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Chimney ducts and draft conditions: the sources of problems

The main function of a flue is apparently to evacuate fumes and gases burned. It is also to create a call of air which feeds the combustion.

A good draw

A good draw depends on a good conduit. And a good conduit depends on its length, its diameter. It is best if it is circular and straight.
And if it has to be deflected, two 45° elbows are recommended rather than just one 90°.
Outside, the wind is an important factor and often contrary to the good draft of the flue, sometimes requiring the addition of an extractor or a regulator.
Finally, note that the creation of outdoor air intake near the home, is also a way to improve the draw.

Operation of the chimney

The primary function of a chimney flue is to create a call for air to fuel the combustion and to evacuate the gas emissions to the outside. Good sorting is essential for good combustion. We usually consider that a good print is 10-20 Pa (1-2 mm).

  • The longer the chimney, the better the draft. One of the solutions to consider to improve the draw is therefore to extend the conduit. The diameter of this duct should never be less than that of the outlet nozzle of your appliance.
  • In principle, a circular duct makes it possible to obtain a better draft than a square duct. The presence of bends on the casing decreases the draft. However, if bends are to be used, it will be preferable to opt for 2 times 45° instead of 90° once.

► The combustion air inlet is essential for the draw
An open fireplace requires about 300 m3 of air per hour, while a closed fireplace requires only about 30 m3. An extractor hood or VMC draws in more air than a chimney. Their presence creates an inverted draw resulting in the presence of smoke in the room. An outside air intake near the unit will allow good combustion.
► The influence of the wind
The draw can be disturbed by the presence of tall trees, cliffs, high buildings. In principle, the solution will be to lengthen the conduit. In extreme circumstances, an extractor must be installed. A regulator can also stabilize the draft.
The draw is hot air rising. A high temperature creates a strong draft, a length and a diameter of ducts adapted to the device will ensure an optimal operation. Pulling too hard can result in too fast suction of hot air into the chimney. A draft regulator or a "restrictor" can solve this problem. The presence of several elbows on the duct reduces the draft.

The sources of problems of wood burning fireplaces

Attention to the size of the stove
An oversized stove can cause creosote and smoke, while an undersized stove induces permanent overheating, which can cause twisting and cracking of parts.
► What are the functions of a stove?

  • Heat source
  • Aesthetics (decoration piece)
  • Main heating or auxiliary heating

► A stove too big does not bring anything good
You will be forced to constantly use your stove set to idle, which will generate smoke, soot, tars and ashes. The window will be blackened.
A stove too small will not bring you anything good either. You will be constantly trying to get a heat that your stove will not produce. In permanent overheating, cracks or twists may occur and will not be supported by the Jøtul warranty (backing plates, baffles, etc.).
If the fire is difficult to light, the causes may be:
  • A lack of air: open the air inlets thoroughly. You can also leave the door open about 1 cm. Make sure the smoke damper is open.
  • Poor ignition materials: use small wood and crumpled newspaper, then add larger pieces of wood. And remember that the best little wood is the thinnest and driest.
  • Negative draft / cold duct: Heat your duct by burning crumpled newspaper and hold it up in the stove until the draft is reversed.
  • The presence of smoke in the room can come from:
  • The quality of the wood: a very damp wood can generate more smoke than can evacuate the chimney.
  • Ventilation systems, such as air conditioning, VMCs or extractor hoods, can meet their air requirements by drawing from the chimney (reverse draft). In this case, you will need an outside air supply to power your home.
  • Handling errors: always open the air inlet and the flue valve before reloading the stove - open the door gently.
  • Conduit: remember that a bend (90°) or a long horizontal pipe reduces draft.
  • A duct that is too short can reduce the draft.
  • A duct that is too cold can cause an inverted draw.
  • The tubing must be of the correct size and properly connected to the unit.
  • The duct may be obstructed by a bird's nest, soot, tar.

Too fast burning can be caused by:
  • The type of wood: large pieces will burn longer than small ones. Burn preferably hardwood that has been cut, split and stored in a dry place for at least 1 year.
  • A draw too strong in the duct
  • Handling errors: The primary air inlet control must be adjusted slowly, from the open position to the closed position, to be closed once the fire is well established.

Never open the ashtray door, it will lead to overheating.
A flashback (gas explosion) can be caused by:
  • Closed air control when loading the stove. You may have to wait up to 15 minutes before closing the air supply.
  • The secondary air supply can be blocked by soot or tar.
  • A wet wood requires a lot of energy to dry before it can burn. Accumulated gases can ignite abruptly and cause an explosion in the combustion chamber.
  • Inadequate print

► Control difficulties (overheating)
  • Damaged seals
  • Fuel: Never burn old pallets, sawdust, garbage or waxed products.
  • Excessive draft in the duct: Use a draft regulator to reduce draft.

