system installed inside.

The effects of wind acting on leaks in the building envelope can cause wild fluctuations of the pressure inside. An open window on the downwind side can cause the pressure inside to become extremely negative. Likewise, an open window on the windward side can pressurize the house. This effect can help to explain many venting failures of wood heating systems and illustrates the importance of looking at the whole house, and not just the top of the chimney, when diagnosing venting failures. That is, wind acting on the building envelope can cause smoke to be sucked out of a stove or fireplace due to negative pressure in the house.

Top view: As wind flows around a house, it is more likely to depressurize than to pressurize the inside because of aerodynamic effects.

The effect of wind on the pressures around and inside a building are complex and unpredictable. In general, however, the leakier the building, the more pronounced and immediate is the effect on pressures inside. The unpredictable effects of wind pressure is one reason why the installation of a specialized chimney cap may not cure a venting problem. The pressure changes inside the house may be either driving or adverse to the desired flow of exhaust gases up the chimney.

The illustration to the left shows why wind is more likely to depressurize than pressurize a house as it flows around it. The air flowing parallel to the sides of the house exerts a pressure lower than atmospheric pressure on the house surfaces. Combined with the negative pressure zone on the downwind side, this means that three of the four sides are likely to experience negative pressure. This is a simplified example. In reality, aerodynamic effects are more complex than this.

Building codes call for the chimney project at least three feet above the highest point at which it touches the roof and that its top must be two feet higher than any roofline or obstacle within a horizontal distance of ten feet. Like all building code provisions, these are the minimums allowable and may need to be exceeded in order to meet performance objectives.

Although the effects of wind are unpredictable, one thing is abundantly clear: wood heater and chimney systems of good design are highly resistant to wind-induced venting failure. A chimney that is installed inside the envelope, that penetrates the roof near the peak and that has a baffled cap is unlikely to be negatively affected by wind.

JG

A fireside chat (rant) about chimneys for architects and builders

By John Gulland

If you design or build houses I need to have a serious talk with you about chimneys. I know that chimneys are hardly the most glamorous aspect of the building business and maybe your eyes glaze over when subject comes up, but I'm here to offer a different take on chimneys, one you probably haven't heard before. So respecting your busy schedule and your lack of interest in chimneys, here is the bottom line: chimneys belong inside houses.

The facts demonstrate without question that masonry chimneys built onto the sides of houses so their profile shows, or metal chimneys enclosed in framed chases, even though they might look alright, don't work well at all.

In fact, I suggest that a chimney hanging off the side of a house like an afterthought is an abomination, functionally and aesthetically.

Chimneys belong inside houses. I'm serious.

But as I look around, it's apparent that the majority of houses less than 50 years old were designed and built by people who don't share my views. Outside chimneys are rampant. They are everywhere, hordes of them in tract developments, and ones and twos stuck on big custom houses. It's not a class thing — the urban rich and the rural poor all seem to get outside chimneys these days.

Another thing I've noticed, a lot of people complain about their fireplaces and wood stoves being fussy and hard to light without getting a room full of smoke. And they complain because when it is not being used, the doors and the hearth are cold. If your houses have fireplaces, you've probably heard the complaints. Hey, you might be one of the complainers.

I know what you're thinking. You think I'm going to make a connection between outside chimneys and annoying fireplaces. Well, there is a connection and I can prove it, if you'll let me explain.

And it's not just fireplaces – wood stoves suffer the same problems when connected to outside chimneys. Although oil furnaces have fans that pump exhaust gases into the chimney, their outside chimneys spill a lot of cold air into basements between firing cycles. Conventional gas furnaces and hot water heaters are famous for spilling their exhaust gas as well as cold air from the chimney into basements. The common feature of all these failures to flow properly is the outside chimney.

I usually talk about fireplaces because they are the object of most complaints. People don't give a damn what their gas furnace is doing, unless chronic backdrafting leads to carbon monoxide poisoning. But when the male of the species has romance on his mind, or more serious still, is about to demonstrate his superior fire-building skills for the neighbors, and the room promptly fills with smoke, the air may be blue with more than smoke. Anyway, the science is the same for all chimney vented combustion equipment and the science says put the chimney inside.

