CAST (Combustion Appliances Safety Test)


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92 terms · Source: BPI technical standards

The combustion appliance safety inspection includes all of the following:

•Carbon monoxide test
•Draft measurement
•Spillage evaluation
•Worst-case depressurization of the combustion appliance zone
•Gas/propane line leak inspection

Carbon Monoxide

odorless, colorless gas. Occurs in buildings due to the incomplete combustion of heating fuels.

What is the ASHRAE suggested maximum 24 hour exposure to CO in room air (ambient CO) ?

9 ppm.

Ambient CO concentrations at 9ppm or higher are due to malfunctioning appliances such as:

Unvented combustion space heaters


the portion of the combustion system that contains heat and by-products of combustion before the intrusion of ambient (dilution) air through the draft hood.

Draft hood or draft diverter

A designed entry point (or port) for ambient (dilution) air within the combustion system.

Vent damper

a mechanical device placed at the base of the vent or chimney that closes the vent when the appliance is not firing.

Vent or chimney

The portion of the combustion system that carries the mixture of flue gases and dilution (ambient air) to outdoors.

Vented appliances

Chimney routes combustion byproducts outside the building envelope. Examples:
•Water heater
•Furnace (including wall heater)
•Boiler (hydronic and forced air systems)

Un-vented appliances

They still vent but inside the building envelope (have no chimney). Examples:
•Gas or propane cooking appliances such as stoves and ranges

Vent types:

Class C = single wall pipe, usually galvanized steel
Class B = double wall pipe, usually galvanized steel jacket with aluminum inner pipe
PVC =used in sealed combustion appliances. (This type cannot be drilled)

Open combustion

Open combustion appliances are those which draw air for combustion from the area in which they are located.

Induced draft or draft assisted appliances

are those which have the draft hood or draft diverter replace by a small fan.

Sealed (or closed) combustion

Sealed combustion appliances have combustion chambers containing the main burners which are sealed to the area they are located in and draw combustion air from outdoors.

Combustion Appliance Zone (CAZ)

the area containing one or more combustion appliances.

CAZ Baseline Configuration

also called the ―base pressure condition or natural condition.

Step to get the baseline configuration

Close all exterior doors, windows, and fireplace damper(s)
•Set all combustion appliances to the pilot setting

CAZ Worst Case Configuration

The baseline configuration with exhaust appliances and interior doors configured such that the CAZ pressure with respect to (WRT) outdoors is as negative as possible.

Draft Pressure

the pressure (force per unit area) that moves combustion byproducts through the flue and chimney and out of the building.

Normal venting

Pressure in vent is more negative than pressure in CAZ

Faulty venting:

Pressure in vent is more positive than pressure in CAZ


this is the temporary flow of combustion byproducts from an open combustion appliance at the draft dilution port (draft diverter).

How do we test for spillage?

We test for this condition with a smoke pencil or other type of smoke generator and stopwatch

When the Spillage test fail?

Vented appliances--regardless of type--that spill flue gases for more than 60 seconds after startup,


continuous spillage when the direction of chimney flow is reversed

Flame rollout

when the pressure in the CAZ is adequately lower than the pressure in the burner compartment, the flame will follow the air supply and burn outward through the secondary air opening

Sealed combustion furnace

Appliances No draft diverter. You can identify this type of unit by the PVC inlet and outlet venting.

The BPI protocol requires the following five tests be performed whenever changes to the building envelope and/or heating system are part of the work scope:

1) carbon monoxide (CO) measurement at each appliance
(2) ambient carbon monoxide (CO) measurement in the living space
(3) draft measurement and
(4) spillage evaluation for atmospherically vented appliances
(5) worst-case negative pressure measurement for each combustion appliance zone (CAZ).
•Additionally, the entire gas/propane line must be examined and all leaks repaired.

Test (1): carbon monoxide (CO) concentration measurement

A sample of undiluted flue gases drawn from the throat or flue of the appliance is tested using a digital gauge and measured in parts per million (ppm).
•In practice, we sample the hot, combustion gas as close to the burner as possible, and before this gas is diluted at the draft diverter port.

What instrument we use to measure the concentration of carbon monoxide (CO)

We use a combustion analyzer (monoxer)

CO concentration testing requires the appliance to have reached steady state conditions.

Steady State conditions are defined by the leveling off of the flue gas temperature. This typically happens after 5-10 minutes of firing.

