ACHRNEWS

Supplement to ‘Ventilation and Heat Exchanger Testing'

April 17, 2006
Typical readings for furnace and boiler burners and heat exchangers, plus measurement procedures.

ATMOSPHERIC DRAFT GAS-FIRED BURNERS

  • Efficiency: 75 to 80 percent

  • Oxygen: 7 to 9 percent

  • CO2: 6.5 to 8 percent

  • Stack temperature: 325° to 500°F

  • Draft: -0.02 to -0.04 inch wc

  • CO: <100 ppm (undiluted)

    Always follow manufacturer's specifications.

    Atmospheric draft appliances tend to have higher excess air readings and higher stack temperatures due to less-complex heat exchanger designs. Remember, there is a positive pressure in the heat exchanger and a negative pressure in the vent connector and stack. A heat exchanger crack in this style furnace will show as an increase in excess air readings when the blower starts and a decrease when the blower stops.

    If the primary air shutters or gas pressure are adjusted, it is imperative that a combustion analysis is performed, as operating characteristics of the furnace have changed. For proper combustion and venting, approximately 30 cubic feet of air is required per 1,000 Btuh; this means a 100,000-Btuh furnace would require 300 cubic feet of ventilation hour ever hour of operation.

    Measurement procedure:
    1. Connect flue gas sampling probe assembly to analyzer per manufacturer's instructions.

    2. Verify condensate trap plug is properly seated in combustion analyzer water trap and thermocouple is not touching side of probe assembly.

    3. In uncontaminated air (outdoor) start the analyzer and allow unit to complete zeroing process. (Never allow the analyzer to zero in the stack (unless manufacturer's design allows this, e.g. Testo 330-2.)

    4. Measurements must be made in each cell of the heat exchanger. If needed a 5/16-inch hole can be made in the front of the draft diverter to allow measurement.

    5. Allow furnace to operate for ten minutes or until stack temperature stabilizes. The furnace must establish draft (measured in the flue pipe) with 10 minutes of operation.

    6. Measure and print combustion results for each cell in the furnace.

    7. Compare results for each cell.

    DRAFT-INDUCED GAS-FIRED BURNERS

  • Efficiency: 80 to 82 percent

  • Oxygen: 7 to 9 percent

  • CO2: 6.5 to 8 percent

  • Stack temp: 325° to 400°F

  • Draft: -0.02 to -0.004 inch wc

  • CO: <100 ppm (undiluted)

    Always follow manufacturer's specifications.

    Draft-induced appliances have similar operating characteristics to atmospheric draft appliances with the exception of lower flue gas temperatures. The stack draft operates identically, but the heat exchanger pressure is now negative. The function of the draft inducer is to pull combustion byproducts through the heat exchanger, not to create a positive pressure in the vent. If the vent pressure is positive the flue pipe is clogged. These furnaces characteristically do not leak flue gas into the house when heat exchanger failure occurs due to the negative heat exchanger pressure. Combustion ventilation air requirements are reduced to 15 cubic feet/1000 Btuh. No dilution air is required for venting.

    Measurement procedure:
    1. Connect flue gas sampling probe assembly to analyzer per manufacturer's instructions.

    2. Verify condensate trap plug is properly seated in combustion analyzer water trap and thermocouple is not touching side of probe assembly.

    3. In uncontaminated air (outdoor) start the analyzer and allow unit to complete zeroing process (never allow the analyzer to zero in the stack (unless manufacturer's design allows this, e.g. Testo 330-2).

    4. Measurements must be made in the vent connector or stack.

    5. Allow furnace to operate for 10 minutes or until stack temperature stabilizes. The furnace must establish draft (measured in the flue pipe) with ten minutes of operation. Verify flue gasses are not spilling from the draft hood of the hot water tank if common vented!

    6. Measure and print combustion results prior to and after making any adjustment to the furnace.

    Note: If the furnace is multistage or modulating, each stage must be checked independently to ensure safe operation through the entire operating range.

    DRAFT-SEALED COMBUSTION INDUCED GAS-FIRED BURNERS (90 PLUS)

  • Efficiency: 88 to 92 percent

  • Oxygen: 5 to 7 percent

  • CO2: 7 to 8.5 percent

  • Stack temp: Less than 125°F

  • Draft: 0.02 to 0.08 inch wc

  • CO: <100 ppm (undiluted)

    Always follow manufacturer's specifications.

    High-efficiency appliances are considerably different in operation. Their heat exchanger pressure is negative, and vent pressure becomes positive. Flue gas temperature drops below 125°. Often outdoor air is used for combustion, allowing these furnaces to operate without indoor air ventilation requirements. If used with a two-pipe configuration, the combustion air temperature must be referenced to get an accurate combustion test result. All burner shields and doors must be in place.