You think I'm stalling. Okay, here's the proof. A chimney is an essentially vertical structure enclosing a space full of air and/or exhaust gas. When it is operating, the contents of the chimney flue are warmer than the outdoor air. Because of its buoyancy, the warm air and/or exhaust gas rises, creating the desired upward flow in the chimney. The flow and the force that cause it are referred to as draft.

Chimneys are in the business of expelling air and/or exhaust gas outside. It is no trivial matter when outside air comes down a chimney into a house. Backdrafting, as it is called by those in the know, is roughly like the wings falling off an AirBus. It is precisely the opposite of the desired behavior. It is a catastrophic event in the life of the chimney.

Most builders and maybe even some architects working in moderate-to-cold climates have heard of the "house as a system" principle which suggests that the house functions as a system rather than as a number of unrelated parts and that its various sub-systems, particularly those that move or contain air, behave in an interactive way — one might say they influence each other. You probably knew that already.

And this: When it's cold outside, the warm air inside makes the house act sort of like a chimney. The warm air in the house wants to rise because it is less dense, more buoyant, than the cold air outside. So, when it is cold out the air pressure high in the house is positive, slightly higher than the atmospheric pressure outside. And the air pressure low in the house is negative, slightly lower than atmospheric pressure. This phenomenon is called stack effect. Somewhere between the high pressure high in the house and low pressure low in the house is a zone of neutral pressure which is called, rather cleverly, the neutral pressure plane.

Now that we have the ingredients assembled, we'll build a truly lousy fireplace just to examine the backdraft phenomenon. This particular one we'll build out of bricks although it could just as easily be a factory-built fireplace and metal chimney enclosed by a framed enclosure or chase.

The fireplace is located on the first floor of a two story house. The first thing we decide is to have the back of the fireplace and its chimney project out from the brick veneer wall of the house. The projection is wide at the bottom and tapers above the fireplace to the outline of the chimney as the brickwork rises. It's a nice architectural element, don't you think, adding interest to an otherwise blank wall? As is normal in this type of construction, there is insulation in the walls of the rooms upstairs between the chimney brick and the drywall.

It is 0°C or 32°F outside and the basement furnace is keeping the house at a comfortable 21°C or 72°F. There is no fire in the fireplace, and hasn't been for days. The couple who bought the house are sitting in the living room near the fireplace and she comments that her ankles are cold. He reaches down to the carpet and verifies that it's cool there. They trace it to the fireplace and start to gripe about the jerk that built the house or the mason, or whoever it is they feel comfortable blaming.

Let's just stop here and take stock. The chimney is brick with a clay tile liner, no insulation. For much of its length there is an insulation barrier preventing the chimney from gaining heat from the house. The chimney gives up its heat to the outside and as the average temperature of the air in the chimney falls, the draft declines and the upward flow in the chimney becomes less stable.

Meanwhile, the house is at a stable temperature from top to bottom which is higher than the average temperature in the chimney now that it has cooled. The negative pressure low in the house due to stack effect is more powerful than the draft being developed in the chimney and the chimney backdrafts. Remember the AirBus? The couple who bought the house are suffering the cold hearth syndrome and are ticked off as cold outside air gushes down the chimney onto the hearth and into the low pressure zone caused by stack effect in the house.

The cold hearth syndrome is caused when the house acts as a better chimney than the chimney. You might think that's a trite little saying and actually that's the reason I like it so much — and the fact that it's true and accurate in every way. The house works better as a chimney because the air inside it stays warm, buoyant and wants to rise, unlike the air in the outside chimney that gives up its heat to the great outdoors.

Another thing worthy of note is that a cold backdraft like this is quite stable. Once the air starts flowing down, the chimney really cools off fast. That is why when you light a fire in a backdrafting fireplace, there's a good chance you'll get a face full of smoke.