Test (3): Draft measurement

We are measuring the draft pressure inside the vent pipe of the combustion appliance with respect to the combustion appliance zone (CAZ).
-Draft tests must be completed for all natural and induced draft space heating systems and water heaters.

How do we check for spillage?

Using tracer smoke, we are checking to see that the appliance combustion gases vent up the chimney rather than out its dilution device into the CAZ.
Failed spillage means the appliance continued to vent to the CAZ after 60 seconds.
-The spillage test must be completed for all natural and induced draft space heating systems and water heaters.

Test (5): worst-case CAZ de-pressurization

The net worst case pressure (Pwc) is the algebraic difference between the worst case pressure and base pressure (under natural conditions—see Key Terms):
»Pwc= P1-Pbase

What should be the action when CAZ depressurization limits are exceeded under worst-case conditions according to the CAZ Depressurization limit Table?

makeup air must be provided or other modifications to the building shell or exhaust appliances must be included in the work scope to bring the depressurization within acceptable limits.

Action to test for Gas leak

1)We use an electronic combustible gas sensor (gas sniffer) to detect gas leaks down to 20 ppm by rotating the sensor probe 360 degrees around the fitting at 1 inch per second.
2)Replace flexible gas lines manufactured before 1973 (the date is stamped on the ring attached to the line) or if the line has soldered connections.

Minimum Draft Pressure Standard (Pascals)
for 10-90 F temperature

(To/ 40) -2.75

if the outside temperature is 85F, then the limit is 85/40-2.75 = -0.63 Pa. If the measured draft pressure is more positivethan this, say -0.5, the system draft test pass or fails?


CO concentration (ppm)
36 to 99

Excessive: medical alert; leave the building.

CO concentration (ppm)
0 to 9

No action. Prolonged exposure to 9ppm is harmful.

Is additional combustion air needed?

Rule of thumb: 50 ft³/1000 BTU

Net worst

Formula: Gross - Base = Net worse case depressurization

The CAZ worst case test fails

if the Net worst case depressurization is more negative than the CAZ Depressurization limit table

When the work scope must include pre and post-installation blower door

When air sealing, enclosed cavity insulation representing 15% or more of the total
building shell area, or sealing of the ducts outside the thermal envelope are

Diagnostic evaluations and inspections must be
aborted if ambient CO concentrations greater than

35 ppm are recorded.

Where should be tested CO?

With the exception of unvented gas or propane cooking appliances, CO must be tested in
all combustion appliances under worst-case conditions and normal draft conditions (when
the appliance fails under worst-case)

What shoud be installed when CAZ depressurization limits are exceeded under worst-case conditions according
to the CAZ Depressurization Limit table?

Make up air

If the CO in any appliance is measured greater than 100 ppm during diagnostic testing

an appliance
clean and tune must be completed as part of the work scope.

Interim Gas/Propane Oven Testing Procedure

1. Remove any items/foil in or on oven.
2. Make sure self cleaning features are not activated, set oven to highest setting.
3. Test oven for CO in the flue, before dilution air.
4. After 5 minutes of operation, check for steady state

Level I Action - 100 ppm to 300 pp
(page 15 standards)

you must install a carbon
monoxide detector and recommendation for service must be made to the consumer

Level II Action - Greater than 300 ppm
(page 15 standards)

the unit must be serviced
prior to work. If greater than 300 ppm after servicing, exhaust ventilation must be
provided with a capacity of 25 CFM continuous or 100 CFM intermittent

What do we inspect on combustion appliances?
(waptac fundamentals)

Chimney • Vent pipe • Fuel leaks • Wiring • Heat exchanger • Distribution (fan and ducts, if applicable)

What do we test for on combustion appliances?
(waptac fundamentals)

Proper combustion air - Measure CO levels in undiluted flue gases. If less than or equal to 25 parts per million (ppm), there is enough combustion air.

Combustion efficiency
(waptac fundamentals)

This is a function of the composition of the flue gases and flue gas temperature.

Primary air
(waptac )

is air mixed with fuel before combustion

Secondary air
(waptac )

is additional air surrounding the flame

Excess air
(waptac )

is air in excess of what is needed for combustion and is released along with the rest of the combustion byproducts.

The Combustion Triangle
(waptac )

Fuel (hydro carbon) and oxygen (O2) must mix with heat for combustion to occur.

Byproducts of combustion are?
(waptac )

heat + water + carbon dioxide + excess air + trace compounds (CO, carbon, sulfur dioxide, etc.).

