Flue gas due point question

Zman,

It is page 40 of the ASHRAE pdf you mention that got me curious about the condensing point of the flue gas in a natural gas boiler here in the Denver area. But the gas here has a Btu content of only about 850 Btu/cf as a result of the lower atmospheric pressure and the addition of air by Xcel. I suspect this may materially alter the condensing point (aka dew point). How can one set up outdoor reset for a cast iron boiler without knowing this?

Bob,

Here is an excerpt from a paper by Rosemarie Halchack, the gas quality engineer at Xcel:



"As noted, natural gas supplies on the Colorado Front Range are blended with air for the purpose of Btu stabilization. The original supply of natural gas to the Denver area contained high levels of nitrogen and was characterized by Wobbe factors of 1200-1250. As additional supplies with Wobbes of 1300-1350 became available from Wyoming, the issue of interchangeability became critical. In the late 1970s, the Institute of Gas Technology performed appliance testing and interchangeability calculations to determine that the new supplies could not be safely substituted for the existing base gas because of the risk of yellow tipping and incomplete combustion.



"The most feasible and economic solution to the interchangeability problem was to blend air into the new supplies to reduce the Wobbe to the required values. Air compressors have been installed at three stations supplying Denver and the surrounding areas. The quality of the gas-air mix is controlled using measurements of oxygen content and Wobbe Index. The air added is typically 5-8 percent of the gas flow, and the composition of the resulting mixture is thus significantly above the upper flammability limit.



"In other situations, Xcel Energy has solved interchangeability problems incurred by new gas supplies by reorificing entire distribution systems. The city of Fort Collins, located 70 miles north of Denver, was reorificed for higher heating value gas in 1968, and the remaining area north of Denver was reorificed in a "High Therm Project" in the early 1980s. In 1995 a reorificing effort in the mountain area near Vail was completed. Reorificing is a time-consuming and labor-intensive project that is undertaken only if no other alternatives are available. The company must enter all homes and businesses to install new orifices in the burners of all appliances and to adjust air shutters and manifold pressures to ensure proper combustion. Follow-up inspections are necessary to verify that the work has been properly performed and minimize the liability associated with improper adjustment."

Return temperature of 130°F looks safe

It looks to me like a return temperature of 130°F is OK in an atmospheric cast iron boiler at 5600 ft with CO2 at 8% or less, assuming 850 Btu/cf gas won't raise the condensing temperature from the theoretical 123°F.



My concern is not space heating, as those zones cycle between 150°F and 170°F. But my indirect can drop the return temperature to 130°F for quite a while, as our water supply is now about 40°F and falling.



Thank you all for the comments.