ISU Extension Pub # AEN-188
Author: Thomas H. Greiner, Extension Agricultural Engineer
Department of Agricultural and Biosystems Engineering, Iowa State University.
September, 1997
CARBON MONOXIDE POISONING
Checking a Gas Burner
For proper combustion, it is vital the proper amount of gas and air be supplied at the correct pressure. The distance above sea level and the type of fuel affect the amount of gas and air entering the burner. At higher elevations the thinner air means less oxygen enters the burner. Typically, above 2,000 feet the amount of fuel must be reduced to compensate for reduced oxygen. Two main fuels, natural gas, with approximately 1,000 BTU/cubic foot, and Liquified Petroleum, with approximately 2,500 BTU/cubic foot, are supplied at greatly different pressures. The two main determinants of gas flow rate are orifice size and manifold gas pressure. Either incorrect orifices or manifold gas pressure will cause combustion problems. Failures occur when incorrect sized orifices are installed; gas pressure regulators are not adjusted correctly or drift out of adjustment, stick, or fail. The following tests help determine if proper combustion is occurring:
* Combustion analysis for carbon monoxide
* Manifold gas pressure check
* Supply gas pressure check
* Determining gas input by “clocking the meter”
* Measuring the orifice size.
COMBUSTION ANALYSIS
Incomplete combustion can quickly be determined by measuring carbon monoxide concentrations in undiluted flue gas using an electronic meter with digital readout. In natural draft appliances the measurement of gases is taken at the top of the heat exchanger before dilution air enters at the draft hood. In multiple cell units, test each cell separately. Test sealed combustion units either from an access in the discharge line, or outdoors by probing into the discharge pipe. Typical concentrations will be less than 20 parts per million (ppm) carbon monoxide, although a few units are designed with up to 100 ppm. The maximum allowed under ANSI is 400 ppm, however at 400 ppm improper combustion is occurring and corrective measures should be taken.
Combustion analyzers with the ability to measure oxygen and/or carbon dioxide concentrations, furnish additional information. Using a full combustion analyzer, set excess air to manufacturer’s specifications. In general, if excess air is less than 30 percent the unit should be cleaned and adjusted.
MANIFOLD GAS PRESSURE CHECK
Manifold gas pressure on most heating units is adjustable. Many manufacturers require installers to make the final gas pressure adjustment after installation. Because the gas pressure regulators on these units are not adjusted at the factory, it is critical the installer make this required adjustment. Excess gas pressure causing over-fueling, or insufficient pressure causing under-fueling, results in major combustion difficulties, including excess carbon monoxide, excess sooting, and/or firing and operation problems. Not all problems are noticeable from observing the unit, especially carbon monoxide production. The correct pressure, to the nearest 0.01 inches of water column, will be on the nameplate. Most manufactures provide a pressure tap on the manifold or the gas pressure regulator. The gas pressure must be set exactly to the manufacturers’ specifications, including any adjustments for high elevations. Typically, manufacturers’ specify that units must not be overfired to reduce the risk of carbon monoxide production. Some under-firing is acceptable on some units. Consult the manufacturer for acceptable limits.
CAUTION: mechanical pressure gauges are easily knocked out of adjustment. Use either a liquid manometer or a sensitive, calibrated mechanical gauge to measure and adjust manifold pressure. Do not use a mechanical gage that reads from 0 to 20 inches water column for adjusting natural gas units. Most of these are not sufficiently sensitive at low pressures.
SUPPLY GAS PRESSURE CHECK.
For the appliance gas pressure regulator to function correctly, gas must be supplied at a higher pressure to the appliance. The range of acceptable supply pressures is marked on the nameplate. Supply pressure must be checked to insure that neither over or under pressure is occurring. Severe overpressure can either damage the appliance or exceed the ability of the regulator to reduce pressure to the manifold, causing over-firing.