When Carbon monoxide is created?
(waptac )

when the ratio of fuel to oxygen is either too high to permit the complete formation of CO2 or the temperature is too low to permit complete burning to occur

what refers the National Fire Protection Association's National Fuel Gas Code (NFPA 54)

(waptac )

that combustion air must be provided for any combustion zone where the collective fuel input exceeds 1,000 BTU per 50 cubic feet

Draft gauge

for testing chimney draft

Smoke tester

for measuring the amount of smoke produced by an oil burner

Digital probe thermometer

for testing temperature rise and fan operating temperatures

How much water do you think is produced in the combustion process?

About 1 gallon per 100,000 BTUs

How many gallons of water per hour does a 25,000 BTU/hr appliance produce?

One-quarter gallon (1 quart) per hour.

Why test gas cook stoves?

Gas- and propane-fired cook stoves release combustion byproducts into the air. They must be tested and repaired if CO exceeds suggested action levels

What is the objective of CAZ testing?
(waptac )

is to determine if combustion appliances will vent under worst case conditions and protect the occupant from the hazards of draft reversal.

For an 80+ efficiency induced draft furnace. Where you should test for CO and draft?

in the vent above the inducer fan

For a 90+ efficiency condensing furnace, where you should test for CO?

at the vent termination. A draft test is not required for a condensing furnace because it operates on positive pressure in the flue.

What is a BTU?

is the amount of energy it takes to heat one pound of water 1 degree Fahrenheit.

A venting system should:

•Carry all combustion byproducts to the outdoors.
• Establish draft quickly.
• Be properly sized with minimal restrictions.
• Be durable (corrosion resistant).
• Not overheat surrounding materials.
• Have adequate make-up air.

A type B-vent is:

a double-wall pipe for gas- or propane-fired combustion appliances. An air gap between the pipes acts as an insulator. The inner vent pipe is aluminum and the outer pipe is galvanized steel.

A type BW-vent is:

is an oval B-vent designed for wall furnaces

A type L-vent is:

is also a double-wall pipe for gas, propane, and oil-fired combustion appliances.
An L-vent is always used with oil-fire equipment.

PVC schedule 40 pipe is used for:

90+ condensing units.

According to The National Fire Protection Association's Standard for the Implementation of Oil-Burning Equipment (NFPA 31)

chimneys must be at least 2 feet higher than any portion of the building within 10 feet. B-vent height may be less with a UL-listed cap.

flame roll-out

a problem when ignition of the appliance is delayed, causing a sudden mini-explosion of unburned natural gas in the burner compartment at the bottom of the tank

Testing CO on Water Heaters

Insert the combustion analyzer probe down into the water heater before the draft diverter and take CO readings on both sides of a baffle that runs up through the center of the flue inside the water tank.

Chimney Chase

The cavity between the chimney and the framing. There is usually a gap of at least 2",
allowing substantial air leakage

Flue gas

Includes all gases which leave the furnace combustion chamber by way of a flue. Flue gas consists of nitrogen, gaseous products of combustion, water vapor and oxygen.

Inches of Water Column (IWC)

A unit used in measuring pressure difference.
1"= 0.25 Pa or 0.036 psi

NFP 211

National Fire Protection Association Standard for chimneys, fireplaces, vents, and solid fuel burning appliances. Includes installation procedures


Metric standard for measuring pressure difference.
248 Pa = 1" WC (water column)

Steady State Efficiency (SSE)

The measurement of heat system balance in the on- cycle when heat into system equals heat out.
1 in = 1 out
(10 minutes)

Winter mode

Closing all exterior doors and opening interior doors

Dilution air

Room air that mixes with flue gases.


A measurable pressure difference caused by combustion
byproducts exhausting through a chimney flue as
influenced by temperature difference, height of the flue,
and the Venturi effect (the reduction in pressure that results
when flow occurs through a constricted section of pipe).

Flame impingement

The striking of flame against an object

Furnace blower

A part of the furnace that produces a current of air. Often referred to as the "blower" or "squirrel cage."


National Fire Protection Association's Standard for the
Implementation of Oil-Burning Equipment, dictating that chimneys must be at least 2 feet higher than any portion of
the building within 10 feet


National Fire Protection Association's National Fuel Gas
Code stating that combustion air must be provided for any combustion zone where the collective fuel input exceeds 1,000 BTU per 50 cubic feet.

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