The Art of Venting
Flue Gases
Or
How Not To Kill Your
Client
Presenter: Earl Hicks

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Review New York WAP policy regarding venting systems
BPI Standards
Review combustion process
Define & understand combustion air
How does a vent system work
Identify venting categories and materials
Using venting rules-of-thumb
Inspection of existing flues
Alternative venting methods

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Ensure safe elimination of flue gases from the building

Building Performance Institute
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Any combustion appliance chimney/vent system that is in use must be visually evaluated for defects
A deteriorated chimney should be repaired or relined and causes corrected before reusing.

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Three essential components for combustion
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Fuel
Oxygen
Heat
Fuel – Fossil fuels
Oxygen – Air is 20.9% O
2
Heat – Pilot, spark, or igniter

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CH
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4
+ 4O
2
+ heat = CO
Complete combustion
2
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Natural gas
+ 2H
2
O + heat
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1 Cu Ft of CH
4
+ 10 Cu Ft of air for complete combustion
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11 Cu Ft of flue gases

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Must supply sufficient air for complete combustion when all appliances are in operation simultaneously.
Must determine whether CAZ is a confined or unconfined space per NFPA.
Tightening a dwelling too much may result in the need to bring in outside air for combustion regardless of NFPA classification.
Must follow established standards for bringing in combustion air.

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Confined Space
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Not enough air in the combustion appliance zone to provide for complete combustion when all appliances are operating and the building is set in worst case criteria.
Unconfined Space
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Enough air is present to provide for complete combustion when all combustion appliances are operating and the house is set for worst case criteria.

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Measure the volume of the CAZ.
Add all input Btu ratings of appliance in the CAZ.
If dryer is in CAZ
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Electric – consider input rate of 100,000
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Gas – consider input rate of 125,000
Divide this total Btu by 20.
The resulting number is the Cu. Ft. separating confined from unconfined space.
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Volume of CAZ below result = confined space
Volume of CAZ above result = unconfined space

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CAZ = 10’L X 14’W X 8’H
Appliances
• Gas furnace – 75,000 Btu input
• Hot water heater – 40,000 Btu input
• Gas dryer - 125,000 Btu input

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CAZ = 10’L X 14’W X 8’H
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Volume = 1120 Cu’
Total appliance input Btu
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240,000 Btu
1/20
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240,000 ÷ 20 = 12,000 cu ft.
Actual volume is below 12,000 cu ft
CAZ is a confined space
Results: Additional combustion air must be brought into the CAZ

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IMPORTANT! The above standard is based on a known infiltration rate of less than .4 air changes per hour
When the ACH is known
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For appliances other than fan-assisted, consult NFPA 54 8.3.2.2(1)
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For fan-assisted appliances, consult NFPA
54 8.3.2.2(2)

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Within the dwelling, from other zones
Ducted in horizontally from OD
Directly from OD above and or below

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Negative pressure within a flue that pulls products of combustion out from the dwelling.

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Delta T, Indoors to Outdoors
Height of Vent
Interior Volume
Restrictions
Atmospheric Conditions
Pressure Imbalances in the CAZ

Air Flow
Combustion Gases

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OD temp >80 0 F, >-1 Pa or - .005”WC
OD temp 30 0 – 80 0 F, >-2.5 Pa or -.01”WC
OD temp below 30 0 F, >-5 Pa or -.02”WC
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(250 pascal = 1”W.C.)

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Return air leaks in the basement
Long horizontal vent connectors
Blocked vents
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Deteriorated flue
Bird nests
Outside masonry flues with fan assisted heating appliances
Fireplaces with no outside combustion air and without front enclosures.
Overly tight houses

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NFPA 54 and 58 criteria
Standardized
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Category I
• Negative pressure, non-condensing
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Category II
• Negative Pressure, condensing
• No longer produced
Category III
• Positive pressure, non-condensing
Category IV
• Positive pressure, condensing

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What we normally think of when we think chimney
Negative pressure sucks products of combustion from the appliance breech and deposits them outdoors

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No longer manufactured
Negative pressure vent with combustion gases at or below the dew point
Heavy gases at the dew point are not buoyant enough to vent with a negative pressure flue

