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Boiler Efficiency Calculation

Savastat LC Energy Saving Boiler Controls

eTracLogic Systems

ECMI and eTracLogic Systems has developed an easy way to determine approximate space-heat boiler efficiency for comparative purposes. It is a valuable way for facility managers to get an idea on how efficient a space-heat boiler (or cooling tower) is operating over a given period of time.  We describe the value as rFactor, (meaning boiler run-time factor, not to be confused with r-value).  The concept is actually quite simple. We wanted to arrive at a value to show how many "miles per gallon" a boiler is getting. We arrived at the conclusion that to determine this, one simply needs to take the daily boiler runtime divided by the Heating Degree Day value for that day.  (a Heating Degree Day is the average temperature for the day subtracted from 65). The resulting value is a ratio or factor which indicates how the boiler is reacting to varying outside temperatures at a specific site.

For example: If the degree day value for a day is 23.5 (an average temperature of 41.5 degrees F) and the boiler ran for 10.98 hours that day, then the rFactor value would be:  10.98 divided by 23.5  = .46723

Note that the one caveat to using the rFactor method is that days for which the boiler runs less than 2 hours should be omitted to get valid average data values.  This is because we have found that dividing into a value (or by a value) approaching 1 significantly skews the data set.

To use the formula for yourself, monitor a boiler's daily run-time, get the Heating Degree Day information for the monitored period, and perform the daily calculations. (this can also easily be done by creating a spreadsheet)

Be aware that rFactors will vary quite a bit from site to site.  We have monitored numerous boilers and have arrived at a basic data set that gives a good indication of how efficient a boiler is operating.  Use this information only as a guideline. Note that higher rFactors indicate better overall boiler efficiency.

More on Improving Boiler efficiency


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