Chimney Physics Russ Dimmitt - Chimney Safety Institute of America - National Fireplace Institute
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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
1
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.
2
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.
3
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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
4
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.
5
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.
6
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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.
7
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
8
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
10
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
11
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
13
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
14
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
16
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
17
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
19
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
20
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
22
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
23
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
25
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
26
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
28
Exterior Venting Systems
• Exterior systems are
surrounded by cold air
• Exterior systems will always
perform more poorly than
an interior design
29
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
31
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
32
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
34
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
35
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
37
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
38
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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132/24/2020
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
40
Troubleshooting
• Air leaks in the flue, such as
missing mortar joints, allows air
to enter inhibiting draft and
cooling flue gases
41
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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142/24/2020
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.
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