Posts tagged with "Chimney Airflow"

Types of Chimneys, Vents and Connectors

Chimneys, Vents and Connectors: a Guide

On part two of our guide to chimneys, vents and connectors, we will be covering the specific types of chimneys, vents and connectors. Refer to our initial chimney terminology guide for more general information on chimneys, vents, connectors and flues.

Types of Chimneys

There are two major types of chimneys: masonry and factory made. Masonry chimneys are made of brick or block and require lining for proper safety. Stainless steel liners are preferred.

Factory-built chimneys are often referred to as “class A chimneys”. This terminology is not official, but chimney professionals use it and understand its meaning. Class A Chimneys always have a stainless steel interior and a galvanized or stainless steel exterior. If the class A chimney runs outside without a chase, stainless steel is always used.

Class A Chimney System PartsClass A chimneys are insulated to prevent the outside of the chimney from becoming excessively hot. There are two types of insulation used: packed pipes and air insulated pipes. Packed pipes have a double wall with insulation between the layers to help absorb the heat. Air insulated chimneys can have up to three or four walls without insulation between them. In these chimneys the air space is used to help absorb the heat. These may also be called “Air Insulated” chimneys.

It’s worth noting that Class A Chimneys must always be used as a whole unit. Mixing parts from different brands/makes is extremely dangerous and strictly prohibited.

Types of Vents

Vents are used for the venting of gas, oil and bio-mass appliances. They are never used as a chimney for a solid fuel such as wood. While using multiple brands isn’t optimal, adapters are sold to allow the use of pipes from multiple vent brands.

Type "B" Gas Vents/ConnectorsType B Vents are factory built double wall vent pipes that are only used to for venting gas. They are always made with a galvanized exterior and an aluminum interior. The air space between walls is fairly small. This vent can be used as a vent or connector, and is quite inexpensive.

Type L vents can be either a vent or a connector, and is made to vent oil. Class A chimneys are still preferred in the market over L vents, and as such L vents availability is limited. Sometimes it is listed for “bio-mass venting”, or venting the products of combusting Pellet Ventpellets, corn, cherry pits, etc.

Pellet vents are technically L vents as well. These vents must be installed through a house or be in a chase. While they have stainless steel interiors, their exterior may be black or galvanized steel.

Types of Connectors

Type C vents are used only as connectors. They are single walled galvanized pipes, and as such often called “galvanized pipes”. They are used only for venting gas or oil. Using a C vent with solid fuel appliances can cause extremely toxic fumes. This is the least expensive of the pipes. Inspectors mandate that when used, C-vent crimps must go away from the appliance towards the chimney or vent. This isn’t an official rule and there’s no specific reason for this to be necessary, but is simply a standard on installation. Inspectors will make you reinstall the vent with the crimps pointing the ‘correct’ way, so it’s best to just install them appropriately from the beginning.

Black Single Wall Stove PipeBlack single walled pipes are also only used as connectors. Sometimes they are referred to as “black galvanized pipe” even though it is not galvanized. While black single wall pipes can be used for solid, gas, or oil venting, it’s expensive and overkill for gas and oil. To prevent condensing creosote from leaking out of the pipes, crimps must point to the stove. This isn’t an official rule, but it is a best practice is you don’t appreciate the smell of creosote.

Double walled stovepipes are used for reduced clearance solid fuel, and used only as a connector. They’re more expensive than single walled stovepipes as they are made of double walled pipe with an air space insulator.

When making decisions about vents, connectors or chimneys, it’s always wise to work with an experienced chimney and vent specialist. They can help you navigate installation to ensure your work passes inspection, looks great, and works well for years to come.

Chimney Airflow Problems

Understanding chimney draft problems is not necessary for most people. Usually, if you notice smoke not rising from your chimney, you can call on a professional to fix the issues.

chimney draft problems

Draft problems can cause you problems!

This information is for those who really like to understand; it may be too much information for many people. I’ll do my best to keep it as interesting as it can be. If you’re a do-it-yourselfer or the owner of an old home, you’ll probably get a lot of out of it.

Understanding Chimney Draft Issues

To understand the problems, you need to understand what draft is. Draft is what we name the effect of how the air flows up the chimney. It’s measured in “inches of water column.” Draft then is the combination of volume, speed, and pressure of the flue gasses. And temperature of the gasses comes into play here as well.
For matters of this discussion, chimney draft is usually thought of as the speed at which the vented gasses travel up the stack, or pressure of the gasses. This can also be referred to as the stack effect. A common question might be “how strong is the stack effect?” Good draft conditions mean that the vented gasses are traveling up the chimney quickly rather than slowly or not at all.

