Chimney Physics

Russ Dimmitt Chimney Safety Institute of America

Learn what The House as a System means specifically to you as a retailer and installer of appliances that must vent to the outside atmosphere. This session will include topics like: • building envelopes, • pressure to measure pressure, • stack effect and what causes it, • house systems and exhaust fans, and • techniques to allow hearth appliances to function properly.

2020 HPBExpo Education Sponsored by:

. 2/24/2020

Chimney Physics Russ Dimmittt Director of Education of the Chimney Safety Institute of America and the National Chimney Sweep Guild

Draft & Flow

• Draft – the force or pressure • Flow – the volume of gases difference between the inside which actually pass through the and the outside of the venting system as a result of draft. system.

Draft

Draft can be further defined as the measure of force which causes gases to move up and out of the chimney and air to be drawn into appliance.

Two major factors influence draft: 1. The difference between the average flue gas temperature and the temperature of the outside air. 2. The height of the chimney.

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Temperature Difference

• Warmer flue gases have greater buoyancy than cooler gases

• Warmer less dense gases rise and are replaced by cooler, heavier gases

• These rising gases create a vacuum at the appliance and combustion air is drawn into the combustion chamber

Temperature Difference

• The greater the temperature difference between the gases inside the flue and the air outside the chimney, the greater the draft will be and air will be drawn more forcefully into the system.

Chimney Height

Taller chimneys contain a taller column of warm, rising gas.

The movement of the gases also increases the draft, however, the height is the critical factor, not the volume or movement of the gases.

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Measuring Draft

Chimney draft is measured in inches of water column.

Good draft would be represented by a negative number because the pressure in the chimney is less than the surrounding atmosphere.

Lower negative numbers represent greater draft.

Measuring Draft

‐.08 in. WC represents greater draft than ‐.04 in. WC

‐.04 in. WC is greater draft than +.04 in. WC

‐.1 in. WC is greater draft than ‐.01 in. WC

Measuring Draft

• Older technology utilized U‐tube manometers or analog manometers to measure draft

• Technicians now use digital manometers to measure draft

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Flow Capacity

Flow is the volume of flue gases which actually move through the venting system as a result of draft.

Three factors which influence flow capacity are: 1. Draft 2. Amount of resistance to flow 3. Size of venting passageways

Resistance to Flow Friction always exists between moving gases and the flue walls. Variables affecting friction – or resistance to flow – include:

Bends & turns in venting system

Changes in size or shape

Surface irregularities (mortar protrusions, etc.)

Appliance air inlet settings

Relationship of Draft and Flow

• Fireplaces require large flow capacity but not strong draft.

• The dilution air sucked into the fireplace promotes rapid combustion which requires large flow capacity to vent the high volume of flue gases. Due to the large flow capacity draft may be weaker

• Closed appliances such as inserts and free standing stoves require strong draft but not large flow capacity

• Because combustion air is limited the combustion is slower and the reduced volume of flue gases require less flow capacity

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Troubleshooting

• A chimney must perform three functions

• Safely exhaust the products of combustion to the outside

• Protect surrounding combustibles from the heat of exhaust gases

• Draw oxygen into the appliance to sustain combustion

Troubleshooting

• When troubleshooting follow a basic three step process

• Gather information on the symptoms of the problem

• Make a tentative diagnoses and test if possible

• Specify effective solutions or refer the customer to and expert to provide solutions

Troubleshooting

• Start gathering information by interviewing the homeowner and making observations of the installation. Some questions that may need to be answered: • Is the problem intermittent or constant?

• When does the problem occur?

• Is the problem worsening or does the severity vary?

• Are there any consistent circumstances that are related to the problem?

• What solutions have been attempted thus far?

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Troubleshooting

• If it can be done without undue risk light the appliance and replicate the problem, make a diagnosis and attempt a trial solution

• Try one solution at a time

• Try cheapest and easiest solutions first

• Be aware multiple problems may exist on any system

Troubleshooting

• Draft is an important element of chimney performance

• Draft is expressed in negative numbers as it is a suction

• Draft must be sufficient to overcome negative pressure in the home

Troubleshooting

• Troubleshooting appliances can be challenging as several factors may be in play that add up to a problem. Some things to investigate: • Chimney height • Vent design • Negative pressure in the home • Fuel choices • Burning habits • Wind • Remember the house will function as a system

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Troubleshooting

• When troubleshooting open fireplace issues it is useful to understand the homeowners expectations

• Performance issues with fireplaces generally are related to smoke spillage • Wind induced • Flow reversals due to pressure issues • Inadequate flow • Inadequate draft

Troubleshooting

• Wind may cause smoke and flue gases to be forced from the chimney into the home

• Wind induced issues are typically sporadic and occur under specific circumstances

• The issue may be caused by high pressure at the termination or wind blowing down the flue

Troubleshooting

• Barriers downwind from the chimney may cause wind to “pile up” and create a high pressure zone at the top of the chimney • Barriers upwind may create turbulence which blows down the chimney • Raising the chimney height may put it above the problem area • Specially designed chimney caps are available to combat wind problems

