Short-Cycling Can Hamper Boiler Efficiency

November 7, 2003
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Most contractors have had to deal with a short-cycling boiler at one time or another, but the short-cycling of boilers has become a hot topic in recent years. Manufacturers frequently encounter it when customers call with warranty complaints. The Radiant Panel Association's advanced training manual, Radiant Precision, covers the subject in some depth.

John Siegenthaler, the highly regarded radiant systems expert who has written extensively on the subject, made boiler short-cycling the subject of his presentation at the recent ISH Show in Las Vegas.

One manufacturer is now adding fuel to the debate by stating that short-cycling is a fuel hog, too. "We are seeing efficiency go down by at least 15 percent, and as much as 40 percent, specifically because of short-cycling," said Walt Rykowski, national sales manager for Patterson-Kelley division of Harsco Corp. (East Stroudsburg, Pa.). He claims that preventing short-cycling should be the single most important energy efficiency requirement in both new and existing systems.

Gauging The Effects

There is no industry standard for calculating the energy effects of short-cycling. There is, however, a long-standing, though unpublicized, recognition of its importance among industry leaders going back at least 50 years.

Gus Rehm, the late chief engineer of the Watertube Division of Cleaver-Brooks, indicated that the efficiency of a short-cycling boiler was at least 15 percentage points less than the lowest efficiency achieved at low fire. Cleaver-Brooks' hot water system design manual, now several decades old, has always included what the company called an "oversized header design" (functionally equivalent to a buffer tank) for use in systems where short-cycling was a possibility.

Five years ago, Mark Evans, a recognized boiler room expert and president of Minneapolis- based Blesi-Evans Co., developed a computer model of cycle efficiency, which confirmed that the energy effects of short-cycling can be reliably estimated.

Patterson-Kelley has recently completed development of a boiler room energy analysis software program that, among its other capabilities, calculates cycle and run time effects on fuel use. Here, too, the prevention of short-cycling can be seen to have a dramatic effect on boiler fuel consumption.

"The mistake people are making is in believing that modulating burners with good turndown ratios solve the problem," said Rykowski. Modulating burners are often essential, he believes, and will probably become an industry standard in the years to come. Using the new software, a recent study for a specific project estimated that just changing from high-low firing to full-modulation with a 5:1 turndown ratio lowered fuel use by 12 percent.

"But," emphasized Rykowski, "the job will still short-cycle because of the way the system is designed. The savings from modulation are significant, but the savings from preventing short-cycling are even greater."

Built-In Problems?

Why is short-cycling so common today? There are several reasons. One is that we live in a world of partial loads, and those partial loads are smaller than ever before due to large internal heat gains within buildings and changes in building construction. It is now common for some buildings to see mild day heating loads that are only 4 or 5 percent of the design heating load.

Another reason is the change from high mass, high water content boilers to low mass designs. The most important reason, though, may be system design practices that some experts think build short-cycling into systems in ways that make it a hard problem to solve.

As an example, Rykowski points to radiant panel projects where different zones require different temperatures. Such jobs often have fin-tube radiation and other high temperature zones in addition to lower temperature radiant panels.

In such cases it is common to operate the boiler at a higher temperature and use control valves or variable-speed injection pumps to step down the temperature for the lower temperature zones. A buffer tank can be installed as a secondary circuit off the "buss bar" primary, and controlled so that it only operates during those periods when short-cycling is most likely to occur.

"So far, so good," said Rykowski. "Siegenthaler has developed the methodology. It's a widely publicized design technique."

But, Rykowski adds, most commercial heating systems don't have anything like a high-flow, "buss bar" primary, and finding a place to tie in the buffer tank so that it can be controlled like it would be in a residential system can be a challenge. "Many of the techniques developed for residential systems don't translate very well to non-radiant, light commercial, or larger commercial projects," he said.

There is a growing consensus that Siegenthaler is right in suggesting that buffer tanks should only see flow when they are actually needed so as to avoid radiation losses during times when they really aren't needed. Rykowski cautions that there are systems in which this isn't practical.

"Remember, the energy loss from short-cycling is so great, and the energy loss from a well insulated buffer tank can be so small, that you do best to take the savings from short-cycle prevention - even if it means you give a little back as a result of heat loss from the buffer tank," he noted. "The tail can't wag the dog."

Reassessing System Design

Engineers at Patterson-Kelley believe the energy effects of short-cycling are so great that the company has been actively developing new boiler system layout and design strategies with the goal of making systems "short-cycle-proof." This has caused engineers to rethink long-standing assumptions about boiler room piping and control methods.

"Some of these new designs actually cost less," said Rykowski. "We think that's going to be important if the boiler industry is going to successfully attack the short-cycling problem."

Why is the issue suddenly so hot? Rykowski thinks it's due to changes in boiler technology.

"When a new boiler comes to market, everyone's first thought is to just substitute the new for the old, but the system it's installed in remains the same," he said. "The new equipment can do amazing things, but it has to be installed in systems that allow those benefits to be realized. If the latest, most sophisticated boiler short-cycles, all bets are off on any efficiency claims."

Rykowski notes that all of Patterson-Kelley's new designs have anti-short-cycle logic programmed into the microprocessors that control the boilers.

"Some of the strategies are quite sophisticated," he said, "and these strategies are in boilers that have modulating burners, too. Still it's not enough. Some system designs cause short-cycling, period."

McKeegan is region manager for the Western United States and Canada and principal systems and applications trainer with Patterson-Kelley. For more information, visit www.pkboilers.com.

Publication date: 11/10/2003

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