Minimum pressure to the appliance depends on pressure supplied to the house, gas pipe sizing, and the total amount of gas flow. Smaller pipes and/or increased gas flow decrease the pressure available to an appliance. Pressure to the appliance must be checked with all appliances operating. If pressure drops below the required minimum, changes to supply piping and supply pressures must be made before adjusting or operating the appliances. Failure to insure proper gas pressure to the appliances can cause serious misadjustment of the appliance gas pressure regulator and major combustion failure.
CLOCKING THE METER
On all appliances with a gas meter, a simple method to determine gas flow is by “clocking” or timing the test dial on the gas meter. Count the number of revolutions the most sensitive dial on the meter makes in one minute. Most metered gas is natural gas, with a heating value of 1,000 BTU/cubic foot. Most residential gas meters have a gas dial with 1/2 cubic foot per revolution. With these assumptions, timing the meter is a two-step procedure.
How to clock the meter:
1. Count the revolutions the 1/2 cubic foot dial makes in one minute.
2. Multiply the revolutions by 30,000 to obtain the firing rate, in BTU/hour.
Firing rate example: in one minute the dial makes 3.2 revolutions. The firing rate is:
(3.2 revolutions) x (30,000 BTU/rev) = 96,000 BTU/hour.
In most cases, the heating value of the gas is so near to 1,000 BTU/cubic foot that no adjustment is needed. For exact results, or when the value is considerably different from 1,000 BTU/cubic foot, proportional adjustments can be made. Divide the actual heating value by 1,000. Multiple that result times the firing rate.
Heating value adjustment example, the actual heating value of the gas is 1050 BTU/cubic foot, not 1000 BTU/cubic foot as assumed.
Adjustment 1050/1000 = 1.050
The adjusted firing rate is (96,000 BTU/hour) x (1.050) = 100,800 BTU/hr.
The 30,000 rule only works with meters with a 1/2 cubic foot dial. The following table gives multiplying factors for different gas meters, and different measurement times. Longer times are recommended for small appliances, or when greater accuracy is desired:
Multiplying Factor
(Number of revolutions) x Factor = BTU/hr
|
Cubic Feet per Revolution |
1 minute |
2 minutes |
3 minutes |
5 minutes |
|
1/2 |
30,000 |
15,000 |
7,500 |
6,000 |
|
1 |
60,000 |
30,000 |
15,000 |
12,000 |
|
2 |
120,000 |
60,000 |
30,000 |
24,000 |
|
5 |
300,000 |
150,000 |
75,000 |
60,000 |
Example of large meter firing rate, in 5 minutes the 5 cubic foot dial makes 0.8 revolution. From the table the factor is 60,000:
(0.8 revolution) x (60,000 BTU/rev) = 48,000 BTU/hr.
Clocking the meter serves as a quick check that both orifice size as well as manifold pressure are correct. NOTE : all other heating systems must be turned off when clocking the meter, and pilot lights on other appliances must be considered.
The firing rate is shown on nameplate. Typically gas flows from 95% to 102% of the rated input are acceptable, although if there is a carbon monoxide problem firing rates should be adjusted to exact manufacturer specifications.
MEASURING ORIFICE SIZE
When gas flow is incorrect, even after adjustment of gas pressure, orifices should be removed and measured.
When to Test
Gas input should be checked, and any necessary adjustments made:
1. At the start-up of a new installation.
2. When taking over service to a new customer.
3. When there is a carbon monoxide problem.
Readings of carbon monoxide in the combustion products should be made in all three of the preceding situations, at least once yearly during annual inspection and cleaning. In homes with service problems, CO readings should be made frequently.
Gas pressures should be checked whenever gas input rates and/or carbon monoxide production are not within specifications. If adjustment of gas pressures does not correct gas input problems or carbon monoxide problems, then gas orifices should be removed and measured. Checking gas burners is important to prevent carbon monoxide exposures and diagnose carbon monoxide problems.