► Insufficient heat
If you feel that the heat is too low, the most common reasons are:
  • Poor wood
  • Bad print
  • Handling errors
  • Construction of the house (insulation, ceiling height,...)
  • The stove is too small

When the wood burns slowly, it releases a low temperature, tar and other organic vapor which, combined with moisture, form creosote. This creosote will coat the walls of the duct.
► Creosote is the fuel of chimney fires
An excessive amount of creosote can come from:
  • A poor quality wood (humidity, sections too big)
  • A bad draw
  • Handling errors
  • The size of the stove (operating temperature too low)

Do not forget to have your pipe swept before the start of the cold season.

(photos / visuals: © Jøtul, except special mention)

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Chimney ducts and draft conditions: the sources of problems

FAQ - 💬

❓ What causes a chimney down draft?

👉 Downdraft is caused by something called the stack effect, which is when warm air rises, leaving less air closer to the floor. In two-story homes or homes with attics, warm air creates a high-pressure point towards the top of your home. Lower pressure points will attempt to regulate by pulling in air.

❓ What affects chimney draft?

👉 The chimney draft is what chiefly determines whether a chimney is doing its job properly. The factors that commonly affect the draft include flue size, air pressure, chimney height, and chimney obstruction. The flue size is crucial as regards proper operation of a solid fuel heating appliance.

❓ What is the most common cause of chimney deterioration?

👉 Over time, the bricks and mortar joints in most chimneys begin to deteriorate. Unfortunately, chimney deterioration is a natural process due to exposure to the elements, such as rain, snow, ice and winds. These elements can impact the chimney from all directions, so deterioration is often inevitable.

❓ Do chimneys cause drafts?

👉 A fireplace is a huge source of drafts from the cold outdoors. A chimney can act as a wind tunnel even when the damper is closed, sucking cold air down into your home through the fireplace opening. A fireplace draft stopper is an efficient way to prevent this from happening.

❓ How can I improve my chimney draft?

👉 How to Increase Draft in Your Chimney

  1. Start with a Hot, Fast Fire. Starting a cold wood stove with a hot, fast fire will get the draft going the quickest. ...
  2. Burn Extremely Low Moisture Wood. ...
  3. Improve Airflow Around the Fire. ...
  4. Warm the Chimney Flue. ...
  5. Provide Enough Air to Replace the Air That is Lost.

❓ How does chimney draft work?

👉 Chimneys operate on the principle that hot air rises above cold air. The movement of hot gases rising from the fire creates a pressure difference between the inside of the flue and the room. This is called a “draught” and it forces air into the fireplace, this air feeds the flames as it rushes past the fire.

❓ How do you fix a chimney draft problem?

👉 Solutions for a Draft Chimney Keep the damper open whenever a fire is burning. If your damper doesn't work properly, get it replaced. Leave a few of the windows cracked to allow fresh air into the home and smoky air out. Limit your use of green wood or wet wood straight from the outdoors.

❓ Does outside temperature affect chimney draft?

👉 once the temperature outside is considerably colder than the air temperature within, the cooler outside air can rush down the chimney shaft to interchange the rising warm air. These back-drafts may be prevented if the damper is kept closed if you are not using the fireplace.

❓ What causes chimney damage?

👉 Water & Excess Moisture Now, one major cause of chimney deterioration is excess exposure to water. Your bricks and mortar are very absorbent, and if they are not adequately protected, they will soak in lots of moisture, which will eventually wreak havoc on your structure as a whole.

❓ What is a common chimney or fireplace problem?

👉 Some common chimney problems are: chimney obstruction, creosote, brickwork problems, chimney crown and cap, and cracks in the flue. Chimney obstruction or blockage is caused by numerous things, including obstruction of the chimney draft.

❓ What is a chimney draft?

👉 The draft is the pressure difference between ambient air and the less dense flue gases within the chimney. The lighter gases are buoyant and rise to be displaced by heavier ambient air. The chimney must contain the hot gases and protect the surrounding materials against combustion.

❓ What causes a chimney to draft?

👉 In order for the fire to burn properly, the chimney must pull combustion air through the fireplace or stove. Here are some common causes of chimney draft problems, and possible solutions. Masonry fireplace flue sizing is determined by the size of the fireplace opening below.

❓ How do I stop a draft from coming from my Chimney?

👉 If opening a door or window near the fireplace or woodstove eliminates the draft problem, or if the problem only occurs when the kitchen stove or bathroom exhaust fans are evacuating air, vent the stove or fireplace to outside air. Chimneys draw a small amount of air, even when there is no fire below: this phenomenon is called ambient updraft.

❓ What is ambient chimney draft?

👉 Ambient chimney draft occurs because the top of the flue extends upward several feet, into a thinner atmosphere than exists at the bottom. Thus, air is drawn up the chimney in much the same way as liquid is drawn up a soda straw when you reduce the air density inside your mouth.

❓ Can a leak between fireplace flues cause an inadequate draft?

👉 But a leak between flues, such as an opening between a basement fireplace and first floor fireplace flues that are supposed to be isolated from one another can also cause inadequate draft for both fireplaces.


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