    The high efficiency of these appliances is achieved by removing the latent (hidden) heat from the flue gasses by condensing the water from the byproducts of combustion. This additional removal of heat through a secondary heat exchanger lowers the flue gas temperature below 125°. High heat extractions, in conjunction with careful control of combustion air, allow these furnaces to operate with high combustion efficiencies and very high thermal efficiencies. Combustion air required by these furnaces is reduced to 10 cubic feet/1,000 Btuh. Provisions for condensate removal must be made for condensing-type furnaces and boilers.

    Measurement procedure:
    1. Connect the flue gas sampling probe assembly to the analyzer per the manufacturer's instructions.

    2. Verify that the condensate trap plug is properly seated in the combustion analyzer water trap, and that the thermocouple is not touching the side of probe assembly.

    3. In uncontaminated air (outdoor), start the analyzer and allow the unit to complete the zeroing process. Never allow the analyzer to zero in the stack (unless the manufacturer's design allows this, e.g. Testo 330-2).

    4. Measurements must be made in the PVC flue pipe, and temperature of combustion air must be referenced in the intake pipe with an auxiliary temperature probe to get accurate combustion efficiency results.

    5. Allow furnace to operate for ten minutes or until stack temperature stabilizes.

    6. Measure and print combustion results prior to and after making any adjustment to the furnace.

    Note: If the furnace is multistage or modulating, each stage must be checked independently to ensure safe operation through the entire operating range.

    GAS-FIRED POWER BURNERS

  • Oxygen: 3 to 6 percent

  • CO2: 8.5 to 11 percent

  • Stack temp (gross): 320° to 570°F

  • Draft: -0.02 to -0.04 inch wc (overfire)

  • Draft: Positive pressure (manufacturer's specifications)

  • CO: <100 ppm (diluted)

    Always follow manufacturer's specifications.

    Power burner appliances tend to have lower excess air readings and higher stack temperatures due to their less-complex heat exchanger designs, although some do approach condensing. Remember, there could be a positive pressure in the heat exchanger and a negative pressure in the stack depending on the design.

    A heat exchanger crack in this style furnace may not show an increase in excess air readings when the blower starts, and a decrease when the blower stops. If the primary air shutters/air band or gas pressure is adjusted, it is imperative that a combustion analysis be performed as operating characteristics of the furnace have changed. For proper combustion and venting, approximately 20 cubic feet of air is required per 1,000 Btuh; a 100,000-Btuh furnace would require 200 cubic feet of ventilation air for every hour of operation. Always set gas manifold pressure per the manufacturer's specifications.

    Measurement procedure:
    1. Connect the flue gas sampling probe assembly to the analyzer per the manufacturer's instructions.

    2. Verify that the condensate trap plug is properly seated in the combustion analyzer water trap, and that the thermocouple is not touching the side of the probe assembly.

    3. In uncontaminated air (outdoor), start the analyzer and allow the unit to complete the zeroing process. Never allow the analyzer to zero in the stack (unless the manufacturer's design allows this, e.g. Testo 330-2).

    4. Measurements must be made in the stack before the barometric damper (if so equipped).

    5. Allow the furnace to operate 10 minutes, or until stack temperature stabilizes. The furnace must establish draft (measured in the flue pipe) with 10 minutes of operation.

    6. Measure and print combustion results prior to and after any adjustments are made.

    OIL-FIRED POWER BURNERS

  • Oxygen: (cast iron cone) 5 to 9 percent

  • Oxygen: (flame retention) 3 to 6 percent

  • CO2: 10 to 12.5 percent

  • Stack temp: 60 to 79 percent AFUE, 400° to 600°F

  • Stack temp: 80-plus AFUE, 330° to 450°

  • Stack temp: 90-plus AFUE, less than 125°

  • Draft: -0.02 inch wc (overfire)

  • Draft (stack): -0.02 inch wc/0.04 inch wc

  • CO: <50 ppm (diluted)

  • Smoke spot: #0-#1

  • Oil pressure: 100 to 150 psi (per manufacturer)

    Always follow manufacturer's specifications.

    Oil power burner appliances tend to have lower excess air readings and higher stack temperatures due to less-complex heat exchanger designs. Some high- and ultra-high-efficiency models do approach and operate in condensing. Remember, these furnaces must operate with a negative pressure in the heat exchanger and a negative pressure in the stack unless otherwise specified by the manufacturer.

    A sizable heat exchanger crack in this style furnace will show as an increase in excess air readings when the blower starts and a decrease when the blower stops. If the primary air shutters/air band or oil pressure is adjusted, it is imperative that a combustion analysis is performed as the operating characteristics of the furnace have changed. For proper combustion and venting, approximately 25 cubic feet of air is required per 1,000 Btuh; a 100,000-Btuh furnace would require 250 cubic feet of ventilation air every hour of operation.

    Measurement procedure:
    1. Connect flue gas sampling probe assembly to analyzer per manufacturer's instructions.

