What's A Wobbe?

In almost all gas appliances, gas flows into the burner through an orifice, which is part of the means to control the input rate of the appliance, Stanonik explained. The usefulness of the Wobbe number is that for any given orifice, all gas mixtures that have the same Wobbe number will deliver the same amount of energy.

The Wobbe number (or Wobbe index) of a fuel gas is found by dividing the higher heating value of the gas in Btu per standard cubic foot (scf) by the square root of its specific gravity with respect to air.

Pure methane has a Wobbe number of 1,363. The Wobbe numbers of natural gas piped to most homes in the United States range from 1,310 to 1,390, although in any one area of the country it will not fluctuate widely around an historical average that is between those two extremes, Stanonik pointed out.

High Wobbe vs. Low Wobbe

If the new gas has a higher Wobbe number, which usually translates to a higher heating value, this will increase the temperature of the heat exchanger and increase the thermal stressing of that component in furnaces, boilers, room heaters, and commercial heaters. This increased level of heat at the burner and in the combustion chamber will raise the temperature of components in direct contact with the burner flame or installed within the appliance.

If those components are heated beyond their temperature ratings, component failure and appliance operational problems will occur. Even if the elevated component temperature does not exceed its temperature rating, it still may be beyond the designed or normal operational temperature range of the component. In that case, premature failure of the component may result.

Common devices that are in direct contact with the flame include flame sensors or other ignition-proving devices, Stanonik explained.

The obvious things a contractor might see are a larger-than-normal flame, he said, soot inside the combustion chamber, flame impingement on components, or a high limit tripping. These symptoms are not specific to gas quality, of course, but gas quality will need to be considered in the troubleshooting process.

If the new gas has a lower Wobbe number, which usually translates to a lower heating value, the gas appliances will be underfired. The appliance will not deliver as much energy as it's designed to do, generally resulting in decreased performance of gas appliances.

Space heating appliances of all types will have to operate longer to provide the heat required for the space; water heaters will have slower recovery times, and cooking appliances will either cook longer or cook unsatisfactorily. It will not result in higher fuel bills, Stanonik added.

"The consumer probably won't see a change in [the energy] bill," he said. "There will be a change in the comfort level. They may notice that it takes longer to get the house warm."

A significant change in the Wobbe number in either direction presents potential problems due to increasing carbon monoxide production, according to GAMA. It can be caused by incomplete combustion that results from major changes in the air-fuel mixture occurring at the burner, or it can be caused by flame impingement on surfaces within the combustion chamber that alters the combustion process.

"On appliances with properly designed and operating venting systems, this is not an immediate problem in and of itself," Stanonik said. "But it is indicative of improper operation of the appliance. If such a condition develops, it should not be allowed to continue as normal operation."

Contractors also can check for a difference in flame characteristics, such as size and color. Stanonik said that underfiring or significant overfiring both may cause a more yellow flame.

"A lot of contractors are used to performing that kind of visual inspection," he said, noting that contractors also should look for sooting in the appliance, specifically on the heat exchanger's surface. "Many of the symptoms are things that contractors will know to look for anyhow."

Relatively Easy to Fix

Can a contractor service equipment so that combustion is complete, to match the new fuel's Wobbe?

"Absolutely yes," answered Stanonik. "If the new fuel is within the acceptable range of interchangeability and if he's aware of the situation, he can adjust the appliance to put it back on rate."

First, the contractor needs to find out what the actual heating value of the gas is for the day he is out providing service. This entails calling the service utility and asking what the heating value is for the gas it is providing that day. Then the contractor clocks the appliance and measures its firing rate using the home's gas meter. Is its firing higher or lower than the normal input rate?

Next, the contractor should adjust the appliance to go back to its proper firing rate with this new fuel.

In times past, Stanonik said, some contractors adjusted the firing rate based on an average heating value of 1,000 Btu per cubic foot.

"In old days it would work, but now that rule-of-thumb adjustment may not be good enough," he said, adding, "This may become another reason why people should really start to make it a practice to have their appliances checked once a year. There is more motivation."

In the end, contractors need to educate their customers about the importance of annual system maintenance.

"As new appliances are installed or serviced, it is vitally important that installers and service technicians make sure that those appliances are operating at their nameplate hourly input rating," Stanonik said. "In order to do that, the technician will need to know the heating value of the gas being delivered at that time. Using an assumed nominal value will not be good enough."

GAMA's Actions

Recognizing the need to understand and manage this situation, a coalition of gas transmission and distribution companies, gas utilities, GAMA, and other interests was formed: the Natural Gas Council (NGC). One of the products of the National Gas Council was the development of a set of interim guidelines for gas interchangeability.

These interim guidelines were submitted as a recommendation to the Federal Energy Regulatory Commission (FERC) in early March. In essence, the interim guidelines state that for any specific service territory, the quality of the new gas supply (as measured by the Wobbe number) should not vary by more than ±4 percent from the gas that has been supplied historically in that service territory.

At the same time as these interim guidelines are being implemented, the industry hopes to learn more about the performance of end-use equipment using the anticipated new natural gas supplies. In GAMA's case, that equipment includes residential and commercial appliances.

For more information, visit www.gamanet.org.

Publication date: 07/25/2005