This is the ratio of BTU output divided by BTU input on a particular boiler. This includes all the heat loss (e.g. radiation and convection losses) from a boiler and is typically used in a manufacturer's catalouge. Fuel-to-steam efficiency is a readily vertifiable number and is often used in efficiency guarantees by various manufacturers. It is the ideal "Fire-rate" efficiency which is tested for about 10 minutes with a stack analyzer. Miura does not think the above efficiencies explain "real-world" efficiencies. Each boiler runs under different operating condition such as load, operating time, radiation heat losses from boiler physical surface area and so on.

Boiler efficiency is one of many factors in controlling overhead costs. It is important to understand what boiler efficiency really means and how to use them if you are serious in reducing overall operating cost in your system.

Let's begin with the common definitions of efficiency as related to the boiler...

This is the effectivness of the burner only and relates to its ability to completely burn the fuel. The boiler or heat exchanger has little influence on combustion efficiency.

We have developed a new definition of "IN-SERVICE EFFICIENCY" to describe these savings. The Definition is: The resulting efficiency of a boiler when the total operation cycles are taken into account such as day, night, weekends, high loads, low loads, standby loads. It is a comprehensive efficiency which is based upon an operating model and is the "bottom line" efficiency, which should be used in any boiler comparison. It reflects how well a particular boiler design handles a particular operating model.

This is the effectiveness of the heat transfer in a heat exchanger. It does not take into account boiler radiation and convection losses.

Be careful when a salesperson mentions this term. This is a general term and must be clearly defined by the manufacturer.

The basic difference between Fuel-to-Steam versus In-Service efficiencies is that the Fuel-to-Steam efficiency is measured at a fixed fire-rate and includes radiation and convection losses; In-Service efficiency measures how well a particular boiler design responds from bank firing (idling) to full fire under a projected operating schedule. For example a 100% fire rate - which includes convection and radiation losses (fuel-to-steam) - the efficiency of a typical firetube boiler for natural gas at an operating steam pressure of 125 PSIG is about 81%. Drop the fire rate (due to convection + radiation losses) to 25% for the same boiler and the efficiency becomes about 78.5%. Likewise an In-Service efficiency of a boiler at 100% load, (meaning full fire 24 hours per day, 365 days per year) will be equal to Fuel-to-Steam efficiency. However, drop the steam load to 25%, (i.e. full fire for six hours and idling for 18 hours) and the In-Service efficiency will be much lower than 78.5% efficiency. Idle the boiler to keep it warm and ready to deliver steam upon demand and the In-Service efficiency becomes zero. In-Service efficiency is the most conservative approach to efficiencies and should be considered in order to project what the bottom line fuel consumption of a boiler should be.