[an error occurred while processing this directive]
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.
: Doug
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.