    2. Verify condensate trap plug is properly seated in combustion analyzer water trap and thermocouple is not touching side of probe assembly.

    3. In uncontaminated air (outdoor), start the analyzer and allow the unit to complete its zeroing process. Never allow the analyzer to zero in the stack (unless the manufacturer's design allows this, e.g. Testo 330-2).

    4. Measurements must be made in the stack before the barometric damper, if so equipped.

    5. Allow the furnace to operate 10 minutes or until stack temperature stabilizes. The furnace must establish draft (measured in the flue pipe) within 10 minutes of operation.

    6. Measure and print combustion results prior to and after any adjustments are made.

    Sidebar: A Field Guide to Furnace Efficiencies

    Determining the gas furnace Annual Fuel Utilization Efficiency (AFUE) in the field is often hard, if not impossible. There are several types of efficiency ratings, including AFUE, combustion efficiency, thermal system operating efficiency, and steady-state operating efficiency.

    An old furnace can have 85 percent combustion efficiency yet have an AFUE of 60 percent because of the amount of heat going up the stack (heat not being transferred by the heat exchanger). A 95-percent-AFUE furnace can be operating in the 60-percent range due to poor installation or operation.

    Furnace efficiency (AFUE) can be generalized by applying the following information, provided the installation is adequate.

  • 60- to 70-percent AFUE:

    Standing pilot, draft diverter, belt-drive blower, and a single upshot burner; single-wall flue pipe.

  • 70- to 78-percent AFUE:

    Intermittent pilot, direct spark or hot surface igniter; draft diverter, with/without flue damper; direct-drive blower; multicell construction with ribbon, slotted, or ported burners; single-wall flue pipe.

  • 80-percent AFUE:

    Intermittent pilot direct spark or hot surface igniter; induced-draft, direct-drive blower, could be multistage and/or variable-speed; jet or in-shot-type burners; single- or double-wall flue pipe. If vented in masonry chimney, chimney must be lined.

  • 90- to 97-percent AFUE:

    Intermittent pilot, direct spark, or hot surface igniter; induced-draft, direct-drive blower, could be multistage or modulating and/or variable speed; jet or in-shot-type burners; secondary heat exchanger; plastic flue pipe.

    Thermal efficiency can be field calculated provided the heating value of the fuel being burned is known, accurate measurement of the airflow across the heat exchanger is made, and the values are input into the sensible heat formula then divided by the appliance input.

    Table 1. Average Btu content of fuels. (Source: U.S. Department of Energy.)

    Sidebar: Calculating Field Thermal Efficiency or Btuh Output

    To calculate field thermal efficiency or Btuh output, measure airflow across the heat exchanger, preferably using a mini-vane anemometer such as a Testo 416, or measure total external static pressure using a Testo 506-3 or 506-2 and the manufacturer's chart to determine cfm. Measure the temperature rise across the heat exchanger, being careful not to let radiant energy affect the temperature reading. It is preferable to make your temperature measurements in several locations and average them. Enter the results into the sensible heat formula.

    Total sensible heat (TSH): This is the heat added that causes a change in temperature of the air without adding or removing humidity.

    TSH formula: TSH = 1.08 x cfm x ∆T

    cfm = Airflow in cubic feet per minute

    ∆T (Delta T) = Change in temperature across the heat exchanger

    If the heat content of the natural gas is verified, the meter can be clocked and the actual input can be divided into the measured output to calculate the actual operating thermal efficiency.

    Sidebar: Problem Notification Form for Customers

    Company Name

    Heat Exchanger/Venting/Combustion Problem:

    While inspecting your furnace, performing the three-part inspection procedure as outlined in Refrigeration Service and Engineering Society (RSES) publication 630-92 9/86 and adopted by the Gas Appliance Manufacturers Association, and/or during venting/combustion air testing outlined in the National Fuel and Gas Code, we found the following:

    _____ The heat exchanger has a visible crack, hole, or opening that may allow flue gasses to enter the home.

    _____ Chemical testing using the salt spray test (chemical test) gave a positive result. This indication shows that flue gasses may enter the home through a crack, hole, or opening that isn't visible.

    _____ There is a venting or combustion air problem.

    Any breach in the heat exchanger or venting/combustion air problem that may allow flue gasses to enter the home creates a potentially dangerous situation for the homeowner. We highly recommend that you allow us to turn off the furnace until this problem can be corrected.

    I understand that by operating this appliance without correcting the problem, I assume full responsibility for any harm or damage that may result by my decision. I will not hold [Company Name], its heirs, or any of its employees, liable for my decision.

    Customer's Name: ______________________________ Date: _________________

    Customer's Signature: ____________________ PRINTED _____________________

    Technician's Signature: ___________________ PRINTED _____________________

    Publication date: 04/17/2006