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Direct sidewall vented without additional apparatus.
Positive pressure requires joints in flue material to be sealed
Because these 80% appliance flue gases are close to the dew point, and the vent material is single wall, corrosion resistant materials must be used
Drains are typically incorporated to remove flue condensation before it enters the heat exchanger

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Has been used to solve installation problems where no appropriate flue is available
Positive pressure requires joints in flue material to be sealed

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Positive pressure condensing appliances, joints must be sealed
90%+ AFUE
Appliances are designed to dispose of flue condensate as well as condensate formed within the secondary heat exchanger
May be able to sidewall vent at reduced distances to openings in the building than NFPA suggests
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Sealed combustion
Combustion air intake in same pressure plane

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Single wall galvanized pipe, 26 gauge
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Only as a connector on 70% AFUE and lower gas appliances and all oil appliances
Connector for oil appliances
Masonry
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Vitreous clay liner
Oil appliances
Transite
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Rated as single wall
Asbestos
Does not meet any venting requirements

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“B” Vent – double wall
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Galvanized steel outside, aluminum inner pipe
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Rated only for gas appliances
Used as a vent connector for all 78% and
80% AFUE appliances
May not be run outside of the building
Used as a liner in an existing flue chase

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Flexible liner
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Aluminum – gas appliances
Stainless Steel – gas and oil appliances
Used to retrofit existing flue passages to meet code or a specific application
“L” Vent – double wall
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Inner and outer pipes are stainless steel
Oil appliances
All Fuel, double wall insulated
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Oil
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Solid Fuels

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Aluminum
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No longer recommended because of corrosion problems
Stainless Steel
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Use type specified by manufacturers to reduce corrosion issues
High temperature plastic
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Was the most common material specified by manufacturers
Have been involved in recall
Manufacturer specific installation protocol
• Sealed joints, High temperature sealant
• Hanger spacing
• Pitch back to appliance ¼” per Ft.

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PVC
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Low cost
Sealed joints
Must be provided with adequate hanging support
Pitch – back to appliance ¼” per
Ft.
CPVC
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Higher operating temperatures than PVC
Follow manufacturer instructions
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Manufacturer specific
Sealed Joints

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Water is a byproduct of combustion
Key for category I appliances is to maintain water in a gaseous state within the flue so that it exits to the outdoors.
Category IV appliances are engineered to remove liquid water from within the secondary heat exchanger and flue.

• 93# (lbs) ~ 10 gallons
• Million Btu = 100,000 Btu @
10 Hrs run time
• 10 Hr run time for natural gas = 10 gallons of water that must be removed from the flue.

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21 Cu Ft flue gases for every 1 Cu Ft of natural gas burned
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Draft diverter mixes the extra ~10 Cu Ft of air with the flue gases before entering the flue
Reduced efficiency means more heat going up the chimney, hot flue gases (450 0 – 600 0 F)
Dilution gases reduce the relative humidity and increases the dew point

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1 Cu Ft CH
4
+ 10 Cu Ft air
+ 10 Cu Ft of dilution air mixed at draft hood
21 Cu Ft of flue gases up chimney under maximized conditions
Draft hood
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 aids in minimizing fluctuation of draft
Prevents backdraft from affecting burner
Spill Switch

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Draft induced fan does not force flue gasses into the vent.
A category I flue of adequate design must be used
More heat in the building and less up the chimney yields a colder chimney
Cycle time is longer than older furnaces to adequately warm the flue and keep moisture in a gaseous state.

 If firing rate = 100,000 Btu/hr and SSE = 75%, then 75,000
Btu/hr go to the distribution system and 25,000 Btu/hr go through the vent.
If the burner on-cycle is 12 minutes (.2 hrs), then during one cycle the vent receives:
.2 hr x 25,000 Btu/hr = 5,000 Btu/cycle
MASONRY CHIMNEY:
A masonry chimney (block or brick + tile liner) requires about
4570 Btu/.ft. to go from 0 o to 120 o . So, on a very cold day, about one foot of chimney will be warmed in one burner cycle:
5,000 Btu/cycle @4,570Btu/ft = 1 ft/cycle
TYPE B-VENT CHIMNEY:
A 6" B-vent chimney requires about 90 Btu/.ft. to go from 0 o to
120 o . So,
5,000 Btu/cycle @ 90Btu/ft = 55 ft/cycle