How Does Chimney Draft Work?

The reason smoke (or other flue gas) goes up the chimney at all is because of the vacuum in the chimney. The question you should ask now is “a vacuum relative to what?” The general answer is that it’s relative to the air in the house. Don’t read too much into that because it gets tricky (for example, how does replacement air get into the house?- because the house environment is a relative vacuum to the outside. Yet the inside of the house is not a vacuum compared to the chimney.) Let’s keep this simple and just talk about the chimney. The pressure in the chimney is typically less than that inside the house. Thus, the draft effect is caused by air inside the chimney being pushed up the chimney by the house air.

And why is there a difference in pressure in and out of the house, or in and out of the chimney? There can be a few reasons, but the biggest and most important reason is the temperature difference from one place to another. Remember that when air is heated it expands? The same amount of air occupies a larger space, or you could say the same amount of space has less air (fewer molecules of air.)

The air outside the house in the winter is colder and heavier than the warmer air in the house. It pushes its way into the house (or is it pulled, depending on how confused you want to be.) The air in the chimney just came from a fire so it’s really hot and expanded and being pushed up the chimney to the cooler air outside where warm air rises, right? That’s buoyancy. Problems occur when these processes don’t happen correctly.

Diagnosing Chimney Draft Problems

Draft is measured with a pressure meter that has a probe which goes into the smokepipe. The meter should register a negative number, and generally speaking for residential heating appliances that number would range between -0.02 to -0.04. Zero or a positive number means the gasses are not going up the chimney. And too large a negative number can have its own set of consequences; but that isn’t usually the problem. Mostly “a draft problem” means the gases are not going up the flue, this is merely a minor chimney repair.

Causes of Draft Problems

Chimney Airflow Issues

Chimney Airflow Issues

Now there are other reasons for draft problems. One is called Dynamic Wind Loading. or “DWL.” DWL is caused when the wind blows on one side of the house and causes a positive pressure, and creates a corresponding negative pressure on the other side of the house.

If the windward side of the house is tight and the lee side (negative pressure side) isn’t, the vacuum resulting from the wind can suck air out of the house. And the most likely source of that air is the chimney; it’ll pull down on the chimney, smoke and all and keep it from exiting your house! Or if a gas furnace is being vented you won’t see smoke but you still get the carbon monoxide.

The way to deal with that is to tighten up the lee side of the house and then put in an outside-air source. There are kits for that or you can just crack a window on the windward side of the house.

Chimney Draft Issues Caused by Fans

Big Kitchen Vent Hood

If your ears pop when you turn on your kitchen fan, you’ll probably have chimney draft issues.

The other large reason for bad draft is when chimneys have to overcome fans in the house. Kitchen fans, bathroom fans, radon fans. It doesn’t take much of a fan to overcome a natural draft appliance (such as a fireplace or woodstove) Again, the best answer is to allow “make up air” into the house.

The problem with that of course is that you don’t want a draft across the floor and you hate to purposefully introduce freezing cold air into the very house you’re trying to heat. It’s a Catch 22, but I can tell you CO poisoning is a bad thing, and smoke in the house is a bad thing. You just may have to make some choices.

Air Flowing Down Your Chimney

Finally, sometimes air actually blows down the chimney, but less frequently than you’d guess- it’s usually something else. But maybe your chimney is short and next to a larger part of the house or a bigger building. The same problem occurs if your house is located at the base of a mountain. If you have this problem, a Vacustack is a good solution if you can’t raise the chimney to the proper height.

How Do You Stop Smoke From Crossing Over From One Fireplace Chimney To Another?

If you have two fireplaces with flues running up the same chimney structure, (like the picture) you are often a candidate for the problem of smoke crossover. Smoke crossover is when one chimney is breathing out smoke from the fire, and the other chimney is breathing in outside air to equalize the pressure in the home. The inhaling chimney flue is also sucking back in smoke from the other nearby chimney.

Many people assume closing the metal damper on the unused fireplace will stop this smoke crossover, but that is not the case. Dampers are not tight enough to stop this air draw. The smoky fireplace needs to be air sealed to stop the smoke. That is why a chimney balloon is needed in the unused fireplace to stop the air draw inward through the second flue.

You would measure your fireplace flue for a chimney balloon and then install that chimney balloon low and tight in the fireplace that is drawing smoke. Air sealing the smoky fireplace with the balloon forces the house to find another location (i.e. windows and doors and other envelope penetrations) to draw outside air in at where there is no smoke. This keeps the fire burning in the one fireplace and keeps the smoke outside where it belongs.

This troubleshooting article was submitted with permission by Chimney Balloon USA.