Although our example uses a brick fireplace, note that a factory-built fireplace with its backside hanging off the side of the house in a flimsy frame chase is every bit as likely to spill cold air, odors and smoke into the room as is a masonry fireplace with its back showing from the outside. The common cause of their failure is their outside location. Bring the same systems inside and they'll work fine.

Here is the harsh reality: When you combine an outside chimney with an appliance installation below the neutral pressure plane of the house, the system will suffer the cold hearth syndrome during cold weather. Period. The result is just as certain for furnaces and water heaters, only it's not called the cold hearth syndrome, it's called a cold basement.

Now, I don't know about you, but I find this astounding. In many areas of North America the majority of chimneys run up outside the building envelope, outside the heated space. And I just showed that if you do this, an appliance installed low in the house will screw up when it is cold outside. Don't you think we should have talked about this sooner? Don't you think someone should have said something?

It's fashionable lately to talk about houses that are so tight that the stove or fireplace — or whatever — "can't get enough air". Meanwhile, the chimney is out in the cold, crippled from the start by its location. The not-enough-air claim is mostly nonsense. Few houses are so tight that a healthy chimney can't pull enough air to run a heating appliance. Open fireplaces, having a huge appetite for house air, are an entirely different matter.

Let's be clear – take the same chimney and move it inside the house envelope, to the warm side of the insulation, and it will be transformed. It will make draft, lots of it, and quick as kindling. This chimney will always perform better than the house and even when there is no fire burning, it will gently tug the air at each leak in the fireplace. When you open the doors to light a fire, air from the room rushes in and up the chimney. When you light the kindling fire the smoke goes up the flue immediately and you'll have a hot bright fire very soon. It's a fine chimney, you'll say with satisfaction.

If you work at it, you can overcome even a good (read inside) chimney, by, for example, turning on a large exhaust system like one of those downdraft kitchen range exhausts for indoor barbecuing. Some of these suckers are powerful enough to make your ears pop, or at least to backdraft a fireplace chimney. Here's my advice for people with chimneys: Barbecue outside. You don't have to like my advice.

If you can't do without the monster kitchen exhaust, you could hire an engineer and have an equally monster fan-forced make-up air system designed and installed, one that is interlocked to turn on when the range fan is switched on.

There are builders who tell me they won't give up the expensive floor space that the fireplace would occupy if it wasn't hanging off the side of the house in a chase. To them I say, fine, then start building chases that are truly inside the building envelope, part of the heated space. Run the insulated chase to the top of the house envelope, seal it properly and do not isolate the chase from the house with insulation. Of course that would be fussy and expensive detail work to do properly. But if you see yourself as a quality builder, you are kidding yourself to do less. You can either act on my advice or you can listen to complaints. Pick one.

One last thing, I'd prefer you didn't shoot the messenger. I've just given you a brief physics lesson on how gravity and temperature affect air flow. You learned that chimneys belong inside houses. Now quit fighting it and quit complaining. Start putting chimneys inside. You'll be a better, more successful builder for it.

JG

A chimney is a long-term investment so it pays to get the right chimney installed properly. It is easy to make mistakes in selecting and locating a chimney, so here is some reliable advice that will help you avoid problems and get maximum performance from your chimney and wood burning appliance.

Summary list of things to consider:

• Match the chimney to your stove or fireplace
• Put the chimney inside the house
• Locate the hearth in the heart of your home
• Your new chimney must meet safety codes
• Give the chimney some of the heat
• Maintain your chimney investment

Match the chimney to your stove or fireplace

To work properly, your new wood burning appliance needs a chimney that suits its characteristics. There are two main factors to match:

Size

In general, the chimney flue (the hole that gases flow through) should match the size of the outlet on the stove. Bigger is definitely not better when it comes to chimneys. In special cases (like with a very tall chimney), a chimney flue can be smaller than the appliance outlet. Always get professional advice before selecting a chimney of a different size than the stove outlet.