 If firing rate = 75,000 Btu/hr and SSE =82%, then 61,500 Btu/hr go to the distribution system and 13,500 Btu/hr go through the vent.
 If the burner on-cycle is 6 minutes (.1hrs), then during one cycle the vent receives:
 .1hr x 13,500 Btu/hr =1,350 Btu/cycle
 EXISTING MASONRY CHIMNEY:
 About four inches of the existing masonry chimney will be warmed during one burner cycle:
 1,350 Btu/cycle @ 4,570Btu/ft = .3 ft/cycle
 TYPE B-VENT CHIMNEY:
 During each burner on cycle, enough heat to warm 15feet of B vent goes into the chimney:
 1,350 Btu/cycle @ 90Btu/ft = 15 ft/cycle

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No exterior masonry chimneys without a properly sized metal liner.
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Flex or “B”
No interior masonry chimneys over 2 stories
No transite chimneys
No unlined masonry chimneys
No masonry chimneys unless common vented with a draft hood type appliance without a vent damper

(cont.)
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Must have double wall “B” vent connector
Furnace must be properly sized
Furnace must be set up correctly
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Temperature rise
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Gas input
Heat anticipator or cycle rate set @ 3 cycles per hour
Vent sizing should be in accordance with tables supplied with the furnace or NFPA
When sidewall venting a power vent kit must be used unless the manufacturer specifies otherwise.

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Use “B” vent as connector from the appliance to the flue
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Reduce heat loss in the connector
Pitch connector down toward appliance ¼” per Ft.
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Warm air rises
Maximum horizontal distance
(Table 13.2.2)
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1 ½ times the diameter of the connector in feet
4” connector = 6’ maximum horizontal distance

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Follow NFPA 54 sizing charts
Never used unlined masonry flue
Don’t use outside masonry flue
Never use Transite
If you take the heating appliance out of the flue and leave the water heater in, you are responsible to ensure the water heater will vent properly
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Line the flue

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NFPA 31 sizing guidelines
Masonry
Type “L”
 stainless steel
All fuel

“L” Vent
What We Should See In The Field
Outside
Masonry
“B” Vent

Barometric damper
(“swinging door”) in an oil-fired warm air furnace vent.
Drill test hole between the breech and the barometric damper

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11 Cu Ft flue gas for every 1 Cu
Ft natural gas (Vs. 21 Cu Ft for
70% AFUE)
Lower flue gas temperatures,
(350 0 F)
No dilution air (no draft hood) so gases are close to the dew point
Water must stay in gaseous state to be removed from the building
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KEEP FLUE GAS AS WARM
AS POSSIBLE

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NFPA manual
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54, Natural Gas
58, LP Gas
31, Oil
211, Solid Fuels
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Tape measure
Flashlight
Mirror
Combustion Analyzer
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Pressure Probe
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Temperature Probe
Boroscope
Digital Cameral

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Inside visual inspection
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General Safety Inspection
Vent connections
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Internal flue inspection
Outside visual inspection
CAZ Test

1” Clearance?
Fire stop?
Combustible?
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Clearance to combustibles
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6” single wall pipe, gas
9” single wall pipe, oil
1” “B” vent, gas
Single wall connectors must not pass through walls.
Spill switches
Flue blockage
Condition of flue materials
Draft under worst case conditions

Corrosion
Pitch

Inspecting An Existing Application
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Remove vent connector
Inspect with mirror & light
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Is the vent straight or is there an offset
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Is there a liner present
Are tiles cracked allowing flue gas to escape
Blockage
Examine termination from outside
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Cap
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Condition of flue

An appliance that produces soot is a cause for concern.
Auditor should call for clean and service.

Check the chimney for accumulated debris.

Oil-fired boiler before cleaning – plugged with soot after several years without service.