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Troubleshooting

• Negative pressure in homes is an ever increasing issue for chimney technicians

• Negative pressure has two basic causes in a home

• Mechanical systems that exhaust air from the home

• Stack effect

Troubleshooting

• As homes are sealed tighter to aid in energy efficiency, it becomes more difficult for natural draft appliances to function

• Exhaust devices such as clothes dryers, attic fans, kitchen range hoods and central vacuum systems all exhaust air from the home. When a home is tight often the easiest way for replacement air to enter the home is down the flue

• Natural draft appliances will lose when competing with mechanical systems

Troubleshooting

• Stack effect is caused when heated air rises and leaves the home

• The home functions as a chimney and the taller the home the greater the stack effect

• Pressure will change throughout levels in the home

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Troubleshooting

• Any appliance in the negative pressure zones of the home will have to overcome the negative pressure to vent

• The neutral pressure plane will follow the leaks, if a window is opened the neutral pressure plane will move to that location

• The lower the location of the neutral pressure plane, the less the negative pressure at the lowest levels

Troubleshooting

• Stack effect can not be eliminated but it’s effects can be minimized or eliminated by sealing leaks at upper levels

• Other appliances operating in the home may overpower an appliance at a lower level causing spillage or back drafting

• Bringing air in a the lowest level possible can minimize stack effect and allow appliances to operate

Interior Venting Systems

• Installations completely within the structure are surrounded by warm air • Interior systems will always perform better than outdoor systems

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Interior Venting Systems

• Promote fast, easy start‐up • Maintain better draft • Reduce condensation and creosote • Rarely have a “smoky tail‐out” • Radiates heat indoors, not outside

Exterior Venting Systems

• Exterior systems are surrounded by cold air • Exterior systems will always perform more poorly than an interior design

Cold Hearth Syndrome

• “Cold Hearth Syndrome” occurs when cold outside air flows down a chimney that is not currently in operation • Common causes are: • Exterior chimneys serving an appliance located below the Neutral Pressure Plane • Building envelope is higher than the chimney

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Exterior Venting Systems

• “Cold‐Hearth Syndrome”

• Makes it hard to initiate a fire

• Promotes condensation of flue gases and increased creosote through cooling effect

• Have a lower draft as a result

Higher than Heated Area

• Whether located indoors or out, the chimney should terminate higher than any portion of the heated area (building envelope) Heated Area

Chimney Termination

The 3‐2‐10 Rule

All residential chimneys must terminate at least 36” above the point where they pass through the roof and 24” above any structure within 10 feet of the chimney.

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Troubleshooting

• Leaky duct work may affect pressure levels in the home in a way unrelated to stack effect

• Operating exhaust fans will exacerbate pressure issues

Troubleshooting

• Mechanical draft inducers may create other problems in the home. Draft inducers are not a solution for pressure issues. They are to be used for sizing or construction issues

• Additional air introduced into the home in a proper location will solve many issues

Troubleshooting

• Inadequate flow is characterized by some of the flue gases evacuating the home but spillage still occurs

• Inadequate flow may be caused by sizing issues, restrictions within the flue or insufficient draft

• Sufficient draft can exist but spillage may still occur due to sizing or restrictions in the flue

• Improperly sized flues or obstacles will prevent adequate flow

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Troubleshooting

• Codes and standards give proper sizing for fireplace flues if they are straight and at least 8 feet in length

• One‐twelfth the fireplace opening for round flues

• One‐tenth the fireplace opening for square or rectangular flues with an aspect ratio of less than two to one

• One‐eighth the fireplace opening for rectangular flues with an aspect ratio of greater than two to one

Troubleshooting

• If a fireplace is smoking and inadequate flue size is suspected, the technician can test the theory with a sheet of metal. By gradually lowering the sheet of metal you can determine at what point the fireplace stops smoking. That reduction is the amount of permanent reduction needed in the fireplace opening either with a smoke guard or masonry work

• Debris including creosote may reduce the flue opening to the point the restriction may cause inadequate flow

Troubleshooting

• Chimney caps and top sealing dampers may cause restrictions at the top of the flue

• Changes in shape, size or direction of the flue may restrict flow

• Issues with construction of a fireplace may create issues, these my be in the smoke chamber shape and size or the firebox

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Troubleshooting

• Corbeled bricks in the smoke chamber can create turbulence and inhibit flow

• Codes and standards call for a smooth smoke chamber often this is not the case

• Parging the smoke chamber will enhance flow

Troubleshooting

• Air leaks in the flue, such as missing mortar joints, allows air to enter inhibiting draft and cooling flue gases

Dangerous!

• Spillage and backdrafting can be dangerous! • Noxious smoke and carbon monoxide can enter the building • Can be life threatening if it is not corrected

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Chimney Physics

• Remember a chimney or vent that has spilled or back drafted should not be trusted as the conditions for failure exist until corrective action is taken and that chimney or vent can not be trusted to perform as intended.