Insulation

High performance stoves and fireplaces need high performance chimneys. And high performance chimneys always have insulation. If you have selected a purely decorative fireplace without gasketed doors and air controls, then an uninsulated chimney like conventional masonry or air cooled metal might be satisfactory. But if you have an advanced stove or fireplace, you'll need a high performance chimney like a 650°C model, a specialized fireplace chimney or a lining system in a masonry chimney.

Put the chimney inside the house

By far the biggest mistake you can make in buying a new chimney is to have it installed up the outside of your house. Outside chimneys allow leakage of cold air and odors into the house when there is no fire burning in the stove, and can lead to smoking when a fire burns. Here is why.

When it is cold outside, the warm air in the house wants to rise, just like hot exhaust rises in a chimney — this is called stack effect. Stack effect creates a slight low pressure area in the lower part of the house and a slight high pressure at the higher levels of the house. A neutral pressure plane (NPP) lies between the high and low pressure zones. If there is no fire in the stove or fireplace and the air in an outside chimney cools to below room temperature, the house becomes a better chimney than the chimney, and sucks cold, smelly air down the flue.

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. The neutral pressure plane always moves towards the big leaks, so the large opening at the chimney top means that the chimney has an NPP that is higher than the one in the house. The result is a gentle upward trickle of air through an inside chimney when no fire burns in the appliance.

If a chimney is kept warm by being inside the house, it will always make more standby draft than the house makes stack effect. The resulting low pressure zone at the fireplace opening means that when you light a fire it will kindle quickly without smoking.

Save yourself a lot of grief: don't fall into the outside chimney trap.

 Locate the hearth in the heart of your home

This is more than a hackneyed homage to homespun values, it is one key to a successful stove or fireplace. This fireplace will not cold backdraft or be fussy to light.By locating your hearth inside the house and not against an outside wall, the chimney will rise through the warm space for most of its length and will exit the roof near its peak. The result is excellent chimney performance, unaffected by cold temperatures or harsh winds. Don't make the mistake of locating a chimney low on the eaves of a cathedral roof, or in a one story section of a two story house. If the chimney exits the warm part of the house below its highest level, the house might function as a better chimney than the chimney.

By locating the stove or fireplace so that the chimney can rise straight up through the tallest part of the house, you will get the very best performance from both the chimney and the appliance.

If the top of the chimney is lower than the highest living space in the house, cold air will leak from the appliance when there is no fire burning in it. Simply raising the chimney rarely helps because of excessive cooling.

Your new chimney must meet safety codes

A high priority is to ensure your new chimney is safe so you won't have nagging worries about fire putting your home and family at risk. Here are the options:

Factory-built metal chimney

For a wood stove you would select a 650°C (HT in the US) metal chimney, which is a high-performance insulated unit. For fireplaces, the manufacturer will specify the correct chimney and your dealer can tell you what the options are.

Masonry chimney

A conventional masonry chimney, made of brick, block or stone could be a suitable option, depending on the appliance you choose. There are several types and brands of liners that can be installed in masonry chimneys to improve their performance. If yours is a high performance appliance, you should consider an upgraded liner. Note that full stainless steel liners are mandatory for fireplace insert installations (Canada only, but strongly recommended).

Give the chimney some of the heat

You might think that any heat flowing into the chimney with the exhaust is waste, but it is not. The chimney needs heat to stay clean and free from corrosive moisture. Don't be afraid to burn the stove or fireplace hot, at least for a brief period, each time you start a fire or reload on coals. This practice heats up the appliance and chimney structure and `primes' the chimney to make reliable draft for the rest of the heating cycle.

Maintain your chimney investment

Chimneys must withstand corrosive compounds, extremes of temperature and repeated heating and cooling cycles. Each time a chimney heats up, its materials expand and when it cools they shrink. This constant movement takes its toll on joints, seals and the structural materials. There is also the potential for the build up of combustible deposits called creosote. To protect your investment, check the chimney for creosote regularly and have it professionally cleaned and inspected annually (fire codes require chimneys to be inspected and cleaned at least once a year). By following this advice, your investment will be protected through preventative maintenance before serious damage occurs.