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Posted by dana on October 08, 2008 at 10:28:55:
In Reply to: Re: Hot-Water Boiler Temp Settings posted by Doug Criner on October 07, 2008 at 15:10:38:
: Thanks. I have a newly purchased flue-gas analyzer, and intend to check the efficiency when it's a little colder and I get longer run times.
: Last year, I measured the flue-gas temperature and made an assumption about excess air. The estimated eff was around 80%. I'll be interested to see what the eff is with the flue-gas analyzer.
: The boiler is natural draft, and draft is controlled with an automatic damper connected to the exhaust flue. So, I can adjust the damper if there is too much excess air. What's a good number - 20%?
: The boiler is a Kewanee fire-tube, natural gas, 168,000 Btu/hr. The house has about 4,000 sq ft. The walls are insulated, but not as much as today's standards.
As much fun as it is to mess around with the instrumentation, it's genrally better to either take a burner tech course or hire a pro for this. Getting a burner adjustment wrong is potentially more dangerous than anything that might happen by merely setting the output & return water temps too low. (But do check those- the longevity of the system could be compromised.)
That said, if everything is in good shape, 80% combustion efficiency is realistic. But if it's short-cycling your in-situ system efficiency is severely eroded, and is nothing like the boiler's steady-state thermal efficiency or it's raw combustion efficiency.
But 168kbtuh-in/135kbtuh-out isn't likely to be outlandishly oversized for a modestly insulated 4000 square foot house, but it depends. A full heat-loss analysis should tell you the ball-park, but if the behavior is still low-duty-cycle or short firings even on the coldest days, measured experience trumps any analysis.
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