Same boiler after cleaning

Connector
Must be
Supported at Base

Termination
Failure

B-vent exposed to the outdoors & not properly supported
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Does not meet code
(13.2.20)
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Sizing tables 13.6 through 13.10 are not to be used for “B” vent exposed to the outdoors below the roofline.

Draft assisted furnace, or only water heater left in flue?
Condensation bleed through

Condensation
Damage

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Place building in winter mode
Place all combustion appliances in pilot mode, or turn off
Energize all exhaust fans
Measure pressure difference between CAZ in relation to outdoors
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Open and close interior doors until the worst case draft condition is reached
Must have draft to continue
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OD temp >80 0 F, >-1 Pa or - .005”WC
OD temp 30 0 – 80 0 F, >-2.5 Pa or -.01”WC
OD temp below 30 0 F, >-5 Pa or -.02”WC

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Used to reduce off cycle losses
Motorized
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End switch safety
Thermal
Spill Switch

Thermal Vent Damper
Bimetal petals warp open when heated

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Follow manufacturer instructions
Use NFPA guidelines if manufacturer instructions are not available

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A chimney shall extend at least 3 ft. above the highest point where it passes through a roof of a building and at least
2 ft. higher than any portion of a building within a horizontal distance of 10 ft.

Sealed Combustion
Category IV

Concentric Vent Category IV

Look for conditions that may affect health or safety of the occupants, the weatherization crew, and YOU.
Dangerous vent, fire hazards, CO, fuel leaks, etc.
Complete a Health and
Safety Warning form if necessary.

Scary, home-made distribution system

REALLY scary homemade vent connector

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Use the appropriate NFPA manual
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54 for Natural gas
58 for LP for Oil
211 for Solid fuels
Find the appropriate chart
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One or more appliances?
“B” Vent or masonry?
“B” Connector or single wall?

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Height of flue
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From top of tallest appliance to the top of the flue termination
Increasing the height increases the draw
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Horizontal distance to flue
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Used with single appliance application
Increased horizontal run decreases draw
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Vertical connector height
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From the appliance breech to the point where flue gases combine
Used with multiple appliances
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Elbows – Charts are listed with up to (2) 90 0 elbows in the vent

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Single draft assisted appliance
50,000 BTU input rate
Total chimney height = 17’
Lateral distance = 3’
“B” vent and connector

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2 category I appliances
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50,000 BTU fan assisted furnace
• Connector rise = 2’
30,000 BTU water heater
• Connector rise = 3’
“B” vent with “B” connectors
Chimney height = 18’

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Follow manufacturer sizing tables
Use NFPA sizing tables, but reduce capacity by 20%
The masonry or original flue is used as a chase for the liner
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If a liner is installed, the remaining space around the liner can not be used to vent other appliances.
More than one liner may be installed in the masonry chase

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“B” vent may be used as a liner
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Drop down an inadequate or improperly sized flue
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Original flue must be straight.
SS flexible liner

1.
2.
3.
4.
5.
The height of a chimney is identified as:
If the chimney height falls between two columns in the NFPA chart, do you round up or down?
For a single category I appliance installation, if the lateral distance falls between two value on the chart, do you round up or down?
Can you use NFPA charts to size a flexible chimney liner?
What is one advantage of using a flexible liner over “B” vent?

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NYS WAP Policy
Operational Requirements
Oxygen Depletion Sensor

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WAP funds cannot be used to purchase or install any type of unvented or ventless combustion appliance including but not limited to unvented kerosene space heaters, unvented natural gas space heaters, unvented propane space heaters, unvented gas fireplaces, and unvented gas fireplace logs.

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Unvented Space Heaters: Educate the client about the potential danger of CO and fire from unvented space heaters. Explain that significant amounts of combustion products including water vapor and CO2 are produced.

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Must supply combustion air while operating
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Open window while operating
Tucson instructions require defining the space as confined / unconfined
Products of combustion remain in the conditioned space
Must provide some measure of safety for oxygen depletion

Fresh Air Requirements
Tucson Heater

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KEEP THE WATER IN A GASEOUS STATE
WHILE IN THE FLUE
Use “B”vent connectors on any new category I gas appliance installation
Most masonry chimney’s will need a liner
Never leave a water heater in a flue alone without ensuring it will vent