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Technical Tips & Information
Boiler Efficiency
With boiler fuel prices up substantially, we have had quite a few calls about fuel costs which lead into discussions about boiler efficiency.Most folks I chat with almost automatically state that their boiler efficiency is 81%. When asked how they know this number, the answer cited is “my boiler service man showed me a print out with 81% on the data sheet”. The simple answer is this number is both wrong and correct. It is correct that a combustion analyzer was used to adjust the fuel air mixture to achieve an 81% combustion efficiency. It is incorrect that this figure is any real measure of the actual boiler fuel to steam efficiency.
For most boilers, the fuel to steam efficiency is in the 50-75% range, not 81% from a stack analyzer. How is this large difference possible?
Let’s assume for the moment that the combustion efficiency is 81% across the entire firing range of the boiler. Since the boiler is hot, it radiates heat and this radiation loss is about 4% for a boiler in very good condition. This rating is at a firing rate of 100%. Since the radiation loss is constant when the boiler is firing, the percentage loss rises as a function of the load on the boiler as follows:
Assume your boiler is firing at a 50% rate, your fuel to steam efficiency is down to 73% (81%-8%). Blow down losses are typically about 3% which means your efficiency is now down to 70%. If one or more of your boilers are operating at low firing rates, then your efficiency can easily be in the 60% range. Boiler design, load management, and age have a very large bearing on your efficiency.
The only way to measure efficiency of a boiler is with meters to monitor fuel flow and water or steam meters to measure output.
As with gas and diesel engines, remarkable advances in electronics and controls have permitted quantum leaps in efficiency of boilers over the past 10-15 years. If you are operating a boiler 20 years or older, you are running the equivalent of an old V8 engine with a four barrel carburetor. Reliable and fast for terrible fuel efficiency. With boiler fuel prices up over 50% in the past months, you might be wise to look at more efficient ways to generate steam or hot water.
Miura Boilers are Japanese designed and North American built boilers which deliver 85% fuel to steam efficiency 24/7. In most cases you can cut your fuel bill by 15% or more by switching to high efficiency PLC operated boilers. At current fuel prices, a completely new boiler room installed will pay back in 10-15 months.