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Posted by jsf on January 07, 2006 at 18:27:13:
In Reply to: Re: Energy Savings 101- Electric vs. Steam? posted by J.C. on December 16, 2005 at 11:59:57:
Hi JC,
Regardless of what Burnham's technician told you, you can easily determine you are getting just over 81% theoretical efficiency from your boiler.
This is calculated from the input BTU/hr rating versus the output rating of the boiler in BTU/hr
142,000/175,000 x 100 = 81%
Actual variance from this depend on boiler and venting system installation and condition, and whether the boiler is oversized or not.
Venting plays a very important issue on efficiency. The vent temperature must be in an ideal range to reach peak efficiency. If the vent is under or over sized the flue temp will not be ideal. If the boiler short cycles and the flue does not reach temp quickly the boiler will spend much of the time running at less than peak efficiency. As an example an oil boiler with a masonry chimney may take 15 minutes to reach steady state conditions with peak efficiency.
Real world testing by various organizations show that actaul installed efficiencies would be in the neighborhood of the mid 70% range. This is much higher than figures reported by service technicians and more importantly HVAC sales people who have a good reason to undersell the efficiencies of older equipment which they want you to replace.
I would use 75% as your multiplier for calculating the actual usable therms you are purchasing actually going to heat your home using gas heat. Whether that heat is distributed where in the home it is needed is something else entirely, and varies widely. I have found steam systems tend to be poorly designed in the past when it comes to good designed distribution of heat.
(based on your own cost per therm calculations) yo